• it is derived from the Greek words en (meaning ‘within’) and zume
• Enzymes are biological catalysts (also known as biocatalysts) that speed up
biochemical reactions in living organisms
• -=Enzymes are protein catalysts that increase the velocity of a chemical
reaction and are not consumed during the reaction.
• can be extracted from cells and then used to catalyse a wide range of
commercially important processes
• Most commonly used enzyme names have the suffix “-ase” attached
to the substrate of the reaction (for example, glucosidase and urease)
or to a description of the action performed (for example, lactate
dehydrogenase and adenylyl cyclase).
• [Note: Some enzymes retain their original trivial names, which give
no hint of the associated enzymic reaction, for example, trypsin and
6. Enzyme names and classification
• Enzymes typically have common names (often called ‘trivial names’) which
refer to the reaction that they catalyse, with the suffix -ase (e.g. oxidase,
dehydrogenase, carboxylase), although individual proteolytic enzymes
generally have the suffix -in (e.g. trypsin, chymotrypsin, papain).
• Often the trivial name also indicates the substrate on which the enzyme acts
(e.g. glucose oxidase, alcohol dehydrogenase, pyruvate decarboxylase).
• However, some trivial names (e.g. invertase, diastase, catalase) provide little
information about the substrate, the product or the reaction involved
• History of Enzymes 1878: Name was used when he was describing the ability
of yeast to produce alcohol from sugars
• 1897: Eduard Buchner began to study the ability of yeast extracts that lacked
any living yeast cells to ferment sugar.
• He found that the sugar was fermented even when there were no living yeast
cells in the mixture.
He named the enzyme that brought about the fermentation of sucrose
• In 1907 he received the Nobel Prize in Chemistry “for his biochemical research
and his discovery of cell-free fermentation".
• 1920s that enzymes were crystallized, revealing that catalytic activity is
associated with protein molecules.
• For the next 60 years or so it was believed that all enzymes were proteins
• 1980s it was found that some ribonucleic acid (RNA) molecules(ribozymes) are
also able to exert catalytic effects and play an important role in gene
They have important roles
i. The production of sweetening agents –used within the food-processing industry
ii. Modification of antibiotics
iii. Enzymes that are used as diagnostic reagents and in clinical therapeutics . Glucose--
iv. They are used in washing powders and various cleaning products-proteases for use in
v. They play a key role in analytical devices and -assays Elisa-that have clinical, forensic
and environmental applications.-Technical advantages-Microorganisms are also very
amenable to genetic modification to produce novel or altered enzymes, using relatively
vi. Enzymes are essential components of animals, plants and microorganisms, due to the
fact that they catalyse and co-ordinate the complex reactions of cellular metabolism
11. Enzymes may be intracellular or extracellular
• Although many enzymes are retained within the cell, and may be located in
specific subcellular compartments, others are released into the surrounding
• The majority of enzymes in industrial use are extracellular proteins from either
fungal sources (e.g. Aspergillus species) or bacterial sources (e.g. Bacillus
species). Examples of these include α-amylase, cellulase, dextranase, proteases
• Many other enzymes for non-industrial use are intracellular and are produced in
much smaller amounts by the cell. Examples of these include asparaginase,
catalase, cholesterol oxidase, glucose oxidase and glucose-6-phosphate
12. Catalytic efficiency
• Enzyme-catalyzed reactions are highly efficient, proceeding from 103–108
times faster than uncatalyzed reactions.
• The number of molecules of substrate converted to product per enzyme
molecule per second is called the turnover number, or k cat, and typically
• Enzymes are highly specific, interacting with one or a few substrates
and catalyzing only one type of chemical reaction.
• The set of enzymes made in a cell determines which reactions occur
in that cell.
14. Holoenzymes, apoenzymes, cofactors, and coenzymes
• Some enzymes require molecules other than proteins for enzymic activity.
• The term holoenzyme refers to the active enzyme with its nonprotein component,
• whereas the enzyme without its nonprotein moiety is termed an apoenzyme and
• If the nonprotein moiety is a metal ion, such as Zn2+ or Fe2+, it is called a cofactor
• If it is a small organic molecule, it is termed a coenzyme.
