2 Структура и функция белка (1160071), страница 8
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Foreach pair of amino acids listed, determine whichmember will be eluted first from an ion-exchangecolumn by a pH 7.0 buffer.(a) Asp and Lys(b) Arg and Met(c) Glu and Val(d) Gly and Leu(e) Ser and Ala8. Naming the Stereoisomers of Isoleucine Thestructure of the amino acid isoleucine is:COO+IH3N—C—HH—C—CH3CH2CH3(a) How many chiral centers does it have?(b) How many optical isomers?(c) Draw perspective formulas for all the opticalisomers of isoleucine.1339. Comparison of the pKa Values of an Amino Acidand Its Peptides The titration curve of the aminoacid alanine shows the ionization of two functionalgroups with pKa values of 2.34 and 9.69, corresponding to the ionization of the carboxyl and theprotonated amino groups, respectively.
The titration of di-, tri-, and larger oligopeptides of alaninealso shows the ionization of only two functionalgroups, although the experimental pKa values aredifferent. The trend in pKa values is summarizedin the table.Amino acid or peptideAlaAla-AlaAla-Ala-AlaAla-(Ala)n-Ala, n > 4PK22.343.123.393.429.698.308.037.94(a) Draw the structure of Ala-Ala-Ala. Identifythe functional groups associated with pKi and pK2.(b) The value of pKi increases in going from Alato an Ala oligopeptide. Provide an explanation forthis trend.(c) The value of pK2 decreases in going from Alato an Ala oligopeptide. Provide an explanation forthis trend.10. Peptide Synthesis In the synthesis of polypeptides on solid supports, the a-amino group of eachnew amino acid is "protected" by a t-butyloxycarbonyl group (see Box 5-2).
What would happen ifthis protecting group were not present?C H A P T E RAn Introduction to ProteinsAlmost everything that occurs in the cell involves one or more proteins.Proteins provide structure, catalyze cellular reactions, and carry out amyriad of other tasks. Their central place in the cell is reflected in thefact that genetic information is ultimately expressed as protein. Foreach protein there is a segment of DNA (a gene; see Chapters 12 and23) that encodes information specifying its sequence of amino acids.There are thousands of different kinds of proteins in a typical cell, eachencoded by a gene and each performing a specific function.
Proteins areamong the most abundant biological macromolecules and are also extremely versatile in their functions.The chapter begins with a discussion of some of the general properties of proteins. This is followed by a short summary of some commontechniques used to purify and study proteins. Finally, we will examinethe primary structure of protein molecules: the covalent backbonestructure and the sequence of amino acid residues. One goal is to discover the relationships between amino acid sequence and biologicalfunction.Properties of ProteinsAn understanding of these important macromolecules must begin withthe fundamentals.
What do proteins do? How big are they? What formsor shapes do they take? What are their chemical properties? The answers serve as an orientation to much that follows.Proteins Have Many Different Biological FunctionsFigure 6-1 Functions of proteins, (a) The lightproduced by fireflies is the result of a light-producing reaction involving luciferin and ATP that is catalyzed by the enzyme luciferase (see Box 13-3).(b) Erythrocytes contain large amounts of theoxygen-transporting protein hemoglobin, (c) Thewhite color of milk is derived primarily from theprotein casein, (d) The movement of cilia in protozoans depends on the action of the protein dynein.(e) The protein fibroin is the major structural component of spider webs, (f) Castor beans contain ahighly toxic protein called ricin.
(g) Cancerous tumors are often made up of cells that have defectsinvolving one or more of the proteins that regulatecell division.134We can classify proteins according to their biological roles.Enzymes The most varied and most highly specialized proteins arethose with catalytic activity—the enzymes.
Virtually all the chemicalreactions of organic biomolecules in cells are catalyzed by enzymes.Many thousands of different enzymes, each capable of catalyzing adifferent kind of chemical reaction, have been discovered in differentorganisms (Fig. 6-la).Transport Proteins Transport proteins in blood plasma bind andcarry specific molecules or ions from one organ to another. Hemoglobinof erythrocytes (Fig. 6-lb) binds oxygen as the blood passes throughthe lungs, carries it to the peripheral tissues, and there releases it toparticipate in the energy-yielding oxidation of nutrients. The bloodChapter 6 An Introduction to Proteinsplasma contains lipoproteins, which carry lipids from the liver to otherorgans.
Other kinds of transport proteins are present in the plasmamembranes and intracellular membranes of all organisms; these areadapted to bind glucose, amino acids, or other substances and transport them across the membrane.Nutrient and Storage Proteins The seeds of many plants store nutrient proteins required for the growth of the germinating seedling. Particularly well-studied examples are the seed proteins of wheat, corn,and rice. Ovalbumin, the major protein of egg white, and casein, themajor protein of milk, are other examples of nutrient proteins (Fig.6-lc).
