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University of Waterloo
KIN 217

Kin 217 – October 3 rd • Question o Consider a reaction A + B -> C + D…if you start with 0.1 mole of A and B then how many molecules of enzyme are required to convert the whole of A and B to C and D (Avagadro's number is 6 x 1023)…assume that no reaction occurs in the absence of enzyme and that the enzyme is stable • One…by definition a catalyst can be reused…the rest of the information is irrelevant • Mechanism of the chymotrysin reaction o Chymotrypsin is an endopeptidase… • So it acts on polypeptides… It hydrolyses peptide bonds adjacent to bulky hydrophobic groups  • If we are asked what an endopeptidase is, what do we look for? "Peptidase"…this means…it acts on a peptide sequence…and "endo"  means inside…"exo" on the outside… • RCONHR' + Enz-OH --> RCOO-Enz + R'NH2 • RCOO-Enz + HOH --> RCOOH + Enz-OH • Two key questions… • Why is that particular serine so reactive? • Why is the ester bond to serine of the acyl intermediate readily hydrolysed? • The catalytic triad (diagram in text, but number unknown) o We see the serine residue up at the top…note that the residues are linked linearly… • The catalytic triad II o At physiological pH… • Aspartate is not dissociated • Serine is not • Histidine is partly dissociated  Can function as a proton donor and as an acceptor (we are talking about the hydrogen on the southern side of the molecule) • Fig 6.10 • The aspartate residue of the catalytic triad (Fig 6.11) o The negative charge (?) is holding the hydrogen down… • Other serine proteases (Fig 6.12) o The catalytic triad is not restricted to chymotrypsin… • Trypsin has aspartate (?) in the pcoket • Elastase has a hydrophobic pocket but is much more restricted…so the phenol group can't fit in there! • Other proteases… o Thiol proteases • Have cysteine at the active site rather than serine o Aspartic proteases • 2 aspartates at the active iste…  One donates a proton, the other accepts one
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