3.5.2 Classification Based Upon Substrate

Introduction

Enzymes can be classified not only on the basis of the type of reaction they catalyze but also on the basis of the substrate they act upon. This is a simpler and more practical way of classifying enzymes. In this system, enzymes are named and grouped according to the type of substrate they break down or act upon.

Definition

Classification Based Upon Substrate: Enzymes can be classified on the basis of substrates they use. Some of the examples are: proteases, lipases, carbohydrases, and nucleases.

In simple words: Enzymes are grouped based on what substance (substrate) they act on.

Core Concept

• Enzymes can be classified on the basis of the substrates they use.
• This classification is based on the type of substrate the enzyme acts upon.
• The main groups are:

Detailed Explanation

1. Proteases

Definition: These enzymes act upon proteins.

In simple words: Proteases are enzymes that break down proteins.

• They break down proteins into smaller peptides and amino acids.
• They are involved in protein digestion.
• They are also involved in protein turnover and regulation.

Examples:

• Stomach (pepsin)
• Small intestine (trypsin, aminopeptidases, carboxypeptidases, erepsin)

• Pepsin: pH 2
• Trypsin: pH 8

2. Lipases

Definition: These enzymes hydrolyze lipids into fatty acids and glycerols.

In simple words: Lipases are enzymes that break down lipids (fats).

• They hydrolyze lipids into fatty acids and glycerol.
• They are involved in lipid digestion.
• They are also involved in lipid metabolism.

Examples:

• Pancreas (pancreatic lipase)
• Blood vessels (lipoprotein lipase)
• Adipose tissue (hormone-sensitive lipase)

Optimum pH: - Pancreatic lipase: pH 8

3. Carbohydrases

Definition: These enzymes cause breakdown of carbohydrates.

In simple words: Carbohydrases are enzymes that break down carbohydrates.

• They break down carbohydrates into simpler sugars.
• They are involved in carbohydrate digestion.
• They are also involved in carbohydrate metabolism.

Examples:

• Mouth (salivary amylase)
• Pancreas (pancreatic amylase)
• Small intestine (maltase, sucrase, lactase)
• Bacteria and fungi (cellulase)

• Salivary amylase: pH 7
• Pancreatic amylase: pH 8
• Maltase, sucrase, lactase: pH 6–7

4. Nucleases

Definition: These are involved in the breakdown of DNA and RNA.

In simple words: Nucleases are enzymes that break down nucleic acids.

• They break down DNA and RNA into nucleotides.
• They are involved in nucleic acid digestion.
• They are also involved in DNA replication and repair.

Examples:

• Pancreas (pancreatic nucleases)
• Small intestine
• Nucleus
• Cytoplasm

Optimum pH: - Pancreatic nucleases: pH 8

Summary Table of Classification Based Upon Substrate

Diagnostic Uses of Enzymes (from textbook)

