Human Enzymes
Introduction
Life depends on thousands of chemical reactions occurring continuously
inside the human body. Digestion, respiration, muscle contraction, nerve
conduction, DNA replication, hormone synthesis, detoxification, and energy
production all require biochemical reactions. However, under normal body
conditions, most of these reactions would occur extremely slowly without the
help of special biological catalysts called enzymes.
Enzymes accelerate biochemical reactions by lowering activation energy
without themselves being consumed in the reaction. They are therefore essential
for maintaining life.
Human physiology is fundamentally dependent on enzymes. Even a slight
deficiency or malfunction of an enzyme can produce severe disease.
Definition of Enzymes
Enzymes are:
Biological catalysts produced by living cells that accelerate biochemical
reactions without undergoing permanent change themselves.
Most enzymes are proteins in nature, although there are a few RNA
molecules with catalytic activity which are called ribozymes.
Characteristics of Enzymes
1. Protein Nature
Most enzymes are globular proteins.
Examples:
- Pepsin
- Trypsin
- Amylase
- Lipase
Exception:
- Ribozymes (RNA enzymes)
2. Biological Catalysts
Enzymes speed up reactions by lowering activation energy.
Ea↓⇒Reaction rate↑
Without enzymes, many reactions required for life would occur too slowly.
3. Highly Specific
Enzymes usually act on specific substrates.
Example:
Types of specificity:
- Absolute specificity
- Group specificity
- Bond specificity
- Stereospecificity
4. Required in Small Amounts
A very small amount of enzyme can catalyze a large amount of substrate.
5. Remain Unchanged After Reaction
Enzymes are not consumed during the reaction and can be reused.
6. Optimum Temperature and pH
Most human enzymes work best at:
- Temperature: around 37°C
- pH: near neutral
Exceptions:
- Pepsin: optimum pH 1.5–2
- Trypsin: optimum pH around 8
7. Sensitive to Environmental Changes
High temperature or extreme pH can denature enzymes.
Chemical Composition of Enzymes
Enzymes may be:
1. Simple Enzymes
Contain only protein.
Example:
- Pepsin
2. Conjugated Enzymes
Contain:
- Protein part → Apoenzyme
- Non-protein part → Cofactor
Complete active enzyme:
- Holoenzyme
Representation
Apoenzyme+Cofactor=Holoenzyme
Cofactors
Cofactors are non-protein substances required for enzyme activity.
Types:
A. Coenzymes
Organic non-protein molecules loosely attached.
Usually vitamin-derived.
Examples:
- NAD⁺
- FAD
- Coenzyme A
B. Prosthetic Groups
Firmly attached non-protein groups.
Example:
- Heme in catalase
C. Metal Ion Activators
Metal ions required for enzyme action.
Examples:
- Mg²⁺
- Zn²⁺
- Fe²⁺
- Cu²⁺
Active Site of Enzyme
The active site is the specific region where substrate binds.
Functions:
- Substrate recognition
- Catalysis
- Product release
Properties:
- Three-dimensional
- Highly specific
- Contains catalytic amino acids
Substrate
The molecule upon which an enzyme acts is called the substrate.
Example:
- Starch is substrate for amylase.
Enzyme-Substrate Complex
When substrate binds enzyme:
E+S⇌ES→E+P
Where:
- E = enzyme
- S = substrate
- ES = enzyme-substrate complex
- P = product
Mechanism of Enzyme Action
Two major theories explain enzyme action.
Proposed by: Emil Fischer
The active site has a rigid shape complementary to substrate.
Like:
- Lock → enzyme
- Key → substrate
Only correct substrate fits.
Limitation:
- Does not explain flexibility of
enzymes.
Proposed by: Daniel Koshland
Active site is flexible and changes shape when substrate approaches.
This model is more accepted today.
Activation Energy
Every reaction requires initial energy called activation energy.
Enzymes lower this activation energy.
Enzymes lower Ea without changing ΔG
This increases reaction rate tremendously.
Factors Affecting Enzyme Activity
1. Temperature
- Rate increases with temperature
up to optimum.
