Table 702

Vitamin/Parent

Heme Lipoic acid

Biopterin

Heme Lipoamide

Tetrahydrobiopterin

Function vision decarboxylations many redox reactions redox reactions (degradative)

redox reactions (biosynthetic)

acylation reactions fatty acid synthesis amino acid metabolism m ethyl group carrier rearrangements carboxylations redox reactions and one carbon carrier post-translational carboxylation of glutamate hormone/regulator antioxidant hormone controlling Ca and P metabolism antioxidant oxygen carrier redox reactions 2-carbon carrier Phe metabolism

Certain proteins are linked to other large molecules, such as lipids, carbohydrates or nucleic acids.

can be made by some animals but not others. Thus vitamin C is a vitamin for humans but is not a dietary requirement for most other mammals.

Nucleoproteins, Lipoproteins and Glycoproteins Are Conjugated Proteins

Conjugated proteins are proteins that are linked to molecules of other types. For example, nucleoproteins are complexes of protein and nucleic acid, lipoproteins are proteins with lipid attached and glycoproteins have carbohydrate components.

conjugated protein Complex of protein plus another molecule glycoprotein Complex of protein plus carbohydrate lipoprotein Complex of protein plus lipid nucleoprotein Complex of protein plus nucleic acid

Nucleoproteins, Lipoproteins and Glycoproteins Are Conjugated Proteins 173

FIGURE 7.21 A Membrane Glycoprotein

A glycoprotein in the cytoplasmic membrane of an animal cell is shown protruding from both sides of the membrane. At the exterior surface, several sugar residues project from the protein into the extra-cellular space.

Nucleoproteins, Lipoproteins and Glycoproteins Are Conjugated Proteins 173

FIGURE 7.21 A Membrane Glycoprotein

A glycoprotein in the cytoplasmic membrane of an animal cell is shown protruding from both sides of the membrane. At the exterior surface, several sugar residues project from the protein into the extra-cellular space.

FIGURE 7.22 Extracellular Enzyme Tethered by Lipid Tail

A lipoprotein is held to the membrane surface by one or more lipid tails that penetrate into the lipid bilayer of the membrane. The lipid tails are related to those of the phospholipids that comprise the membrane itself.

Tethered extracellular enzyme

Tethered extracellular enzyme

FIGURE 7.22 Extracellular Enzyme Tethered by Lipid Tail

A lipoprotein is held to the membrane surface by one or more lipid tails that penetrate into the lipid bilayer of the membrane. The lipid tails are related to those of the phospholipids that comprise the membrane itself.

Many glycoproteins are found at the surface of cells. They carry short carbohydrate chains consisting of several sugar molecules, which usually project outward from the cell (Fig. 7.21). The sugar chain is usually linked via the hydroxyl group of serine or threonine or the amide group of asparagine. Glycoproteins often function in cell-to-cell adhesion, especially in organisms (animals) that lack rigid cell walls. In addition, the carbohydrate portion of glycoproteins is often the key factor in cellular recognition. For example, sperm recognize egg cells by binding to the carbohydrate part of a surface glycoprotein. Recognition by the immune system often depends on the precise structure of the carbohydrate chains of glycoproteins. For example, the A- and B-antigens of the well-known ABO blood typing system are actually protein-borne carbohydrate chains that differ in the presence or absence of a single sugar.

Many lipoproteins are attached to membranes by their lipid tails (Fig. 7.22). An example is the b-lactamase found in many gram-positive bacteria, such as Bacillus.The b-lactamase protects the cell by destroying antibiotics of the b-lactam family, which includes penicillin. Since the target for penicillin action is the cell wall, the protective enzyme needs to be outside the cell. The lipid tail makes sure it does not drift away into the surrounding medium.

Proteolipids are a specialized subclass of lipoproteins that are extremely hydrophobic and are insoluble in water. They are soluble in organic solvents and are found in the hydrophobic interior of membranes. These properties are not solely due to attached lipid groups. In part, their hydrophobicity is due to a high percentage of b-lactamase Enzyme that destroys antibiotics of the b-lactam class that includes penicillins and cephalosporins proteolipid A type of lipoprotein that is extremely hydrophobic and found in the interior of membranes hydrophobic amino acid residues. Instead of being hidden inside the protein, many of these are exposed on the surface.

Proteins play a wide variety of functional roles in the cell.

Most metabolic reactions are catalyzed by proteins known as enzymes.

Some proteins carry nutrients or other molecules across membranes or around the organism.

Proteins Serve Numerous Cellular Functions

Proteins make up about 60% of the organic matter of living organisms. They are responsible for most of the metabolic reactions and many of the structural components of cells. Not surprisingly, there is colossal variety in the functional role of proteins. Nonetheless, proteins may be subdivided into several major categories:

1. Enzymes

2. Structural proteins

3. Binding proteins (transport, carrier, and storage proteins)

4. Mechanical proteins

5. Information processing proteins

Enzymes are proteins that catalyze chemical reactions. These are discussed in more detail below. Many of the characteristics of enzymes, such as the presence of binding pockets for small molecules and the ability to change shape, are shared by other proteins.

Many sub-cellular structures consist largely or partly of structural proteins. The filaments of the flagella with which bacteria swim around, the microtubules used to control traffic flow inside cells of higher organisms, the fibers in connective tissue, and the outer coats of viruses (see Ch. 17) are examples of structures built using proteins.

Specialized proteins are known that control the structure of water. Fish that live in polar regions have antifreeze proteins to keep their blood from freezing. These proteins bind to ice surfaces and prevent the growth of ice crystals. Conversely, surface proteins of certain bacteria, known as ice nucleation factors, promote the formation of ice crystals and are important in causing frost damage to plants. Damaging plant cells releases nutrients on which the bacteria can grow and may also allow colonization of plant tissue by the bacteria.

Binding proteins bind small molecules but unlike enzymes they do not carry out a chemical reaction. Nonetheless, they also need "active sites" to accommodate the small molecules. Transport proteins or carrier proteins are involved in moving their substrates around within the organism. Transport proteins or permeases are located in membranes and transport their target molecules across the membrane. Nutrients, such as sugars, must be transported into the cells of all organisms, whereas waste products are deported. Many permeases consist of a bundle of a-helical segments (often 7 or 11) that crosses the membrane and is joined by regions of random coil (Fig. 7.23). Most permeases require energy to operate.

In contrast to permeases, carrier proteins are soluble, not membrane bound. Most transport nutrients within the bodies of multicellular organisms. These carrier proteins are sometimes extracellular and found in the bloodstream or other body fluids. In other active site Special site or pocket on a protein where other molecules are bound and the chemical reaction occurs antifreeze protein Protein that prevents freezing of blood, tissue fluids or cells of organisms living at sub-zero temperatures binding protein Protein whose role is to bind another molecule carrier protein Protein that carries other molecules around the body or within the cell enzyme A protein that catalyzes a chemical reaction ice nucleation factor Protein found on surface of certain bacteria that promotes the formation of ice crystals permease A protein that transports nutrients or other molecules across a membrane structural protein A protein that forms part of a cellular structure transport protein Protein that transports another molecule across membranes or from one cell to another

Proteins Serve Numerous Cellular Functions 175

Alpha helical segment

Alpha helical segment

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