Kin 217 – October 3 rd
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
• 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