Answer
(a) Little or no effect. The binding pocket of trypsin contains an aspartate residue, Asp 189. The aspartate residue electrostatically interacts with the positively charged residues that are recognized and bound by trypsin. The mutation of Asp 102 to Asn 102 is not in trypsin's binding pocket so this mutation should have little effect on binding.
(b) Catalysis would be much slower. Asp 102 is one of the three residues that are part of the catalytic triad of serine proteases. Asp 102 orients His 57 making the histidyl residue a better proton acceptor through electrostatic effects. Mutating the Aspto an Asn residue removes the negatively charged side chain of Asp. This destroys the electrostatic effect and makes the His 57 a poorer proton acceptor. The first step of the mechanism for serine proteases is the deprotonation of Ser 195 by His 57 to generate the nucleophilic alkoxide ion which then attacks the carbonyl carbon of the scissile bond. The result is that trypsin would be a poor catalyst.
Work Step by Step
(a) Little or no effect. The binding pocket of trypsin contains an aspartate residue, Asp 189. The aspartate residue electrostatically interacts with the positively charged residues that are recognized and bound by trypsin. The mutation of Asp 102 to Asn 102 is not in trypsin's binding pocket so this mutation should have little effect on binding.
(b) Catalysis would be much slower. Asp 102 is one of the three residues that are part of the catalytic triad of serine proteases. Asp 102 orients His 57 making the histidyl residue a better proton acceptor through electrostatic effects. Mutating the Aspto an Asn residue removes the negatively charged side chain of Asp. This destroys the electrostatic effect and makes the His 57 a poorer proton acceptor. The first step of the mechanism for serine proteases is the deprotonation of Ser 195 by His 57 to generate the nucleophilic alkoxide ion which then attacks the carbonyl carbon of the scissile bond. The result is that trypsin would be a poor catalyst.