• Coenzymes that only transiently associate with the enzyme are called
15. • If the coenzyme is permanently associated with the enzyme and
returned to its original form, it is called a prosthetic group (FAD .
Coenzymes commonly are derived from vitamins.
• For example, NAD+ contains niacin, and FAD contains riboflavin
• Enzyme activity can be regulated, that is, increased or
decreased, so that the rate of product formation responds
to cellular need.
17. Location within the cell
• Many enzymes are localized in specific organelles within the cell.
• Such compartmentalization serves to isolate the reaction substrate or
product from other competing reactions.
• This provides a favorable environment for the reaction and organizes the
thousands of enzymes present in the cell into purposeful pathways.
18. HOW ENZYMES WORK
• The mechanism of enzyme action can be viewed from two different perspectives.
• The first treats catalysis in terms of energy changes that occur during the reaction.
• That is, enzymes provide an alternate, energetically favorable reaction pathway
different from the uncatalyzed reaction.
• The second perspective describes how the active site chemically
• facilitates catalysis.
19. Action of Enzymes
• As catalysts, enzymes are only required in very low concentrations, and
they speed up reactions without themselves being consumed during the
• We usually describe enzymes as being capable of catalysing the
conversion of substrate molecules into product molecules as follows:
Substrate ↔ Product
20. Active sites
• Enzyme molecules contain a special pocket or cleft called the active site.
• The active site, formed by folding of the protein, contains amino acid side
chains that participate in substrate binding and catalysis.
• The substrate binds the enzyme, forming an enzyme–substrate (ES) complex.
Binding is thought to cause a conformational change in the enzyme (induced fit
model) that allows catalysis.
• ES is converted to an enzyme–product (EP) complex that subsequently
dissociates to enzyme and product
22. Effect of an enzyme on reducing the activation energy required to start a
reaction where (a) is uncatalysed and (b) is enzyme-catalysed reaction
24. Enzymes are affected by pH and temperature
• Various environmental factors are able to affect the rate of enzyme-
catalysed reactions through reversible or irreversible changes in the
• The effects of pH and temperature are generally well understood.
• Most enzymes have a characteristic optimum pH at which the velocity of
the catalysed reaction is maximal, and above and below which the
26. • Thermal denaturation is time dependent, and for an enzyme the term
‘optimum temperature’ has little real meaning unless the duration of
exposure to that temperature is recorded.
• The thermal stability of an enzyme can be determined by first
exposing the protein to a range of temperatures for a fixed period of
time, and subsequently measuring its activity at one favourable
temperature (e.g. 25°C).
• Enzymes are sensitive to inhibitors
• Substances that reduce the activity of an enzyme-catalysed reaction are known as
• They act by either directly or indirectly influencing the catalytic properties of the active
• Inhibitors can be foreign to the cell or natural components of it.
• Those in the latter category can represent an important element of the regulation of cell
• Many toxins and also many pharmacologically active agents (both illegal drugs and
prescription and over-the counter medicines) act by inhibiting specific enzyme-catalysed
30. Allosteric regulators and the control of enzyme activity
• Allosteric enzymes are key regulatory enzymes that control the activities of
metabolic pathways by responding to inhibitors and activators
• allosteric enzymes there is an area where activity is lower than that of an
equivalent ‘normal’ enzyme, and also an area where activity is higher than
that of an equivalent ‘normal’ enzyme, with a rapid transition between these
• This is rather like a switch that can quickly be changed from ‘off’ (low activity)
to ‘on’ (full activity
31. • Most allosteric enzymes are polymeric—that is, they are composed of at least two
(and often many more) individual polypeptide chains.
• They also have multiple active sites where the substrate can bind.
• Allosteric enzymes have an initially low affinity for the substrate, but when a
single substrate molecule binds, this may break some bonds within the enzyme and
thereby change the shape of the protein such that the remaining active sites are
able to bind with a higher affinity.
