Wednesday, 25 January 2017

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]

2 comments:

  1. Enzymes are proteins (some may be catalytic RNA) that have unique capacity of speeding up chemical reaction within cells. They accelerate the velocity of the reaction without being altered after the reaction. They function via a variety of mechanisms, enzyme catalytic characteristics

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