2 Структура и функция белка (1160071), страница 27
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Calculate the minimum number of amino acids necessary for one segment of ahelix to traverse the membrane completely. Estimate the fraction of the bacteriorhodopsin proteinthat occurs in a-helical form. Justify all your assumptions. (Use an average amino acid residueweight of 110.)9. Biosynthesis of Collagen Collagen, the mostabundant protein in mammals, has an unusualamino acid composition. Unlike most other proteins, collagen is very rich in proline and hydroxyproline (see p.
172). Hydroxyproline is not one ofthe 20 standard amino acids, and its incorporationin collagen could occur by one of two routes: (1) proline is hydroxylated by enzymes before incorporation into collagen or (2) a Pro residue is hydroxylated after incorporation into collagen. Todifferentiate between these two possibilities, thefollowing experiments were performed. When[14CJproline was administered to a rat and the collagen from the tail isolated, the newly synthesizedtail collagen was found to be radioactive.
If, however, [14C] hydroxyproline was administered to arat, no radioactivity was observed in the newlysynthesized collagen. How do these experimentsdifferentiate between the two possible mechanismsfor introducing hydroxyproline into collagen?10. Pathogenic Action of Bacteria That Cause GasGangrene The highly pathogenic anaerobic bacterium Clostridium perfringens is responsible for gasgangrene, a condition in which animal tissue structure is destroyed. This bacterium secretes an enzyme that efficiently catalyzes the hydrolysis of thepeptide bond indicated in red in the sequence:—X—Gly—Pro—Y— H2° >—X—COO" + H3N—Gly—Pro-Ywhere X and Y are any of the 20 standard aminoacids.
How does the secretion of this enzyme contribute to the invasiveness of this bacterium inhuman tissues? Why does this enzyme not affectthe bacterium itself?11. Formation of Bends and Intrachain CrossLinkages in Polypeptide Chains In the followingpoly peptide, where might bends or turns occur?Where might intrachain disulfide cross-linkages beformed?12345678910Ile-Ala -His -Thr-Tyr-Gly -Pro-Phe-Glu-Ala 11121314151617181920Ala-Met-Cys-Lys-Trp-Glu-Ala-Gln-Pro-Asp2122232425262728Gly-Met-Glu-Cys-Ala-Phe-His-Arg12. Location of Specific Amino Acids in GlobularProteins X-ray analysis of the tertiary structure ofmyoglobin and other small, single-chain globularproteins has led to some generalizations about howthe polypeptide chains of soluble proteins fold.With these generalizations in mind, indicate theprobable location, whether in the interior or on theexternal surface, of the following amino acid residues in native globular proteins: Asp, Leu, Ser,Val, Gin, Lys.
Explain your reasoning.Chapter 7 The Three-Dimensional Structure of Proteins13. The Number of Polypeptide Chains in an Oligomeric Protein A sample (660 mg) of an oligomeric protein of Mr 132,000 was treated with anexcess of l-fluoro-2,4-dinitrobenzene under slightlyalkaline conditions until the chemical reaction wascomplete. The peptide bonds of the protein werethen completely hydrolyzed by heating it with concentrated HC1. The hydrolysate was found to contain 5.5 mg of the following compound:O2NHowever, 2,4-dinitrophenyl derivatives of thea-amino groups of other amino acids could not befound.(a) Explain why this information can be used todetermine the number of polypeptide chains in anoligomeric protein.(b) Calculate the number of polypeptide chainsin this protein.14. Molecular Weight of Hemoglobin The first indication that proteins have molecular weightsgreatly exceeding those of the (then known) organic compounds was obtained over 100 years ago.For example, it was known at that time that hemoglobin contains 0.34% by weight of iron.(a) From this information determine the minimum molecular weight of hemoglobin.(b) Subsequent experiments indicated that thetrue molecular weight of hemoglobin is 64,500.What information did this provide about the number of iron atoms in hemoglobin?19715.
