Directions: The next questions are based on the passage below. Answer them to th
ID: 509008 • Letter: D
Question
Directions: The next questions are based on the passage below. Answer them to the best of your ability. An Unexpected Event When Chymotrypsin Performs Its physiological Role Ivan G. Daney J. Chem, Educ. (2000), 77, p. 422. When students are taught about the general properties of derivatives of carboxylic acids in introductory organic chemistry courses, they are probably told that the reactivity of derivatives of carboxylic acids decreases in the order acid chlorides acid anhydrides esters amides, and thus that, for reactions generally camied out in organic chemistry laboratories, amides can be synthesized from esters, but the synthesis of esters from amides is dificult. It seems a pity that authors of introductory textbooks of biochemistry, and probably therefore biochemistry teachers, donotrefer to these general properties of carboxylic acid derivatives when discussing the mechanism of the chymotrypsin-catalyzed hydrolysis of peptide (substituted amide) bonds. Ifreference were made to these properties, then students could be told that one ofthe events that occur during the chymotrypsin-catalyzed hydrolysis of peptide bonds is an example of an exception to the rule of thumb that amides can't readily be converted to esters. In forming an acyk me intentmediate, chymotrypsin converts an unreactive stable substituted amide present in the substrate molecule to a more reactive ester intermediate, in which the acylgroup of the substrate is covalently bound to a serime residue at the active site ofthe enzyme. Once fomed, this more reactive ester intermediate present at the enzyme's active site undergoes hydrolysis. Thus, one of the major steps performed when chymotrypsin camies out its main perceived "physiological role", namely, the hydrolysis peptide bonds in proteins in the small intestine, ofcertain is the unexpected formation ofan ester from a substituted amide.Explanation / Answer
14a.
Peptide bond contains –CO-NH- linkage which connects two amino acid units together.
Structure (a) is zwitter ionic form of amino acid. It does not contain peptide bond.
Structure (b) contains amine and amide functional groups present in single molecule. It is an amide of amino acid. Therefore, it does not contain peptide bond.
Structure (c) contains two amino acids connected together by a –CO-NH- linkage. Thus, this molecule contains peptide bond.
Therefore, option (c) is the correct answer.
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14b.
The order of reactivity of esters and amides given in textbooks is correct. However, in case of enzymes, their activity does not depend totally on reactivity of esters and amides. There are various other factors which determine the reactivity of enzyme catalyzed reactions. Thus, options (a), (b) and (d) are incorrect.
Correct answer is option (c). A violation of the relative reactivity rules provides the insight into the way enzyme reactions occur.
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14c.
An amide is the stable and unreactive species. As compared with other carboxylic acid derivatives such as acid chloride, acid anhydrides and esters; amide are considered as most stable and unreactive species.
Therefore, correct answer is option (a). An amide is a stable and unreactive species.
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14d.
A molecule of serine contains hydroxyl group as the side chain. In chymotrypsin molecule the free hydroxyl group of serine reacts with carbonyl group of amide to form ester and amine molecule is given out.
Therefore, correct answer is option (a). A molecule of serine at the active site of chymotripsin reacts at the peptide carbonyl carbon.
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14e.
Chymotripsin hydrolyzes the peptide bonds in proteins molecules. Peptide bonds are substituted amides (-CO-NH-).
Therefore, correct answer is option (b). Chymotripsin catalyzed hydrolysis of substituted amides.
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