Enzyme specificity, Factors affecting enzyme activity >> Lecture
Enzymes show different degrees of specificity:
Relative, low or bond
In this type the enzyme acts on substrates
that are similar in structure and contain the same type of bonds
a. Amylase, which acts on
1-4 glycosidic, bonds in starch, dextrin and glycogen.
b. Lipase that hydrolyzes ester bonds in different triglycerides
or group specificity
In this type of specificity, the enzyme
is specific not only to the type of bond but also to the structure
a. Pepsin is an endopeptidase that hydrolyzes
central peptide bonds in which the amino group belongs to aromatic
amino acids e.g. phenyl alanine, tyrosine and tryptophan.
b. Trypsin is an endopeptidase that hydrolyzes central peptide
bonds in which the amino group belongs to basic amino acids
e.g. arginine, lysine and histidine.
c. Chymotrypsin is an endopeptidase that hydrolyzes central
peptide bonds in which the carboxyl group belongs to aromatic
d. Aminopeptidase is an exopeptidase
that hydrolyzes peripheral peptide bond at the amino terminal
(end) of polypeptide chain.
e. Carboxypeptidase is an exopeptidase
that hydrolyzes peripheral peptide bond at the carboxyl terminal
of polypeptide chain.
Absolute, high or substrate specificity
In this type of specificity, the enzyme
acts only on one substrate e.g.
a) Uricase, which acts only on uric acid.
b) Arginase, which acts only on arginine.
c) Carbonic anhydrase, which acts only on carbonic acid.
d) Lactase, which acts on lactose.
e) Sucrase, which acts on sucrose.
f) Maltase, which acts on maltose.
There are two types of dual specificity:
A- The enzyme may act on two substrates by one reaction type.
e.g. xanthine oxidase enzyme acts on xanthine and hypoxanthine
(two substrates) by oxidation (one reaction type).
B- The enzyme may act on one substrate by two different reaction
types e.g. isocitrate dehydrogenase enzyme acts on isocitrate
(one substrate) by oxidation followed by decarboxylation (two
different reaction types).
Factors Affecting the Rate of Enzyme Action
1. Effect of enzyme concentration
The rate of enzyme action is directly proportional to the concentration
of enzyme provided that the condition of the reaction remains
constant and sufficient substrate is supplied.
2. Effect of substrate concentration
The rate of reaction increases as the substrate
concentration increases until a certain point (Vmax) at which
the reaction attains maximal velocity.
Any increase in substrate concentration after this point does
not cause further increase in the rate of the reaction because
at Vmax enzyme molecules are completely saturated with substrate
The substrate concentration that causes the reaction to proceed
at its half maximal velocity (1/2 Vmax) is called Michaelis constant
Enzymes that have low Km have high affinity to
the substrate and act at maximal velocity at low substrate concentration
e.g. hexokinase enzyme that acts on glucose in the fasting state
(low glucose concentration).
Enzymes with high Km have low affinity to substrate and need high
concentration of substrates e.g. glucokinase which needs high
concentration of glucose so it acts maximally in the fed state.
At very low temperature, enzymes are inactive.
Enzyme activity increases gradually with the rise in temperature
until a temperature at which the enzyme attains its maximal activity,
this temperature is called optimum temperature, which lies between
37 - 40 °C in humans.
Optimum temperature is the temperature at which the enzyme attains
its maximal activity.
The rise in temperature from low temperature to optimum temperature
causes an increase in the rate of reaction due to:
a. The rise in temperature increases the initial energy of substrate
leading to a decrease in the activation energy and lower the energy
barrier of the reaction.
b. Also, the rise in temperature increases collision of the molecules
i.e. more molecules become in the bond forming or bond breaking
The rise in temperature above the optimum temperature
leads to a decrease in the rate of enzyme activity.
At higher temperature (60 - 65 °C in humans) irreversible loss
of enzyme activity occurs due to denaturation of the enzymes,
which are protein in nature.
Each enzyme has an optimum pH at which it attains
its maximal activity e.g.
a. Optimum pH of pepsin is 1.5 - 2 (acidic).
b. Optimum pH of pancreatic lipase is 7.5 - 8 (alkaline).
c. Optimum pH of salivary amylase is 6.8 (slightly acidic).
Any change of pH below or above the optimum pH decreases the rate
of enzyme action due to:
a. Changes in pH lead to changes in the ionization state of the
substrate or the enzyme or both.
b. Also, extreme changes in pH lead to denaturation of the enzyme
that is protein in nature.
At the beginning, the rate of reaction increases but by time
the rate of reaction decreases due to:
a. Depletion of substrate.
b. Accumulation of end products.
c. Change in pH of the reaction, which becomes different from
the optimum pH of the enzyme.
6. Concentration of coenzymes
In the conjugated protein enzymes that need coenzymes, the
increase in the coenzyme concentration causes an increase in
the rate of enzyme action.
7. Concentration of metal ion activators
The increase in metal ion activators increases the rate of
enzyme action. Many enzymes are activated by metal ions e.g.
a. Chloride ions activate salivary amylase.
b. Calcium ions activate thrombokinase enzyme.
8. Presence of inhibitors
Inhibitors decrease or even abolish enzyme activity.
Enzyme inhibitors may be:
a. Competitive inhibitors
b. Noncompetitive inhibitors