• Therefore allosteric enzymes are often described as moving from a tensed state or
T-state (low affinity) in which no substrate is bound, to a relaxed state or R-state
(high affinity) as substrate binds
Allosteric regulators and the control of enzyme activity
32. FACTORS AFFECTING REACTION VELOCITY
A. Substrate concentration
• The rate or velocity of a reaction (v) is the number of substrate molecules converted
to product per unit time.
• Velocity is usually expressed as μmol of product formed per minute.
• The rate of an enzyme-catalyzed reaction increases with substrate concentration
until a maximal velocity (Vmax) is reached.
• The leveling off of the reaction rate at high substrate concentrations reflects the
saturation with substrate of all available binding sites on the enzyme molecules
33. • B. Temperature
• Increase of velocity with temperature:
• The reaction velocity increases with temperature until a peak velocity is reached .
• This increase is the result of the increased number of molecules having sufficient energy to
pass over the energy barrier and form the products of the reaction.
• Decrease of velocity with higher temperature:
• Further elevation of the temperature causes a decrease in reaction velocity as a result of
temperature induced denaturation of the enzyme
• Nb; The optimum temperature for most human enzymes is between 35°C
• Human enzymes start to denature at temperatures above 40°C, but thermophilic
bacteria found in the hot springs have optimum temperatures of 70°C.
FACTORS AFFECTING REACTION VELOCITY
34. • C. pH
• The concentration of protons (H+) affects reaction velocity in several ways
• catalytic process usually requires that the enzyme and substrate have specific chemical
groups in either an ionized or un-ionized state in order to interact .e.g. catalytic activity
may require that an amino group of the enzyme be in the protonated form (–NH3+).
• Effect of pH on enzyme denaturation:
• Extremes of pH can also lead to denaturation of the enzyme, because the structure of the
catalytically active protein molecule depends on the ionic character of the amino acid side
FACTORS AFFECTING REACTION VELOCITY
35. PH cont’d
• Variable pH optimum: The pH at which maximal enzyme activity is achieved
is different for different enzymes and often reflects the [H+] at which the
enzyme functions in the body.
• For example, pepsin, a digestive enzyme in the stomach, is maximally active
at pH 2, whereas other enzymes, designed to work at neutral pH, are
denatured by such an acidic environment
36. Name of the enzyme Optimum pH
Acid phosphatase 5
Alkline phosphatase 9-10
1. Metabolic pathways are controlled by regulating enzyme activity. If enzyme activity is
not regulated, it can harm cellular activities and may lead to the development of
55. For detailed description of this topic, please click on:
56. Nature of Enzymes
• Enzymes are specific catalysts
• enzymes also possess remarkable specificity in that they generally catalyse
the conversion of only one type (or at most a range of similar types) of
substrate molecule into product molecules. E.g. glucose oxidase shows
almost total specificity for its substrate, β-D-glucose
57. Nature of Enzymes
• Two categories dependant on: the presence and absence of a nonprotein
i. Simple enzyme: made up of only protein molecules not bound to
any nonproteins, e.g. Pancreatic Ribonuclease.
ii. Holo enzyme(apoenzyme) -made up of protein groups and non-
protein component. non-protein component here includes- (Fe
2+, Mn 2+, or Zn 2+ ions).
a) Coenzymes are derivatives of vitamins without which the enzyme cannot
exhibit any reaction. One molecule of coenzyme is able to convert a large
number of substrate molecules with the help of enzyme.
i. Coenzyme accepts a particular group removed from the substrate or
donates a particular group to the substrate
ii. Coenzymes are called co substrate because the changes that take place
in substrates are complimentary to the changes in coenzymes.
iii. The coenzyme may participate in forming an intermediate enzyme-
substrate complex Example: NAD, FAD, Coenzyme A
59. Metal ions in enzymes
Many enzymes require metal ions like ca2+, K+, Mg2+, Fe2+, Cu2+, Zn2+,
Mn2+ and Co2+ for their activity.
Metal-activated enzymes-form only loose and easily dissociable complexes
with the metal and can easily release the metal without Denaturation.
Metalloenzymes hold the metal tightly on the molecule and do not release
it even during extensive purification.
