Cytoplasm - Clinical Tree

The basic functional unit of complex organisms is the cell. Cells that have or serve a common purpose congregate to form tissues, which, in animals — specifically, mammals — are placed in four categories (epithelium, connective tissue, muscle, and nervous tissue). These tissues assemble to form organs, which, in turn, are collected into the various organ systems of the body. Each organ system performs a collection of associated functions, such as digestion, reproduction, and respiration.

Although there are more than 200 different types of cells that comprise the body, each having a different function, they all possess certain unifying characteristics and, thus, can be described in general terms ( Figs. 2.1 to 2.4 ). Every cell is surrounded by a plasma membrane, possesses organelles that permit it to discharge its functions, synthesizes macromolecules for its own use or for export, produces energy, and is capable of communicating with other cells. The number and disposition of the organelles varies not only with the cell in question but also with the particular stage in life cycle of that cell.

Protoplasm , the living substance of the cell, is subdivided into two compartments: the cytoplasm , extending from the plasma membrane to the nuclear envelope, and the karyoplasm , the material forming the contents of the nucleus. The cytoplasm is detailed in this chapter; the nucleus is discussed in Chapter 3 .

The cytoplasm is composed mostly of water , in which various inorganic and organic chemicals are dissolved and/or suspended. This fluid suspension is called the cytosol ( intracellular fluid ), and it is that portion of the cytoplasm that is left after all organelles, the cytoskeleton, and inclusions are removed from the cytoplasm. Organelles are metabolically active structures that perform distinctive functions ( Figs. 2.5 and 2.6 ). The cytoskeleton , a system of tubules and filaments, maintains the shapes of cells and enables them to move and form the intracellular pathways within cells. Inclusions consist of metabolic by-products, storage forms of various nutrients, or inert crystals and pigments.

Organelles

Organelles are metabolically active cellular structures that execute specific functions.

Although some organelles were discovered by light microscopists, their structure and function were not elucidated until the advent of electron microscopy, separation techniques, and sensitive biochemical and histochemical procedures. As a result of the application of these methods, it is now known that the membranes of organelles are composed of a phospholipid bilayer , which not only partitions the cell into compartments but also provides large surface areas for the biochemical reactions essential for the maintenance of life.

Cell Membrane

The cell membrane forms a selectively permeable barrier between the cytoplasm and the external milieu.

Each cell is bounded by a cell membrane (plasma membrane; plasmalemma) that

Maintains the structural integrity of the cell
Controls movements of substances in and out of the cell (selective permeability)
Regulates cell–cell interactions
Recognizes, via receptors, antigens and foreign cells, as well as altered cells
Acts as an interface between the cytoplasm and the external milieu
Establishes transport systems for specific molecules
Sustains a potential difference between the intracellular and extracellular aspects of the membrane
Transduces extracellular physical or chemical signals into intracellular events

Cell membranes are not visible with the light microscope. In electron micrographs, the plasmalemma is about 7.5 nm thick and appears as a trilaminar structure of two thin, dense lines with an intervening light area. Each layer is about 2.5 nm in width, and the entire structure is known as the unit membrane ( Fig. 2.7 ). The inner (cytoplasmic) dense line is its inner leaflet ; the outer dense line is its outer leaflet .

Fig. 2.7, A junction between two cells demonstrates the trilaminar structures of the two cell membranes (×240,000).

Molecular Composition

The plasmalemma is composed of a phospholipid bilayer and associated integral and peripheral proteins.

Each leaflet is composed of a single layer of phospholipids and associated proteins , usually in a 1:1 proportion by weight. In certain cases, such as myelin sheaths, however, the lipid component outweighs the protein component by a ratio of 4:1. The two leaflets, the phospholipid bilayer with associated proteins , form the basic structure of all membranes of the cell ( Fig. 2.8 ). Although the two leaflets appear indistinguishable from each other, their phospholipid compositions are different, making the two leaflets asymmetrical .

Fig. 2.8, Three-dimensional diagrammatic representation of the fluid mosaic model of the cell membrane.

