Introduction to Enzyme and Coenzyme Chemistry - E-Library Home

3 Enzymes are Wonderful
Catalysts
3.1 Introduction
The function of enzymes is to catalyse biochemical reactions. Each enzyme has
evolved over millions of years to catalyse one particular reaction, so it is
perhaps not surprising to Wnd that they are extremely good catalysts when
compared with man-made catalysts.
The hallmarks of enzyme catalysis are: speed, selectivity and speciWcity.
Enzymes are capable of catalysing reactions at rates well in excess of a million-fold
faster than the uncatalysed reaction, typical ratios of k cat =k uncat being
10 6 ---10 14 . Figure 3.1 shows an illustration of the speed of enzyme-catalysed
glycoside hydrolysis. The rate of acid-catalysed glycoside catalysis is accelerated
10 3 -fold by intramolecular acid catalysis, but enzyme-catalysed glycoside hydrolysis
is 10 4 -fold faster still – some 10 7 faster than the uncatalysed reaction
carried out at pH 1.
Rate of hydrolysis
HO
HO
OH
O
OH
H +
OPh
acid-catalysed
hydrolysis
k obs = 1.9 10 −6 s −1 in 0.1 M HCl
HO
HO
OH
O
OH
H
O
O
O
intramolecular
catalysis
k uni = 1.4 10 −3 s −1
Enz
OH
OH O O
H
O
HO
OPh
β-galactosidase k cat = 40 s −1
OH
Figure 3.1 Rate acceleration of glycoside hydrolysis by intramolecular and enzyme catalysis.
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