60. Metal ions in enzymes
Metal ions promote enzyme action by
•Maintaining or producing the active structural conformation of the enzyme
(e.g. glutamine synthase)
•Promoting the formation of the enzyme-substrate complex (Example: Enolase
and carboxypeptidase A.)
•Acting as electron donors or acceptors (E.g. : Fe-S proteins and cytochromes)
•Causing distortions in the substrate or the enzyme E.g. phosphotransferases).
62. The six major classes of enzymes with some example are
• They catalyze oxidation and reduction reactions
• They catalyze transfer of groups
• They catalyze hydrolysis of peptide, ester, glycosyl etc. bonds
• 4. Lyases
• They catalyze removal of groups from substrates by mechanisms other than hydrolysis forming
63. The six major classes of enzymes with some example are
• 5. Isomerases
• They catalyze interconversion of optical, functional and geometrical isomers.
• 6. Ligases
• They catalyze linking together of two compounds. The linking is coupled to
the breaking of phosphate from ATP
64. clinically important enzymes
• Some of the clinically important enzymes which are routinely measured in clinical
chemistry laboratory are:
1. Alkaline phosphatase
• This enzyme catalyzes the hydrolysis of organic esters at alkaline pH 9.0, hence
the name alkaline phosphatase. The normal level is 20-90 units/L. The level of
the enzyme is elevated in rickets, obstructive jaundice, hyper para thyroidism,
metastatic cancer, bone cancer and osteomalacia. In obstructive jaundice
2. Acid phosphatase
• This enzyme catalyzes the hydrolysis of organic esters at acidic pH (5.0) hence
the name acid phosphatase. The normal level of the enzyme is 2.5-12.0 units/L.
It is increased in prostrate cancer. Small increase are seen in bone disease and
65. 3. Transminases
Aspartate aminoa transferase (AST) and alanine amino transferase
(ALT) are two transaminases most frequently measured. Normal levels
are 3-20 U/L for AST and 4-20 U/L for ALT (Units-U).
66. clinically important enzymes
4. γ-glutamyl trans peptidase (GGT)
• It is involved in the degradation of glutathione. Its level is increased in alcoholic cirrhosis.
The normal plasma level of GGT is less than 30 units/L. Since this enzyme is secreted into
bile by liver, like alkaline phosphatase γ-glutamyl trans peptidase level increases in
cholestatic or obstructive jaundice. It is also elevated in brain lesions.
5. Creatine phosphokinase (CK)
• The normal level of this enzyme in plasma is 12-60 U/l. Since skeletal muscle is rich in CK
serum CK level raises in disease effecting skeletal muscle. Its level is elevated in muscular
dystrophy, polymyositis, severe muscle exercise, muscle injury, hypothyroidism, epileptic
seizures and in tetanus.
• CK level is also elevated in diseases affecting cardiac muscle because of its high content in
it. CK level is elevated in myocardial infarction
67. clinically important enzymes
6. Lactate dehydrogenase (LDH)
• The LDH normal level is 70-90 units/L. LDH levels are elevated in myocardial
infraction. The serum LDH level raises within 24 hours after infraction, reaches
peak level around 2-3 days and returns to normal in a week. Serum LDH level is
also elevated in pernicious anemia, megaloblastic anemia, acute hepatitis, blood
cancer and in progressive muscular dystrophy.
7. Isocitrate dehydrogenase
• The normal level of this enzyme in plasma is 1-5 Units/L. Its level is elevated in
inflammatory diseases of liver like infective heptatitis, toxic hepatitis. In
obstructive jaundice, its level
68. clinically important enzymes
• The normal range of this enzyme in plasma is 800-1800 Units/L. This enzyme is
secreted by pancreas and salivary glands. So, its level is elevated mainly in acute
pancreatitis and parotitis.
• Its level raised in other conditions like intestinal obstruction and in mumps.
• It is an enzyme produced by pancreas. It is secreted into duodenum through
pancreatic duct. The normal level of this enzyme is up to 150 Units/L. It is mainly
elevated in acute pancreatitis and pancreas cancer.
• It is also elevated in patients with abdominal lesions, perforated peptic ulcer,
intestinal obstruction and in acute peritonitis.