- Beyond optimum, denaturation
occurs.
Human enzymes:
- Optimum ≈ 37°C
2. pH
Each enzyme has optimum pH.
Examples:
- Pepsin → acidic
- Trypsin → alkaline
3. Substrate Concentration
Increasing substrate increases rate until saturation.
4. Enzyme Concentration
More enzyme → faster reaction (if substrate available).
5. Presence of Inhibitors
Certain substances decrease enzyme activity.
Enzyme Inhibition
1. Competitive Inhibition
Inhibitor resembles substrate and competes for active site.
Example:
- Malonate inhibits succinate
dehydrogenase.
Characteristics:
- Reversible
- Can be overcome by increasing
substrate concentration
2. Non-Competitive Inhibition
Inhibitor binds elsewhere and changes enzyme structure.
Example:
- Heavy metals
Cannot be reversed by increasing substrate.
These enzymes possess:
- Active site
- Regulatory site
Binding of molecules at regulatory site changes enzyme activity.
Important in:
- Metabolic regulation
- Feedback inhibition
Isoenzymes (Isozymes)
Different molecular forms of same enzyme catalyzing same reaction.
Example:
- Lactate dehydrogenase (LDH)
Clinical importance:
- Diagnosis of heart and liver
diseases
Zymogens (Proenzymes)
Inactive precursors converted into active enzymes.
Examples:
|
Zymogen |
Active Enzyme |
|
Pepsinogen |
Pepsin |
|
Trypsinogen |
Trypsin |
|
Prothrombin |
Thrombin |
Importance:
- Prevents self-digestion of
tissues
Enzyme Nomenclature
Most enzymes end with suffix:
- “-ase”
Examples:
- Lipase
- Lactase
- Oxidase
Older names do not always follow this rule:
- Pepsin
- Trypsin
- Ptyalin
Basis of Enzyme Naming
Enzymes may be named according to:
1. Substrate
- Urease → acts on urea
- Lactase → acts on lactose
2. Type of Reaction
- Oxidase
- Dehydrogenase
- Hydrolase
3. Source
- Gastric lipase
- Salivary amylase
IUBMB Classification of Enzymes
The International Union of Biochemistry and Molecular Biology (IUBMB)
developed a standardized classification system called the EC system (Enzyme
Commission system).
Each enzyme receives an EC number.
Example:
- Lactate dehydrogenase → EC
1.1.1.27
Structure of EC number: EC;x.x.x.x
Where:
- First digit → major class
- Second → subclass
- Third → sub-subclass
- Fourth → specific enzyme number
Major Classes of Enzymes
Traditionally six major classes are emphasized for NEET preparation.
1. Oxidoreductases (EC 1)
Catalyze oxidation-reduction reactions.
Examples:
- Dehydrogenases
- Oxidases
- Catalase
Function:
- Cellular respiration
- Energy production
Example reaction:
AH2+B→A+BH2
2. Transferases (EC 2)
Transfer functional groups from one molecule to another.
Examples:
- Kinases
- Transaminases
Importance:
- Amino acid metabolism
- Phosphorylation reactions
3. Hydrolases (EC 3)
Catalyze hydrolysis reactions.
Examples:
- Lipase
- Amylase
- Proteases
Major digestive enzymes belong here.
4. Lyases (EC 4)
Add or remove groups without hydrolysis.
Examples:
- Aldolase
- Decarboxylase
5. Isomerases (EC 5)
Catalyze isomerization reactions.
Examples:
- Phosphoglucose isomerase
- Racemases
6. Ligases (EC 6)
Join two molecules using ATP.
