Isozymes are enzymes that differ in amino acid sequence but catalyze the same chemical reaction, while allozymes represent enzymes from different alleles of the same gene. An example of an isozyme is glucokinase, a variant of hexokinase that is not inhibited by glucose 6-phosphate and serves different functions in specific organs. Enzyme inhibitors can be competitive, noncompetitive, uncompetitive, or irreversible depending on whether they bind to the enzyme's active site and how their binding affects the enzyme's kinetic parameters. Many drugs act as enzyme inhibitors to treat diseases, such as statins that competitively inhibit HMG-CoA reductase to lower cholesterol or ACE inhibitors that lower blood pressure by inhibiting

1. Enzyme inhibition

2. Isozymes
Isozymes are enzymes that differ in amino acid sequence but catalyze the same
chemical reaction.
Allozymes represent enzymes from different alleles of the same gene, and isozymes
represent enzymes from different genes that process or catalyse the same reaction
These enzymes usually display different kinetic parameters (e.g. different Km values),
or different regulatory properties.

3. An example of an isozyme is glucokinase , a variant of hexokinase which is not
inhibited by glucose 6-phosphate.
Its different regulatory features and lower affinity for glucose (compared to
other hexokinases), allows it to serve different functions in cells of specific
organs, such as
1. control of insulin release by the beta cells of the pancreas,
2. initiation of glycogen synthesis by liver cells.
Both of these processes must only occur when glucose is abundant.

4. The enzyme Lactate Dehydrogenase is made of two(H-form and M-Form)
different sub units, combines in different Combinations in depending on the
tissue in which it is present

5. Proenzymes (Zymogens)
A zymogen is an inactive enzyme precursor.
A zymogen requires a biochemical change (such as a hydrolysis reaction revealing
the active site, or changing the configuration to reveal the active site) for it to
become an active enzyme.
The biochemical change usually occurs in a lysosome where a specific part of the
precursor enzyme is cleaved in order to activate it.
The inactivating piece which is cleaved off can be a peptide unit, or can be
independently folding domains comprising more than 100 residues.
Although they limit the enzyme's ability, these n-terminal extensions of the enzyme
or a “prosegment” often aid in the stabilizing and folding of the enzyme they inhibit.

6. The pancreas secretes zymogens partly to prevent the enzymes from
digesting proteins in the cells in which they are synthesised.
Enzymes like pepsin are created in the form of pepsinogen, an inactive
zymogen.
Pepsinogen is activated when chief cells release it into HCl which partially
activates it.
Partially activated pepsinogen completes the activation by removing the
peptide turning the pepsinogen into pepsin.
Accidental activation of zymogens can happen when the secretion duct in the
pancreas is blocked by a gallstone resulting in acute pancreatitis.

7. Examples of zymogens:
Angiotensinogen
Trypsinogen
Chymotrypsinogen
Pepsinogen
Proelastase
Prolipase
Procarboxy peptidase

8. Enzyme Inhibition

9. Introduction
Any substance that can diminish the velocity of an enzyme-catalyzed reaction
is called an inhibitor.
Irreversible inhibitors bind to enzymes through covalent bonds.
Reversible inhibitors typically bind to enzymes through noncovalent bonds,
thus dilution of the enzyme–inhibitor complex results in dissociation of the
reversibly bound inhibitor, and recovery of enzyme activity.
The two most commonly encountered types of reversible inhibition are
competitive and noncompetitive.

10. Competitive Inhibitors
This type of inhibition occurs when the inhibitor binds reversibly to the same site that
the substrate would normally occupy and, therefore, competes with the substrate for
that site.
1. Effect on Vmax (Vmax remains same)
As the effect of a competitive inhibitor is reversed by increasing [S]. So At a sufficiently
high substrate concentration, the reaction velocity reaches the Vmax as observed in the
absence of inhibitor.
2. Effect on Km: (Km is increased)
A competitive inhibitor increases the apparent Km for a given substrate.
This means that, in the presence of a competitive inhibitor, more substrate is needed to
achieve 1⁄2Vmax.