Each phospholipid molecule of the lipid bilayer is amphipathic because it is composed of a polar head , located at the surface of the membrane, and two long nonpolar fatty acyl (fatty acid) tails, usually consisting of chains of 16 to 18 carbon atoms, projecting into the center of the plasmalemma (see Fig. 2.8 ). The nonpolar fatty acyl tails of the two layers face each other within the membrane and form weak noncovalent bonds with each other, holding the bilayer together.

The polar heads are composed of glycerol , to which a positively charged nitrogenous group is attached by a negatively charged phosphate group . The two fatty acyl tails, only one of which is usually saturated, are covalently bound to glycerol. Other amphipathic molecules—such as glycolipids, glycosphingolipids, and cholesterol —are also present in the cell membrane. The unsaturated fatty acyl molecules increase membrane fluidity, whereas cholesterol decreases it (although cholesterol concentrations much lower than normal increases membrane fluidity). In fact, certain regions of the plasmalemma are so well endowed with glycosphingolipids and cholesterol that they create a bulge in the cell membrane. These thickened microdomains are known as lipid rafts , which form a slight bulge into the extracellular space. Frequently, lipid rafts possess protein components that participate in diverse signaling events. Therefore, lipid rafts appear to facilitate and enhance the possibilities of communications between a variety of cells.

The protein components of the plasmalemma either span the entire lipid bilayer as integral proteins or are attached to the cytoplasmic aspect (and, at times, the extracellular aspect) of the lipid bilayer as peripheral proteins . Because most integral proteins pass through the thickness of the membrane, they are also referred to as transmembrane proteins . Those regions of transmembrane proteins that project into the cytoplasm or the extracellular space are composed of hydrophilic amino acids, whereas the intramembrane region consists of hydrophobic amino acids. Transmembrane proteins frequently form ion channels and carrier proteins that facilitate the passage of specific ions and molecules across the cell membrane.

Many of these transmembrane proteins are quite long and are folded so that they make several passes through the membrane. Thus, they are known as multipass proteins and are frequently attached to the inner leaflet (and infrequently to the outer leaflet) by prenyl groups or fatty acyl groups. The cytoplasmic and extracytoplasmic aspects of these proteins commonly possess receptor sites that are specific for particular signaling molecules . Once these molecules are recognized at these receptor sites, the integral proteins can alter their conformation and can perform a specific function.

Because integral membrane proteins have the ability to float like icebergs in the sea of phospholipids, this model is referred to as the fluid mosaic model of membrane structure. However, the integral proteins frequently possess only limited mobility, especially in polarized cells, in which particular regions of the cell serve specialized functions.

Peripheral proteins do not usually form covalent bonds with either the integral proteins or the phospholipid components of the cell membrane. Although they are usually located on the cytoplasmic aspect of the cell membrane, they may also be on the extracellular surface. These proteins may form bonds, either with the phospholipid molecules or with the transmembrane proteins. Frequently, they are associated with the secondary messenger system of the cell (discussed below) or with the cytoskeletal apparatus.

Using freeze-fracture techniques, one can cleave the plasma membrane into its two leaflets in order to view the hydrophobic surfaces ( Figs. 2.9 and 2.10 ). The outer surface of the inner leaflet is referred to as the P-face (closer to the p rotoplasm ); the inner surface of the outer leaflet is known as the E-face (closer to the e xtracellular space). Electron micrographs of freeze-fractured plasma membranes show that the integral proteins, visualized by shadowing replica, are more numerous on the P-face than on the E-face (see Fig. 2.10 ).

Fig. 2.9, Schematic diagram of the E-face and P-face of the cell membrane.

$Fig. 2.10, Freeze-fracture replica of a cell membrane. The E-face (closer to the extracellular space) is on the right, and the P-face (closer to the protoplasm ) is on the left. Note that the integral proteins are more numerous on the P-face (left-hand side) than on the E-face (right-hand side). (×168,000)$

Glycocalyx

Glycocalyx, composed usually of carbohydrate chains, coats the cell surface.

A fuzzy coat, referred to as the cell coat or glycocalyx , is often evident in electron micrographs of the cell membrane. This coat is usually composed of carbohydrate chains that are covalently attached to transmembrane proteins and/or phospholipid molecules of the outer leaflet (see Fig. 2.8 ). Its intensity and thickness vary, but it may be as thick as 50 nm on some epithelial sheaths, such as those lining regions of the digestive system.