Examples:
- DNA ligase
- Glutamine synthetase
Digestive Enzymes in Humans
Enzymes of Mouth
|
Enzyme |
Source |
Function |
|
Salivary amylase (ptyalin) |
Salivary glands |
Starch → Maltose |
|
Lingual lipase |
Tongue glands |
Fat digestion |
Enzymes of Stomach
|
Enzyme |
Function |
|
Pepsin |
Proteins → Peptides |
|
Gastric lipase |
Fat digestion |
|
Rennin (infants) |
Milk coagulation |
Pancreatic Enzymes
|
Enzyme |
Function |
|
Trypsin |
Protein digestion |
|
Chymotrypsin |
Protein digestion |
|
Pancreatic amylase |
Starch digestion |
|
Lipase |
Fat digestion |
|
Nucleases |
Nucleic acid digestion |
Intestinal Enzymes
|
Enzyme |
Function |
|
Maltase |
Maltose → Glucose |
|
Sucrase |
Sucrose → Glucose + Fructose |
|
Lactase |
Lactose → Glucose + Galactose |
|
Peptidases |
Peptides → Amino acids |
Physiological Importance of Enzymes
Enzymes are indispensable for normal human physiology.
1. Digestion
Digestive enzymes break complex food into absorbable units.
Examples:
- Amylase
- Pepsin
- Lipase
2. Energy Production
Enzymes regulate:
- Glycolysis
- Krebs cycle
- Electron transport chain
Without enzymes, ATP generation would stop.
3. DNA Replication and Repair
Examples:
- DNA polymerase
- DNA ligase
4. Muscle Contraction
ATPase enzymes are essential for muscle movement.
5. Blood Clotting
Several enzymes participate in coagulation cascade.
Example:
- Thrombin
6. Detoxification
Liver enzymes metabolize toxins and drugs.
Example:
7. Immune Defense
Lysozyme destroys bacterial cell walls.
8. Hormone Synthesis
Enzymes participate in steroid and peptide hormone synthesis.
Clinical Importance of Enzymes
Enzyme levels are important diagnostic markers.
|
Enzyme |
Disease Association |
|
Liver disease |
|
|
Creatine kinase |
Muscle damage |
|
Troponin-associated enzymes |
Myocardial infarction |
|
Amylase, lipase |
Pancreatitis |
Enzyme Deficiency Disorders
|
Disorder |
Deficient Enzyme |
|
Lactase |
|
|
Phenylalanine hydroxylase |
|
|
Tay-Sachs disease |
Hexosaminidase A |
|
Albinism |
Tyrosinase deficiency |
Difference Between Enzymes and Hormones
|
Feature |
Enzymes |
Hormones |
|
Nature |
Mostly proteins |
Proteins/steroids/amines |
|
Function |
Catalysis |
Regulation |
|
Site of action |
Usually local |
Distant organs |
|
Reusability |
Reusable |
Metabolized |
Important NEET Points to Remember
- Enzymes are biological catalysts.
- Most enzymes are proteins.
- Ribozymes are RNA enzymes.
- Enzymes lower activation energy.
- Active site binds substrate.
- Apoenzyme + cofactor =
holoenzyme.
- Digestive enzymes are hydrolases.
- Pepsin works best in acidic pH.
- Trypsin works best in alkaline
pH.
- Enzymes are highly specific.
- IUBMB classification uses EC
numbers.
- Six major enzyme classes are
important for NEET.
Flowchart Summary
Enzyme Classification
Enzymes
│
├── Oxidoreductases
├── Transferases
├── Hydrolases
├── Lyases
├── Isomerases
└── Ligases
Mnemonic for Enzyme Classes
“Only The Hungry Like Ice-cream Lollies”
|
Letter |
Class |
|
O |
Oxidoreductases |
|
T |
Transferases |
|
H |
Hydrolases |
|
L |
Lyases |
|
I |
Isomerases |
|
L |
Ligases |
Conclusion
Enzymes are among the most important biomolecules in the human body. They
regulate digestion, metabolism, energy production, DNA replication, muscle
activity, blood clotting, detoxification, and virtually every biochemical
process essential for life. Their extraordinary specificity and catalytic
efficiency make them indispensable for maintaining homeostasis and survival.
A strong understanding of enzyme structure, nomenclature, classification,
mechanisms, and physiological importance forms a fundamental basis for NEET
biology, human physiology, biochemistry, and medical sciences.
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