Introduction to Enzyme and Coenzyme Chemistry - E-Library Home

38 Chapter 3
pKa
Tyrosine CH 2
O H
CH O −
2
+ H + ~10
Lysine
(CH 2 ) 4 NH + 3
(CH 2 ) 4 NH 2 + H + ~9
Cysteine
CH 2 S H
CH 2 S − + H + 8−9
Histidine
CH CH 2 +
2
NH
N + H + 6−8
HN
HN
O
O
Aspartate/glutamate
+ H + 4−5
O H
O −
Figure 3.9 Amino acid side chains used for acid/base catalysis.
S
X:
H
AEnz
S
+
XH
− AEnz
Figure 3.10 General acid catalysis.
(1) Enz-B −
H
O
(2) Enz-B − H O
H
O
Figure 3.11 General base catalysis.
enzyme acetoacetate decarboxylase contains an active site lysine residue which
forms an imine linkage with its substrate – its pK a value was found to be 5.9,
much less than the expected value of about 9. When an active site peptide was
obtained containing the catalytic lysine, it was found to be adjacent to another
lysine residue. So it is likely that the proximity of another positively charged
residue would make the protonated form thermodynamically less favourable,
and hence reduce the pK a . The same eVect can be observed in the pK a values of
ethylenediamine (see Figure 3.13), where the pK a for the monoprotonated form
is 10.7 as usual, but the pK a for the doubly protonated form is 7.5.