NE224 Lecture Notes - Lecture 9: Pancreatic Ribonuclease, Ribonuclease, Leaving Group

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Acid-base catalysis (i) specificity of substrate binding (ii) optimal arrangement of catalytic groups (i) acid-base catalysis (ii) covalent catalysis (iii) metal ion catalysis (iv) electrostatic catalysis (v) proximity and orientation effects (vi) preferential binding of the transition state. 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. 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. 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.

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