The most important function of the glycocalyx is protection of the cell from interaction with inappropriate proteins, chemical injury, and physical injury. Other cell coat functions include cell–cell recognition and adhesion, as occurs between endothelial cells and neutrophils, as well as T cells and antigen-presenting cells; facilitating blood clotting and inflammatory responses; and assisting in reducing friction between blood and the endothelial cells lining blood vessels.

Membrane Transport Proteins

Membrane transport proteins are of two types, channel proteins and carrier proteins; they facilitate the movement of aqueous molecules and ions across the plasmalemma.

Although the hydrophobic components of the plasma membrane limit the movement of polar molecules across it, the presence and activities of specialized transmembrane proteins facilitate the transfer of these hydrophilic molecules across this barrier. These transmembrane proteins and protein complexes form channel proteins and carrier proteins , which are specifically concerned with the transfer of ions and small molecules across the plasma membrane.

A number of small nonpolar molecules (e.g., benzene, oxygen, nitrogen) and uncharged polar molecules (e.g., water, glycerol) can move across the cell membrane by simple diffusion down their concentration gradients. Enhanced movement of most ions and small molecules across a membrane requires the aid of either channel proteins or carrier proteins. This process is known as facilitated diffusion . Because both types of diffusion occur without any input of energy other than that inherent in the concentration gradient, they represent passive transport ( Fig. 2.11 ). By expending energy, cells can transport ions and small molecules against their concentration gradients. Only carrier proteins can mediate such energy-requiring active transport . The several channel proteins involved in facilitated diffusion are discussed first, followed by consideration of the more versatile carrier proteins.

Fig. 2.11, Types of transport. (A) Passive transport: diffusion, ion channel–mediated diffusion, and carrier-mediated diffusion. (B) Active transport: coupled transport. Symport and antiport.

Channel Proteins

Channel proteins may be gated or ungated; they are incapable of transporting substances against a concentration gradient.

Channel proteins participate in the formation of hydrophilic pores, called ion channels , across the plasmalemma. There are the more than 100 different types of ion channels. Some of these are specific for one particular ion; others permit the passage of several different ions and small water-soluble molecules. Although these ions and small molecules follow chemical or electrochemical concentration gradients for the direction of their passage, cells have the capability of preventing these substances from entering these hydrophilic tunnels by means of controllable gates that block their opening. Most channels are gated channels ; only a few are ungated . Gated channels are classified according to the control mechanism required to open the gate.

Voltage-Gated Channels

Voltage-gated channels go from the closed to the position, permitting the passage of ions from one side of the membrane to the other. The most common example is depolarization in the transmission of nerve impulses. In some channels, such as Na ⁺ channels, the open position is unstable and the channel goes from an open to inactive position , in which the passage of the ion is blocked and for a short time (a few milliseconds) the gate cannot be opened again. This is the refractory period (see Chapter 9 on the nervous system). The velocity of response to depolarization may also vary; some of those channels are referred to as velocity-dependent .

Ligand-Gated Channels

Channels that require the binding of a ligand (signaling molecule) to the channel protein to open their gate are known as ligand-gated channels . Unlike voltage-gated channels, these channels remain open until the ligand dissociates from the channel protein; they are referred to as ion channel–linked receptors . Some of the ligands controlling these gates are neurotransmitters, whereas others are nucleotides. These ligands can be neurotransmitters (neurotransmitter-gated channels), such as acetylcholine; and nucleotides (nucleotide-gated channels), such as cyclic AMP and cyclic GMP.

Mechanically Gated Channels

In mechanically gated channels, an actual physical manipulation is required to open the gate. An example of this mechanism is found in the hair cells of the inner ear. These cells, located on the basilar membrane, possess stereocilia that are embedded in a matrix known as the tectorial membrane . Movement of the basilar membrane causes a shift in the positions of the hair cells, resulting in the bending of the stereocilia. This physical distortion opens the mechanically gated channels of the stereocilia.

G-Protein–Gated Ion Channels

Certain gated ion channels (e.g., muscarinic acetylcholine receptors of cardiac muscle cells) require the interaction between a receptor molecule and a G-protein complex (discussed later in this chapter) with the resultant activation of the G protein. The activated G protein then interacts with the channel protein, modulating the ability of the channel to open or close.

