2 Структура и функция белка (1160071)

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Лекция 2.Структура и функция белкаP A R TStructure and CatalysisIn Part I we contrasted the complex structure and function of livingcells with the relative simplicity of the monomeric units from whichthe enzymes, supramolecular complexes, and organelles of the cells areconstructed. Part II is devoted to the structure and function of themajor classes of cellular constituents: amino acids and proteins (Chapters 5 through 8), fatty acids, lipids, and membranes (Chapters 9 and10), sugars and polysaccharides (Chapter 11), and nucleotides and nucleic acids (Chapter 12). We begin in each case by considering the covalent structure of the simple subunits (amino acids, fatty acids, monosaccharides, and nucleotides).

These subunits are a major part of thelanguage of biochemistry; familiarity with them is a prerequisite forunderstanding more advanced topics covered in this book, as well asthe rapidly growing and exciting literature of biochemistry.After describing the covalent chemistry of the monomeric units, weconsider the structure of the macromolecules and supramolecular complexes derived from them.

An overriding theme is that the polymericmacromolecules in living systems, though large, are highly orderedchemical entities, with specific sequences of monomeric subunits giving rise to discrete structures and functions. This fundamental themecan be broken down into three interrelated principles: (1) the uniquestructure of each macromolecule determines its function; (2) noncovalent interactions play a critical role in the structure and function ofmacromolecules; and (3) the specific sequences of monomeric subunitsin polymeric macromolecules contain the information upon which theordered living state depends. Each of these principles deserves furthercomment.The relationship between structure and function is especially evident in proteins, which exhibit an extraordinary diversity of functions.One particular polymeric sequence of amino acids produces a strong,fibrous structure found in hair and wool; another produces a proteinthat transports oxygen in the blood.

Similarly, the special functions oflipids, polysaccharides, and nucleic acids can be understood as a directmanifestation of their chemical structure, with their characteristicmonomeric subunits linked in precise functional groups or polymers.Lipids aggregate to form membranes; sugars linked together becomeenergy stores and structural fibers; nucleotides in a polymer becomethe blueprint for an entire organism.As we move from monomeric units to larger and larger polymers,the chemical focus shifts from covalent bonds to noncovalent interactions.

The covalent nature of monomeric units, and of the bonds thatconnect them in polymers, places strong constraints upon the shapesFacing page: End-on view of the triple-strandedcollagen superhelix. Collagen, a component of connective tissue, provides tensile strength and resiliency. Its strength is derived in part from the threetightly wrapped identical helical strands (shown ingray, purple, and blue), much the way a length ofrope is stronger than its constituent fibers. Thetight wrapping is made possible by the presence ofglycine, shown in red, at every third position alongeach strand, where the strands are in contact. Glycine's small size allows for very close contact.109L10Part II Structure a n d Catalysisassumed by large molecules.

It is the numerous noncovalent interactions, however, that dictate the stable native conformation and providethe flexibility necessary for the biological function of these large molecules. We will see that noncovalent interactions are essential to thecatalytic power of enzymes, the arrangement and properties of lipids ina membrane, and the critical interaction of complementary base pairsin nucleic acids.The principle that sequences of monomeric subunits are information-rich emerges fully in the discussion of nucleic acids in Chapter 12.However, proteins and some polysaccharides are also information-richmolecules.

The amino acid sequence is a form of information that directs the folding of the protein into its unique three-dimensional structure, and ultimately determines the function of the protein. Some polysaccharides also have unique sequences and three-dimensionalstructures that can be recognized by other macromolecules.For each class of molecules we find a similar structural hierarchy,in which subunits of fixed structure are connected by bonds of limitedflexibility, to form macromolecules with three-dimensional structuresdetermined by noncovalent interactions. Together, the molecules described in Part II are the "stuff" of life.

We begin with the amino acids.C H A P T E RAmino Acids and PeptidesProteins are the most abundant macromolecules in living cells, occurring in all cells and all parts of cells. Proteins also occur in great variety; thousands of different kinds may be found in a single cell. Moreover, proteins exhibit great diversity in their biological function.

Theircentral role is made evident by the fact that proteins are the mostimportant final products of the information pathways discussed inPart IV of this book. In a sense, they are the molecular instrumentsthrough which genetic information is expressed. It is appropriate tobegin the study of biological macromolecules with the proteins, whosename derives from the Greek protos, meaning "first" or "foremost."Relatively simple monomeric subunits provide the key to the structure of the thousands of different proteins. All proteins, whether fromthe most ancient lines of bacteria or from the most complex forms oflife, are constructed from the same ubiquitous set of 20 amino acids,covalently linked in characteristic linear sequences.

Because each ofthese amino acids has a distinctive side chain that determines itschemical properties, this group of 20 precursor molecules may be regarded as the alphabet in which the language of protein structure iswritten.Proteins are chains of amino acids, each joined to its neighbor by aspecific type of covalent bond. What is most remarkable is that cellscan produce proteins that have strikingly different properties and activities by joining the same 20 amino acids in many different combinations and sequences. From these building blocks different organismscan make such widely diverse products as enzymes, hormones, antibodies, the lens protein of the eye, feathers, spider webs, rhinoceroshorns (Fig.

5-1), milk proteins, antibiotics, mushroom poisons, and amyriad of other substances having distinct biological activities.Protein structure and function is the topic for the next four chapters. In this chapter we begin with a description of amino acids and thecovalent bonds that link them together in peptides and proteins.Figure 5—1 The protein keratin is formed by allvertebrates. It is the chief structural component ofhair, scales, horn, wool, nails, and feathers. Theblack rhinoceros is nearing extinction in the wildbecause of the myths prevalent in some parts of theworld that a powder derived from its horn has aphrodisiac properties. In reality, the chemical properties are no different from those of powdered bovinehooves or human fingernails.Ill112Part II Structure and CatalysisAmino AcidsCOO"COO"H3N—C—HH3N—C—HRAmino acidHGlycineFigure 5-2 General structure of the amino acidsfound in proteins.

With the exception of the natureof the R group, this structure is common to all thea-amino acids. (Proline, because it is an imino acid,is an exceptional component of proteins.) The a carbon is shown in blue. R (in red) represents the Rgroup or side chain, which is different in eachamino acid. In all amino acids except glycine(shown for comparison) the a-carbon atom has fourdifferent substituent groups.coo-coo-(a)COO"Amino Acids Have Common Structural FeaturesAll of the 20 amino acids found in proteins have a carboxyl group andan amino group bonded to the same carbon atom (the a carbon) (Fig.5-2). They differ from each other in their side chains, or R groups,which vary in structure, size, and electric charge, and influence thesolubility of amino acids in water. When the R group contains additional carbons in a chain, they are designated /3, y, 8, e, etc., proceedingout from the a carbon.

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