Enzyme catalysis, classes, mechanism of action, application

1. CATALYSIS LECTURE
Part-IV: ENZYME CATALYSIS
Dr.Rabiul Hussain
School of Material Science & Engineering
Jimma Institute of Technology, Jimma University
E-mail: rabiul786@gmail.com
Ph.No. +251-0966882081 (Ethiopia)
+91-9508832510 (India)

2. ENZYME CATALYSIS
• Numerous organic reactions are taking place in the body of animals and
plants to maintain the life process. These reactions being slow remarkably
catalysed by the organic compounds known as Enzymes.
• Enzymes are protein molecules which act as catalysts to speed up organic
reactions in living cells. The catalysis brought about by enzymes is known
as Enzyme Catalysis.
• Importance: Enzyme catalytic processes are extensively used in food
industries for centuries such as in production of bread, beverages,
yoghurt, cheese, vinegar etc. Enzymes have properties similar to
homogeneous catalysts. New enzymes are continuously being discovered.
At present, more than 3000 enzymes are reported.

3. What is an Enzyme?
• Enzymes are large macromolecular polypeptide (polymers of amino acid
monomers) proteins. Molecular weight is in the range of 104-106 .
• Each enzyme has a unique three-dimensional structure with a binding site
or active site that is chemically and geometrically compatible with a
particular reactant molecule and thereby can give upto 100 % selectivity.
• Enzymes are formed in living systems by condensation and/or dehydration
of amino acids that have the composition of H2N-CHR-COOH, to form
peptide C-N bonds. Large structure contains hundreds of amino acids and
there is enormous number of possible structures. Only few are
characterized and well known.
• Enzymes are synthesized by living organisms and can be extracted from
their biological source, purified and used in laboratory and industrial
processes. Enzymes can also be synthesized in vitro that is in an artificial
environment outside the living organism. Enzymes are only active within a
limited range of pH and temperature.

4. Structure of Enzymes
• Enzymes are made from just 20 -amino acid building blocks (structures
and abbreviations are shown in Table 5.1).
• Each amino acid has a unique side chain, or residue, which can be polar,
aliphatic, aromatic, acidic, or basic.
• The amide bonds (peptide bonds) make up the enzyme’s backbone, and
the residues determine the ultimate structure and catalytic activity of the
enzyme.

6. Nomenclature and Classification
• The classification and naming of enzymes are somewhat complex. The first
few enzymes found were given trivial (i.e., nonsystematic) names, e.g.,
pepsin, trypsin,lysozyme, and chymotrypsin.
• Nowadays enzymes were given name by adding the suffix -ase to the
name of the substrate (and sometimes the reaction as well).
• For example, lipase catalyzes the hydrolysis of a lipid triglyceride, sucrase
catalyzes the hydrolysis of sucrose into glucose and fructose, and glucose
isomerase catalyzes the isomerization of glucose to fructose.
• The International Enzyme Commission established a four-stage
classification system for enzymes which is based on six main groups (Table
5.2). Each enzyme is assigned an identifier, called an enzyme number. This
identifier comprises four numbers separated by periods, denoting the
main group, subclass, sub-subclass, and serial number.

7. Classification of Enzymes
• Enzymes are classified into six different groups based on the type of
reactions catalyzed:

8. MECHANISM OF ENZYME CATALYSIS
• The long chains of the enzyme (protein) molecules are coiled on each
other to make a rigid colloidal particle with cavities on its surface. These
cavities which are of characteristic shape and a bound in active groups
(NH2, COOH, SH, OH)] are termed Active centres.
• The molecules of substrate which have complementary shape, fit into
these cavities just as key fits into a lock (Lock-and- Key theory).
• By virtue of the presence of active groups, the enzyme forms an
activated complex with the substrate which at once decomposes to yield
the products. Thus the substrate molecules enters the cavities, forms
complex and reacts, and at once the products get out of the cavities.
• Michaelis and Menten (1913) proposed the following mechanism for
enzyme catalysis .

9. Mechanism of Enzyme Catalysis

10. CHARACTERISTICS OF ENZYME CATALYSIS
• In general, enzyme behave like inorganic heterogeneous catalysts.
However, they are unique in their efficiency and high degree of specificity.
• (1) Enzymes are the most efficient catalysts known: The enzyme
catalysed reactions proceed at fantastic high rates in comparison to those
catalysed by inorganic substances. Thus one molecule of an enzyme may
transform one million molecules of the substrate (reactant) per minute.
• (2) Enzyme catalysis is marked by absolute specificity: An enzyme as a
rule catalyses just one reaction with a particular substance. For example,
urease (an enzyme derived from soya bean) catalyses the hydrolysis of
urea and no other amide, not even methylurea.

11. CHARACTERISTICS OF ENZYME CATALYSIS
• (3) The rate of enzyme catalysed reactions is maximum at the optimum
temperature: The rate of an enzyme catalysed reaction is increased with
the rise of temperature but up to a certain point. Thereafter the enzyme is
denatured as its protein structure is gradually destroyed. Thus the rate of
reaction drops and eventually becomes zero when the enzyme is
completely destroyed. The rate of an enzyme reaction with raising of
temperature gives a bell-shaped curve. The temperature at which the
reaction rate is maximum is called the optimum temperature.

12. CHARACTERISTICS OF ENZYME CATALYSIS
• (4) Rate of enzyme catalysed reactions is maximum at the optimum pH:
The rate of an enzyme catalysed reaction varies with pH of the system.
The rate passes through a maximum at a particular pH, known as the
optimum pH. The enzyme activity is lower at other valuesof pH. Thus
many enzymes of the body function best at pH of about 7.4, the pH of the
blood and body fluids.

13. CHARACTERISTICS OF ENZYME CATALYSIS
• (5) Enzymes are markedly inhibited or poisoned: The catalytic activity of
an enzyme is often reduced (inhibited) or completely destroyed(poisoned)
by addition of other substances. These inhibitors or poisons interact with
the activefunctional groups on the enzyme surface. For example, heavy
metal ions (Ag+, Hg2+) react with the –SH groups of the enzyme and
poison it.
• The physiological activity of many drugs is related to their action as
enzyme inhibitors in the body. Thus sulpha drugs, penicillin, and
streptomycin inhibit the action of several bacteria and have proved
effective in curing pneumonia, dysentery, cholera, and many other
infectious diseases.
• (6) Catalytic activity of enzymes is greatly enhanced by the presence of
Activators or Coenzymes : Activators are metal ions Na+, Mn2+, CO2+,
Cu2+, etc., which get weakly bonded to enzyme molecules and promote
their catalytic action. Thus it has been found that the addition of sodium
chloride (Na+) makes amylase catalytically very active.

14. Industrial bio-catalytic process

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DR. RABIUL HUSSAIN : 966882081