Ungated Channels

One of the most common forms of an ungated channel is the potassium (K ⁺ ) leak channel , which permits the movement of K ⁺ ions across it and is instrumental in the creation of an electrical potential (voltage) difference between the two sides of the cell membrane. Because this channel is ungated, the transit of K ⁺ ions is not under the cell’s control; rather, the direction of ion movement reflects its concentration on the two sides of the membrane.

Aquaporins

Currently, there are at least 13 different types of aquaporins , a family of multipass proteins that form channels designed for the passage of water from one side of the cell membrane to the other. Some of these channels are pure water transporters (e.g., AqpZ), whereas others transport glycerol (GlpF). These aquaporins discriminate in the transport of the two molecules by restricting the pore sizes in such a fashion that glycerol is too large to pass through the pore of the AqpZ channel. An interesting property of aquaporins is that they are completely impermeable to protons, so that streams of protons cannot traverse the channel even though they readily pass through water molecules via the process of donor-acceptor configurations. Aquaporins interfere with this donor-acceptor model by forcing the water molecules to flip-flop halfway along the channel, so that water molecules enter the channel face up (hydrogen side up and oxygen side down—that is, the oxygen enters first, followed by the two hydrogens), flip over, and leave the channel face down (so that the two hydrogen molecules leave first, followed by the oxygen). Properly functioning aquaporins in the kidney may transport as much as 20 L of water per hour, whereas improperly functioning aquaporins may result in diseases such as diabetes insipidus and congenital cataracts of the eye.

Carrier Proteins

Carrier proteins can use ATP-driven transport mechanisms to ferry specific substances across the plasmalemma against a concentration gradient.

Carrier proteins are multipass membrane transport proteins that possess binding sites for specific ions or molecules on both sides of the phospholipid bilayer. When an ion or molecule specific to the particular carrier protein binds to the binding site, the carrier protein undergoes reversible conformational changes; as the ion or molecule is released on the other side of the membrane, the carrier protein returns to its previous conformation.

As stated earlier, transport by carrier proteins may be passive , along an electrochemical concentration gradient, or active , against a gradient, thereby requiring energy expenditure by the cell. Transport may be uniport , a single molecule moving in one direction, or coupled , two different molecules moving in the same ( symport ) or opposite ( antiport ) directions (see Fig. 2.11 ). Coupled transporters convey the solutes either simultaneously or sequentially.

Primary Active Transport by Na ⁺ -K ⁺ Pump

Normally, the concentration of Na ⁺ is much greater outside the cell than inside, and the concentration of K ⁺ is much greater inside the cell than outside. The cell maintains this concentration differential by expending adenosine triphosphate ( ATP ) to drive a coupled antiport carrier protein known as the Na ⁺ - K ⁺ pump. This pump transports K ⁺ ions into and Na ⁺ ions out of the cell, each against a steep concentration gradient. Na ⁺ -K ⁺ ATPase has been shown to be associated with the Na ⁺ -K ⁺ pump. When three Na ⁺ ions bind on the cytosolic aspect of the pump, ATP is hydrolyzed to adenosine diphosphate ( ADP ) and the released phosphate ion is used to phosphorylate the ATPase, resulting in alteration of the conformation of the pump, with the consequent transfer of Na ⁺ ions out of the cell. Binding of two K ⁺ ions on the external aspect of the pump causes dephosphorylation of the ATPase with an ensuing return of the carrier protein to its previous conformation, resulting in the transfer of the K ⁺ ions into the cell. Thus, the expenditure of a single ATP molecule provides the energy for the transfer of three Na ⁺ ions and two K ⁺ ions across the cell membrane.

The constant operation of this pump reduces the intracellular ion concentration, resulting in decreased intracellular osmotic pressure. Because the binding sites on the external aspect of the pump bind not only K ⁺ but also the glycoside ouabain , this glycoside inhibits the Na ⁺ -K ⁺ pump.