Comparison of Fetal and Maternal HemoglobinStudies of oxygen transport in pregnant mammalshave shown that the O2-saturation curves of fetaland maternal blood are markedly different whenmeasured under the same conditions. Fetal erythrocytes contain a structural variant of hemoglobin,hemoglobin F, consisting of two a and two y subunits (a2y2)> w n e r e a s maternal erythrocytes contain the usual hemoglobin A (a2p2).ioo r(a) Which hemoglobin has a higher affinity foroxygen under physiological conditions, hemoglobinA or hemoglobin F? Explain.(b) What is the physiological significance of thedifferent oxygen affinities? Explain.C H A P T E REnzymesWe now come to the most remarkable and highly specialized proteins,the enzymes.
Enzymes are the reaction catalysts of biological systems.They have extraordinary catalytic power, often far greater than that ofsynthetic catalysts. They have a high degree of specificity for theirsubstrates, they accelerate specific chemical reactions, and they function in aqueous solutions under very mild conditions of temperatureand pH. Few nonbiological catalysts show all these properties.Enzymes are one of the keys to understanding how cells surviveand proliferate.
Acting in organized sequences, they catalyze the hundreds of stepwise reactions in metabolic pathways by which nutrientmolecules are degraded, chemical energy is conserved and transformed, and biological macromolecules are made from simple precursors. Some of the many enzymes participating in metabolism are regulatory enzymes, which can respond to various metabolic signals bychanging their catalytic activity accordingly. Through the action ofregulatory enzymes, enzyme systems are highly coordinated to yield aharmonious interplay among the many different metabolic activitiesnecessary to sustain life.The study of enzymes also has immense practical importance.
Insome diseases, especially inheritable genetic disorders, there may be adeficiency or even a total absence of one or more enzymes in the tissues(see Table 6-6). Abnormal conditions can also be caused by the excessive activity of a specific enzyme. Measurements of the activity of certain enzymes in the blood plasma, erythrocytes, or tissue samples areimportant in diagnosing disease. Enzymes have become importantpractical tools, not only in medicine but also in the chemical industry,in food processing, and in agriculture.
Enzymes play a part even ineveryday activities in the home such as food preparation and cleaning.The chapter begins with descriptions of the properties of enzymesand the principles underlying their catalytic power. Following is anintroduction to enzyme kinetics, a discipline that provides much of theframework for any discussion of enzymes. Specific examples of enzymemechanisms are then provided, illustrating principles introduced earlier in the chapter. We will end with a discussion of regulatory enzymes.An Introduction to EnzymesMuch of the history of biochemistry is the history of enzyme research.Biological catalysis was first recognized and described in the early1800s, in studies of the digestion of meat by secretions of the stomach198Chapter 8 Enzymesand the conversion of starch into sugar by saliva and various plantextracts.
In the 1850s Louis Pasteur concluded that fermentation ofsugar into alcohol by yeast is catalyzed by "ferments." He postulatedthat these ferments, later named enzymes, are inseparable from thestructure of living yeast cells, a view that prevailed for many years.The discovery by Eduard Buchner in 1897 that yeast extracts can ferment sugar to alcohol proved that the enzymes involved in fermentation can function when removed from the structure of living cells. Thisencouraged biochemists to attempt the isolation of many different enzymes and to examine their catalytic properties.James Sumner's isolation and crystallization of urease in 1926 provided a breakthrough in early studies of the properties of specific enzymes. Sumner found that the urease crystals consisted entirely ofprotein and postulated that all enzymes are proteins.
Lacking otherexamples, this idea remained controversial for some time. Only later inthe 1930s, after John Northrop and his colleagues crystallized pepsinand trypsin and found them also to be proteins, was Sumner's conclusion widely accepted. During this period, J.B.S. Haldane wrote a treatise entitled "Enzymes." Even though the molecular nature of enzymeswas not yet fully appreciated, this book contained the remarkable suggestion that weak-bonding interactions between an enzyme and itssubstrate might be used to distort the substrate and catalyze the reaction. This insight lies at the heart of our current understanding ofenzymatic catalysis.
The latter part of the twentieth century has seenintensive research on the enzymes catalyzing the reactions of cell metabolism. This has led to the purification of thousands of enzymes (Fig.8-1), elucidation of the structure and chemical mechanism of hundredsof these, and a general understanding of how enzymes work.199James Sumner1887-1955J.B.S. Haldane1892-1964Most Enzymes Are ProteinsWith the exception of a small group of catalytic RNA molecules (Chapter 25), all enzymes are proteins.
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