The ferritin found in some bacteria and in plant and animaltissues stores iron.Contractile or Motile Proteins Some proteins endow cells and organisms with the ability to contract, to change shape, or to move about.Actin and myosin function in the contractile system of skeletal muscleand also in many nonmuscle cells.
Tubulin is the protein from whichmicrotubules are built. Microtubules act in concert with the proteindynein in flagella and cilia (Fig. 6-ld) to propel cells.Structural Proteins Many proteins serve as supporting filaments,cables, or sheets, to give biological structures strength or protection.The major component of tendons and cartilage is the fibrous proteincollagen, which has very high tensile strength. Leather is almost purecollagen. Ligaments contain elastin, a structural protein capable ofstretching in two dimensions.
Hair, fingernails, and feathers consistlargely of the tough, insoluble protein keratin. The major component ofsilk fibers and spider webs is fibroin (Fig. 6-le). The wing hinges ofsome insects are made of resilin, which has nearly perfect elastic properties.Defense Proteins Many proteins defend organisms against invasionby other species or protect them from injury. The immunoglobulins orantibodies, specialized proteins made by the lymphocytes of vertebrates, can recognize and precipitate or neutralize invading bacteria,viruses, or foreign proteins from another species.
Fibrinogen andthrombin are blood-clotting proteins that prevent loss of blood whenthe vascular system is injured. Snake venoms, bacterial toxins, andtoxic plant proteins, such as ricin, also appear to have defensive functions (Fig. 6-lf). Some of these, including fibrinogen, thrombin, andsome venoms, are also enzymes.Regulatory Proteins Some proteins help regulate cellular or physiological activity. Among them are many hormones. Examples includeinsulin, which regulates sugar metabolism, and the growth hormone ofthe pituitary. The cellular response to many hormonal signals is oftenmediated by a class of GTP-binding proteins called G proteins (GTP isclosely related to ATP, with guanine replacing the adenine portion ofthe molecule; see Figs.
1-12 and 3-16b.) Other regulatory proteinsbind to DNA and regulate the biosynthesis of enzymes and RNA molecules involved in cell division in both prokaryotes and eukaryotes (Fig.6-lg).Other Proteins There are numerous other proteins whose functionsare rather exotic and not easily classified. Monellin, a protein of anAfrican plant, has an intensely sweet taste. It is being studied as a135136Part II Structure and Catalysisnonfattening, nontoxic food sweetener for human use.
The bloodplasma of some Antarctic fish contains antifreeze proteins, which protect their blood from freezing.It is extraordinary that all these proteins, with their very differentproperties and functions, are made from the same group of 20 aminoacids.Proteins Are Very Large MoleculesHow long are the polypeptide chains in proteins? Table 6-1 shows thathuman cytochrome c has 104 amino acid residues linked in a singlechain; bovine chymotrypsinogen has 245 amino acid residues. Probably near the upper limit of size is the protein apolipoprotein B, acholesterol-transport protein with 4,536 amino acid residues in a single polypeptide chain of molecular weight 513,000.
Most naturally occurring polypeptides contain less than 2,000 amino acid residues.Table 6-1 Molecular data on some proteinsMolecularweightInsulin (bovine)Cytochrome c (human)Ribonuclease A (bovine pancreas)Lysozyme (egg white)Myoglobin (equine heart)Chymotrypsin (bovine pancreas)Chymotrypsinogen (bovine)Hemoglobin (human)Serum albumin (human)Hexokinase (yeast)Immunoglobulin G (human)RNA polymerase (E.
coli)Apolipoprotein B (human)Glutamate dehydrogenase(bovine liver)5,73313,00013,70013,93016,89021,60022,00064,50068,500102,000145,000450,000513,0001,000,000Number ofresidues51104124129153241245574-550-800-1,320-4,1004,536-8,300Number ofpolypeptidechains2111131412451-40Some proteins consist of a single polypeptide chain, but others,called multisubunit proteins, have two or more (Table 6-1). The individual polypeptide chains in a multisubunit protein may be identical ordifferent. If at least some are identical, the protein is sometimes calledan oligomeric protein and the subunits themselves are referred to asprotomers.
The enzyme ribonuclease has one polypeptide chain.Hemoglobin has four: two identical a chains and two identical j3chains, all four held together by noncovalent interactions.The molecular weights of proteins, which can be determined byvarious physicochemical methods, may range from little more than10,000 for small proteins such as cytochrome c (104 residues), to morethan 106 for proteins with very long polypeptide chains or those withseveral subunits. The molecular weights of some typical proteins aregiven in Table 6-1. No simple generalizations can be made about themolecular weights of proteins in relation to their function.One can calculate the approximate number of amino acid residuesin a simple protein containing no other chemical group by dividing itsmolecular weight by 110. Although the average molecular weight of theChapter 6 An Introduction to Proteins13720 standard amino acids is about 138, the smaller amino acids predominate in most proteins; when weighted for the proportions in which thevarious amino acids occur in proteins (see Table 5-1), the average molecular weight is nearer to 128.
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