Secondary Active Transport by Coupled Carrier Proteins

The ATP-driven transport of Na ⁺ out of the cell establishes a low intracellular concentration of that ion. The energy reservoir inherent in the sodium ion gradient can be used by carrier proteins to transport ions or other molecules against a concentration gradient. Frequently, this mode of active transport is referred to as secondary active transport , distinct from primary active transport , which uses the energy released from the hydrolysis of ATP. The carrier proteins that participate in secondary active transport are either symports or antiports.

ATP-Binding Cassette Transporters (ABC-Transporters)

These highly conserved transporters occur in the largest numbers of all carrier proteins. They are present in both prokaryotic organisms, such as bacteria, and in all eukaryotic organisms. The major difference is that in prokaryotic organisms, the ABC-transporters move substances in both directions (into and out of the cell) whereas in eukaryotic cells, the transport is in a single direction only, namely, out of the cell; only the eukaryotic transporters are discussed here.

ABC-transporters are transmembrane proteins, thus, protruding through both sides of the cell membrane. The intracellular portion of the transporters possesses binding sites (known as ATP-binding cassettes ) for two ATP molecules. When ATP is not present, the intracellular binding sites for specific molecules are exposed and the particular ion or molecule adheres to the binding site. When the ATP molecules bind to the ATP-binding cassettes, the transporter’s conformation becomes altered and the ion or molecule is permitted to leave at the transporter’s extracellular surface. It should be stated that not all ABC-transporters are located on the plasmalemma; many are present on the membranes of intracellular membranous organelles, such as the trans-Golgi network, rough endoplasmic reticulum, and mitochondrion.

Clinical Correlations

A member of the ABC-transporters, the cystic fibrosis transmembrane conductance regulator protein ( CFTR protein , coded for by a mutated form of the CFTR gene) is responsible for the formation of abnormal chloride channel proteins, especially in the respiratory system. The channels formed by these proteins do not permit Cl ⁻ ions to pass through them to leave the cell; thereby, the increased negative charge due to the increased concentration of chloride ions in the cytoplasm attracts Na ⁺ ions into the cells. The elevated NaCl content of the cell attracts water from the extracellular milieu into the cell, increasing the viscosity of the mucus lining the respiratory tract. The thickened mucus blocks the smaller bronchioles, leading to infection, debilitated lung function, and, eventually, death.

Many ABC-transporters transport various hydrophobic toxic substances and drugs out of the cell. Various cancer cells possess specific ABC-transporters, known as multidrug resistance proteins ( MDR proteins ), that drive anticancer drugs out of the cell, providing the malignant cells with increased resistance to chemotherapeutic agents.

Cell Signaling

Cell signaling is the communication that occurs when signaling cells release signaling molecules that bind to cell surface receptors of target cells.

When cells communicate with each other, the one that sends the signal is called the signaling cell ; the cell receiving the signal is called the target cell . Transmission of the information may occur either by the secretion or presentation of signaling molecules , which contact receptors on the target cell membrane (or intracellularly either in the cytosol or nucleus) or by the formation of intercellular pores known as gap junctions , which permit the movement of ions and small molecules (e.g., cyclic adenosine monophosphate [cAMP]) between the two cells. Gap junctions are discussed in Chapter 5 .

The signaling molecule, or ligand , may be either secreted and released by the signaling cell or may remain bound to its surface and presented by the signaling cell to the target cell. A cell-surface receptor usually is a transmembrane protein; an intracellular receptor is a protein that resides in the cytosol or in the nucleus of the target cell. Ligands that bind to cell-surface receptors usually are polar molecules; those that bind to intracellular receptors are hydrophobic and, thus, can diffuse through the cell membrane ( Table 2.1 ).

TABLE 2.1

Types of Signaling

Signaling Type	Description
Synaptic signaling	The signaling molecule, a neurotransmitter, is released so close to the target cell that only a single cell is affected by the ligand.
Paracrine signaling	The signaling molecule is released into the intercellular environment and affects cells in its immediate vicinity.
Autocrine signaling	The signaling cell is also the target cell.
Endocrine signaling	The signaling molecule enters the bloodstream to be ferried to target cells situated at a distance from the signaling cell.

Signaling Molecules

Signaling molecules bind to extracellular or intracellular receptors to elicit a specific cellular response.

