1. Which of the following statements is most FALSE regarding the activity of His
ID: 151230 • Letter: 1
Question
1. Which of the following statements is most FALSE regarding the activity of His12 in RNase A? a.His12 initially acts as a general base, removing a proton from the 2'-OH group of the ribose sugar. b.The protonation state of His12 is finally restored by donating a proton to the RNA leaving group c.His12 removes a proton from water, generating a nucleophilic hydroxide group d.His12 does not bind directly to the substrate to form an enzyme-substrate covalent intermediate2.Due to transition state stabilization, the enzyme proline racemase would be predicted to bind to an intermediate with a tetrahedral carbon at the reaction site rather than a trigonal planar carbon a. True b. False 1. Which of the following statements is most FALSE regarding the activity of His12 in RNase A? a.His12 initially acts as a general base, removing a proton from the 2'-OH group of the ribose sugar. b.The protonation state of His12 is finally restored by donating a proton to the RNA leaving group c.His12 removes a proton from water, generating a nucleophilic hydroxide group d.His12 does not bind directly to the substrate to form an enzyme-substrate covalent intermediate
2.Due to transition state stabilization, the enzyme proline racemase would be predicted to bind to an intermediate with a tetrahedral carbon at the reaction site rather than a trigonal planar carbon a. True b. False a.His12 initially acts as a general base, removing a proton from the 2'-OH group of the ribose sugar. b.The protonation state of His12 is finally restored by donating a proton to the RNA leaving group c.His12 removes a proton from water, generating a nucleophilic hydroxide group d.His12 does not bind directly to the substrate to form an enzyme-substrate covalent intermediate
2.Due to transition state stabilization, the enzyme proline racemase would be predicted to bind to an intermediate with a tetrahedral carbon at the reaction site rather than a trigonal planar carbon a. True b. False
Explanation / Answer
RNase A is a ribonuclease secreted by the pancreas. The Histidine at twelth position acts as a general base and accepts a proton from 2` OH of the RNA sugar ring. The 2`O then acts as a nucleophile and attacks the phosphorus atom thereby forming 2`,3- cyclic ribonucleotide intermediate. His119 acts as a general acid and donates its proton to the oxygen atom.
In the second step, His12 now donates it acquired proton to the 2`,3- cyclic ribonucleotide intermediate and His119 accepts a proton from water molecule and the resulting hydroxyl group then attacks the 2`,3- cyclic ribonucleotide intermediate.
The dinucleotide or substarte binds to His12 by non-covalent interactions.
Hence options a, b & d are correct. Option c is a false statement.
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