1. Which of the following statements is most FALSE regarding the activity of His

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 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

Ans1 - a

His12 acts as a general base, accepting the 2'-OH proton of the sessile ribonucleic sugar ring.

This promotes a nucleophilic attack by the 2'-O on the more positively-charged phosphorus (P) atom thereby creating a 2’,3’-cyclic ribonucleotide phosphate intermediate. The creation of this intermediate is facilitated by the imidazole of His119 which acts as a general acid, donating its proton to the oxygen atom in the susceptible P-O-R' bond.

Ans2- (b)False

The racemization of L-proline to D-proline proceeds through a planar transition state, where the tetrahedral -carbon becomes trigonal as a proton leaves the L-proline.

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