Most signaling molecules are hydrophilic (e.g., acetylcholine ) and cannot penetrate the cell membrane. Therefore, they require receptors on the cell surface. Other signaling molecules are either hydrophobic, such as steroid hormones , or are small nonpolar molecules, such as nitric oxide ( NO ), both of which have the ability to diffuse through the phospholipid bilayer. These ligands require the presence of an intracellular receptor. Hydrophilic ligands have a very short life span (a few milliseconds to minutes at most), whereas steroid hormones last for extended time periods (several hours to days).

Binding of signaling molecules to their receptors activates an intracellular second messenger system , initiating a cascade of reactions that result in the required response. A hormone, for example, binds to its receptors on the cell membrane of its target cell. The receptor alters its conformation, with the resultant activation of adenylate cyclase , a transmembrane protein whose cytoplasmic region catalyzes the transformation of ATP to cAMP , one of the most common second messengers.

The second messenger, cAMP, activates a cascade of enzymes within the cell, multiplying the effects of a very few molecules of hormones on the cell surface. The specific intracellular event depends on the enzymes located within the cell; for instance, cAMP activates one set of enzymes within an endothelial cell and another set of enzymes within a follicular cell of the thyroid gland. Therefore, the same molecule can have a different effect in different cells. The system is known as a second messenger system because the hormone is the first messenger that activates the formation of cAMP, the second messenger. Other second messengers include calcium (Ca ²⁺ ), cGMP, inositol triphosphate (IP ₃ ), and diacylglycerol.

Steroid hormones (e.g., cortisol) can diffuse through the cell membrane. Once in the cytosol, they bind to steroid hormone receptors (members of the intracellular receptor family ), and the ligand-receptor complex activates gene expression, or transcription (the formation of messenger ribonucleic acid [ mRNA ]). Transcription may be induced directly, resulting in a fast primary response , or indirectly, bringing about a slower secondary response. In the secondary response , the mRNA codes for the protein that is necessary to activate the expression of additional genes.

Cell-Surface Receptors

Cell-surface receptors are of three types: ion channel–linked, enzyme-linked, and G-protein–linked.

Most cell-surface receptors are integral glycoproteins that function in recognizing signaling molecules and in transducing the signal into an intracellular action. The three main classes of receptor molecules are ion channel–linked receptors, enzyme-linked receptors, and G-protein–linked receptors.

Enzyme-Linked Receptors

Enzyme-linked receptors are transmembrane proteins whose extracellular regions act as receptors for specific ligands. When a signaling molecule binds to the receptor site, the receptor’s intracellular domain becomes activated so that it now possesses enzymatic capabilities. These enzymes then either induce the formation of second messengers, such as cGMP, or permit the assembly of intracellular signaling molecules that relay the signal intracellularly. This signal then elicits the required response by activating additional enzyme systems or by stimulating gene regulatory proteins to initiate the transcription of specific genes.

G-Protein–Linked Receptors

G-protein–linked receptors are multipass proteins whose extracellular domains act as receptor sites for ligands. Their intracellular regions have two separate sites: one that binds to G proteins and another that becomes phosphorylated during the process of receptor desensitization.

Most cells possess two types of GTPases (monomeric and trimeric), each of which has the capability of binding guanosine triphosphate ( GTP ) and guanosine diphosphate ( GDP ). Trimeric GTPases, G proteins , are composed of a large α subunit and two small subunits , β and γ , and can associate with G-protein–linked receptors. There are several types of G proteins, including:

Stimulatory ( G _s )
Inhibitory (G _i )
Pertussis toxin-sensitive ( G _o )
G _olf
Pertussis toxin-insensitive ( G _Bq )
Transducin ( G _t )
G _12/13

G proteins act by linking receptors with enzymes that modulate the levels of the intracellular signaling molecules (second messengers) cAMP or Ca ²⁺ .

Signaling via G _s and G _i Proteins

G _s proteins ( Fig. 2.12 ) are usually present in the inactive state, in which a GDP molecule is bound to the α subunit. When a ligand binds to the G-protein–linked receptor, it alters the receptor’s conformation, permitting it to bind to the α subunit of the G _s protein , which, in turn, exchanges its GDP for a GTP . The binding of GTP causes the α subunit to dissociate not only from the receptor but also from the other two subunits and to bind with adenylate cyclase , a transmembrane protein. This binding activates adenylate cyclase to form many molecules of cAMP from ATP molecules. As the activation of adenylate cyclase is occurring, the ligand uncouples from the G-protein–linked receptor, returning the receptor to its original conformation without affecting the activity of the α subunit. Within a few seconds, the α subunit hydrolyzes its GTP to GDP, detaches from adenylate cyclase (thus, deactivating it), and reassociates the β and γ subunits.

