CHEM237 Lecture 12: CHEM237-9-Ch11-12-Enzymes2-annot.PDF
10 May 2016
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2 related properties make enzymes powerful catalysts: Use of amino acid side chains to protonate and deprotonate the substrate, such as: glu, asp, his, lys, arg, ser, thr. Catalyzes the hydrolysis of 5"-3" phosphate bond of rna. Enzyme has a bell-shaped ph profile, characteristic of involvement of 2 ionizable groups in catalysis. 2 essential his residues, his 12 and his119, act in concerted manner as general acid-base catalytic groups: The ph dependence of v max/k m in the rnase. Formation of 2"3" cyclic intermediate: his 12 abstracts h+ from rna 2"oh, forming o- nucleophile which attacks p, his119 protonates 5"o leaving group. Hydrolysis of 2"3" cyclic intermediate: his119 deprotonates h2o, forming oh- nuclephile which attacks. Figure 15-3 hydrolysis of rna is a two-step process with the intermediate formation of a 2 ,3 -cyclic nucleotide. Formation of a covalent enzyme-substrate intermediate in the reaction.