12. Example
Malonic acid resembles with succinic acid inhibits succinate dehydrogenase whose
true substrate is succinic acid

13. Statindrugsas examplesof competitiveinhibitors
This group of antihyperlipidemic agents competitively inhibits the first committed
step in cholesterol synthesis.
This reaction is catalyzed by hydroxymethylglutaryl–CoA reductase (HMG-CoA
reductase).
Statin drugs, such as atorvastatin (Lipitor) and pravastatin (Pravachol),1 are
structural analogs of the natural substrate for this enzyme, and compete
effectively to inhibit HMG-CoA reductase.
By doing so, they inhibit de novo cholesterol synthesis, thereby lowering plasma
cholesterol levels

15. 2. Noncompetitive inhibition
This type of inhibition is recognized by its characteristic effect on Vmax.
Noncompetitive inhibition occurs when the inhibitor and substrate bind at
different sites on the enzyme.
The noncompetitive inhibitor can bind either free enzyme or the ES complex,
thereby preventing the reaction from occurring.
1. Effect on Vmax:
Noncompetitive inhibition cannot be overcome by increasing the concentration of
substrate. Thus, noncompetitive inhibitors decrease the apparent Vmax of the
reaction.
2. Effect on Km:
Noncompetitive inhibitors do not interfere with the binding of substrate to
enzyme. Thus, the enzyme shows the same Km in the presence or absence of the
noncompetitive inhibitor

17. Examples of Noncompetitive inhibitors
Some inhibitors act by forming covalent bonds with specific groups of enzymes.
For example, lead forms covalent bonds with the sulfhydryl side chains of cysteine in
proteins.
The binding of the heavy metal shows noncompetitive inhibition.
Ferrochelatase, an enzyme that catalyzes the insertion of Fe2+ into protoporphyrin (a
precursor of heme), is an example of an enzyme sensitive to inhibition by lead.

18. Other examples of noncompetitive inhibition are certain insecticides, whose
neurotoxic effects are a result of their covalent binding at the catalytic site of
the enzyme acetylcholinesterase (an enzyme that cleaves the
neurotransmitter, acetylcholine).
[Note: Even though covalent bonds are formed, if active enzyme can be
recovered, the inhibition is reversible.]

19. Uncompetitive Inhibition
Inhibitor combines with enzyme substrate complex to form enzyme substrate
inhibitor complex
Seen where two or more substrates are involved

20. Irreversible inhibition
Strong covalent bonds are produced between enzyme and inhibitor.
Iodoacetate reacts with thiol group of enzymes to produce irreversible inhibition
Acetylcholinesterase by di- isopropyl –phosphor fluoride
Enzyme containing SH groups inhibited by heavy metals

21. Enzyme inhibitors as drugs
At least half of the ten most commonly dispensed drugs in the United States act as
enzyme inhibitors.
For example, the widely prescribed β-lactam antibiotics, such as penicillin and
amoxicillin, act by inhibiting enzymes involved in bacterial cell wall synthesis.
Drugs may also act by inhibiting extracellular reactions. This is illustrated by
angiotensin-converting enzyme (ACE) inhibitors.
They lower blood pressure by blocking the enzyme that cleaves angiotensin I to
form the potent vasoconstrictor, angiotensin II.
3 These drugs, which include captopril, enalapril, and lisinopril, cause vasodilation
and a resultant reduction in blood pressure.

22. Diagnostic Value of Serum Enzyme Levels
Amylase: Increased in parotitis and acute pancreatitis
Cholineasterase: Decreased in liver disease and increased level in
nephrotic syndrome
Alkaline phosphatase: Increased in rickets, hyperparathyroidism,
obstructive jaundice
Acid Phosphatase: Increased in prostate carcinoma
Aspartate aminotransferase: Increased in myocardial infraction and acute
liver damage
Alanine aminotransferase: Increased in acute liver damage
Lactate dehydrogenase: Increased in acute myocardial infarction
Isocitrate dehydrogenase: Increased in liver disease
Creatine kinase: Increased in disease of muscle and myocardial infarction