Fig. 2.12, G-protein–linked receptor. When the signaling molecule contacts its receptor, the α subunit dissociates from the G protein and contacts and activates adenylate cyclase, which converts adenosine triphosphate (ATP) to cyclic adenosine monophosphate (cAMP). GDP, guanosine diphosphate; GTP, guanosine triphosphate; PPi, pyrophosphate.

G _i protein behaves similarly to G _s , but instead of activating adenylate cyclase, it inhibits it so that cAMP is not being produced. The lack of cAMP prevents the phosphorylation—thus, activation—of enzymes that would elicit a particular response. Hence, a particular ligand binding to a particular receptor may activate or inactivate the cell depending on the type of G protein that couples it to adenylate cyclase.

Cyclic AMP and Its Role as a Second Messenger

cAMP is an intracellular signaling molecule that activates cAMP-dependent protein kinase ( A-kinase ) by binding to it. The activated A-kinase dissociates into its regulatory component and two active catalytic subunits . The active catalytic subunits phosphorylate other enzymes in the cytosol, initiating a cascade of phosphorylations and resulting in a specific response. Elevated levels of cAMP in some cells result in the transcription of those genes whose regulatory regions possess cAMP response elements ( CREs ). A-kinase phosphorylates—and, thus, activates—a gene-regulatory protein known as CRE-binding protein (CREB) whose binding to the CRE stimulates the transcription of those genes.

As long as cAMP is present at a high enough concentration, a particular response is elicited from the target cell. In order to prevent responses of unduly long duration, cAMP is quickly degraded by cAMP phosphodiesterases to 5′-AMP, which is unable to activate A-kinase. Moreover, the enzymes phosphorylated during the cascade of phosphorylations become deactivated by becoming dephosphorylated by another series of enzymes ( serine/threonine phosphoprotein phosphatases ).

Signaling via G _o Protein

When a ligand becomes bound to G _o -protein–linked receptor , the receptor alters its conformation and binds with G _o . This trimeric protein dissociates, and its subunit activates phospholipase C , the enzyme responsible for cleaving the membrane phospholipid phosphatidylinositol bisphosphate ( PIP ₂ ) into IP ₃ and diacylglycerol . IP ₃ leaves the membrane and diffuses to the endoplasmic reticulum, where it causes the release of Ca ²⁺ —another second messenger—into the cytosol. Diacylglycerol remains attached to the inner leaflet of the plasma membrane and, with the assistance of Ca ²⁺ , activates the enzyme protein kinase C ( C-kinase ). C-kinase, in turn, initiates a phosphorylation cascade, whose end result is the activation of gene-regulatory proteins that initiate transcription of specific genes.

IP ₃ is rapidly inactivated by being dephosphorylated, and diacylglycerol is catabolized within a few seconds after its formation. These actions ensure that responses to a ligand are of limited duration.

Ca ²⁺ and Calmodulin

Because cytosolic Ca ²⁺ acts as an important second messenger, its cytosolic concentration must be carefully controlled by the cell. These control mechanisms include the sequestering of Ca ²⁺ by the endoplasmic reticulum, specific Ca ²⁺ -binding molecules in the cytosol and mitochondria, and the active transport of this ion out of the cell.

When IP ₃ causes elevated cytosolic Ca ²⁺ levels, the excess ions bind to calmodulin , a protein found in high concentration in most animal cells. The Ca ²⁺ -calmodulin complex activates a group of enzymes known as Ca ²⁺ -calmodulin–dependent protein kinases ( CaM-kinases ). CaM-kinases have numerous regulatory functions in the cell, such as initiation of glycogenolysis, synthesis of catecholamines, and contraction of smooth muscle.

You're Reading a Preview

Become a Clinical Tree membership for Full access and enjoy Unlimited articles

Become membership

If you are a member. Log in here