Properties of Enzymes

Characteristics of enzymes
1) biological catalysts
2) not consumed during a chemical reaction
3) speed up reactions from 1000 - 1017, with a mean increase in rate of 00,000
4) exhibit stereospecificity --> act on a single stereoisomer of a substrate
5) exhibit reaction specificity --> no waste or side reactions
Nomenclature
Typically add “-ase” to name of substrate
e.g. lactase breaks down lactose (dissacharide of glucose and galactose)
IUBMB classifies enzymes based upon the class of organic chemical reaction catalyzed:
1) oxidoreductase - catalyze redox reactions
dehydrogenases, oxidases, peroxidases, reductases
2) transferases - catalyze group transfer reactions; often require coenzymes
3) hydrolases - catalyze hydrolysis reactions
4) lyases - lysis of substrate; produce contains double bond
5) isomerases - catalyze structural changes; isomerization
6) ligases - ligation or joining of two substrates with input of energy, usually from
ATP hydrolysis; often called synthetases or synthases
Enzyme kinetics:
A mathematical and graphical study of the rates of enzyme-catalyzed reactions.
k
S -----------> P
k
A + B ---> C
The velocity of this reaction can be summarized by the following equation:
v = k[S] or v = k[A][B]
This reaction is considered a first order reaction, determined by the sum of the exponents
in the rate equation --> number of molecules reacting.
There are also bimolecular reactions, which involve two substrates; good example of group
transfer reactions.
S1 + S2 ---> P1 + P2
2
v = k[S1][S2] first order for each reactant;
but second order overall
For enzyme-catalyzed reactions:
E + S -----> ES -----> E + P
The rate or velocity is dependent upon both [enzyme] and [substrate].
In reality, enzyme-catalysed reactions are not that simple:
k1 kcat
E + S ES E + P
k-1
k1 and k-1 govern the rates of association and dissociation of ES
kcat is the turnover number or catalytic constant
VES = k1[E][S]
VE+S = k-1[ES]
VE+P = kcat[ES]
Usually an enzyme’s velocity is measured under initial conditions of [S] and [P].
These same reactions can be described graphically:
velocity
[S]
At low [S], vo increases as [S] increases.
At high [S], enzymes become saturated with substrates, and the reaction is
independent of [S] --> display saturation kinetics.
Vmax = kcat[ES]
or because the [S] is irrelevant at high [S]
Vmax = kcat [E]
The graph is a graph of a hyperbola, and the equation for a hyperbola is
y = ax
b + x where a is the asymptote
b is value at a/2
3
Substituting our equation parameters,
Vo = Vmax[S]