Question

Using chemical structures and a reaction mechanism to illustrate your answer, provide an example of concerted acid-base catalysis in enzyme catalysis AND briefly explain why this is not possible in the absence of an enzyme.

Answer 1

Enzymes are chemical catalyst in realtion to the substrate used to enhance the chemical reaction.They are different types of catalysed reactions where the enzyme catalysed reactions highly vary and have high effectiveness of chemical catalysts.The specificity of substrate binding contributes to the rate of enzyme catalysed reactions.When the reaction rate is affected by concentration of hydronium or OH ions from water, the reaction is considered to be specifically acid or base catalysed reaction.In this type of catalysis the reaction rate is affected by prototrophic groups located on side chain of amino acid residues.

Ribonucleases or RNases are example for such reaction where this group of enzymes that catalyzes the hydrolysis of phospodiester bond in RNA molecules

MECHANISM OF ACTION OF RIBONUCLEASE

The imidazole group og his12 acts as general base catalyst,abstracts a proton from 2oh group of sugar,forming a an nucleophillic akoxide ion.Cleavage of 5-3 phosphodiester bond takes place which yeild 2-3 nucleoside monophosphate inetrmediate.The alkoxide ion pick ups proton from His 119 hence his119 acts as acid catalyst.Hi 119 whose imidazole group is now unprotonated abstracta a proton from water.The 2 oxygen leavimg group in this step retrives the proton from immidazolium ion group of His12.This mechanism has been shown in below figure.The absence of the ezymes in the case of ribonuclaease enzyme does not allow the cleavage of phosphodiester bond in the hydrolysis reaction.