Intermediate Level

Classification in Detail

Proteases in Detail

Lipases in Detail

Carbohydrases in Detail

Nucleases in Detail

Diagnostic Uses of Enzymes

Important Points

• 1. Enzymes can be classified on the basis of substrates they use.
• 2. Some of the examples are: proteases, lipases, carbohydrases, and nucleases.
• 3. 1-Proteases:
• - These enzymes act upon proteins.
• - Examples are:
• - Pepsin and Trypsin (both digest large polypeptides into small polypeptides or peptones)
• - Aminopeptidases and Carboxypeptidases (both digest peptones into dipeptides)
• - Erypsin (digest dipeptides into amino acids)
• 4. 2-Lipases:
• - These enzymes hydrolyze lipids into fatty acids and glycerols.
• - Examples are pancreatic lipases.
• 5. 3-Carbohydrases:
• - These enzymes cause breakdown of carbohydrates.
• - Examples are:
• - Amylase (digest starch or glycogen into maltose)
• - Cellulase (digest cellulose into cellobiose, a disaccharide)
• - Maltase (digest maltose into glucoses)
• - Sucrase (digest sucrose into glucose and fructose)
• - Lactase (digest lactose into galactose and glucose)
• 6. 4-Nucleases:
• - These are involved in the breakdown of DNA and RNA.
• - Examples are:
• - RNAases (digest RNA into ribonucleotides)
• - DNAases (digest DNA into deoxyribonucleotides)
• - ATPases (cause hydrolysis of ATP in muscles, etc.)
• 7. Diagnostic uses of enzymes:
• - Aldolase: Progressive muscular dystrophy, viral hepatitis, and advanced cancer of the prostate
• - Creatine Phosphokinase: Damage to muscle cells
• - Gamma-glutamyl Transpeptidase: Assessing liver function
• - Lactic Dehydrogenase: Differentiating heart attack, anemia, lung injury, or liver disease
• - Lipase: Damage to the pancreas
• 8. Venoms as enzyme inhibitors:
• - Snake venom is highly modified saliva that is produced by special glands of certain species of snakes.
• - Snake venom is a combination of many toxins (proteins) and different enzymes.
• - Snake venom inhibits cholinesterase to make the prey lose control of its muscles.
• - Venom is an inhibitor for an essential enzyme cytochrome oxidase in the cells.
• - Three distinct types of venom:
• 1. Hemotoxic venoms act on the heart and cardiovascular system.
• 2. Neurotoxic venom acts on the nervous system and brain.
• 3. Cytotoxic venom has a localized action at the site of the bite.
• - Venom occupies the active site of the enzyme or combining with the iron which may be present in the prosthetic group or which may be required as an enzyme activator.

Remember

• - Proteases = proteins; examples: pepsin, trypsin, erepsin
• - Lipases = lipids; example: pancreatic lipase
• - Carbohydrases = carbohydrates; examples: amylase, maltase, sucrase, lactase
• - Nucleases = nucleic acids; examples: RNAase, DNAase
• - Diagnostic enzymes: Aldolase, Creatine Phosphokinase, Gamma-glutamyl Transpeptidase, Lactic Dehydrogenase, Lipase
• - Venoms: Hemotoxic, Neurotoxic, Cytotoxic

Common Mistakes

• 1. Confusing proteases and lipases – Proteases act on proteins; Lipases act on lipids.
• 2. Confusing carbohydrases and nucleases – Carbohydrases act on carbohydrates; Nucleases act on nucleic acids.
• 3. Thinking all proteases work at the same pH – Pepsin works at pH 2; Trypsin works at pH 8.
• 4. Forgetting the examples – Each group has specific examples.
• 5. Confusing the diagnostic uses of enzymes – Each diagnostic enzyme has a specific clinical use.
• 6. Thinking venoms only inhibit one enzyme – Venoms can inhibit multiple enzymes.
• 7. Confusing the types of venoms – Hemotoxic, Neurotoxic, Cytotoxic.
• 8. Thinking all carbohydrases work in the same location – Different carbohydrases work in different locations.
• 9. Forgetting that cellulase is not produced by humans – Cellulase is produced by bacteria and fungi.
• 10. Confusing the products of enzyme action – Each enzyme produces specific products.
• 11. Thinking ATPases are nucleases – ATPases are involved in ATP hydrolysis, not nucleic acid breakdown.
• 12. Forgetting the clinical significance of diagnostic enzymes – Diagnostic enzymes are important in medicine.

Quick Revision

• Enzymes can be classified on the basis of substrates they use. Some of the examples are: proteases, lipases, carbohydrates and nucleases.
• 1-Proteases: These enzymes act upon proteins. Examples are: pepsin and trypsin (both digest large polypeptides into small polypeptides or peptones), aminopeptidases and carboxypeptidases (both digest peptones into dipeptides) and erypsin (digest dipeptides into amino acids).
• 2-Lipases: These enzymes hydrolyze lipids into fatty acids and glycerols. Examples are pancreatic lipases.
• 3-Carbohydrases: These enzymes cause breakdown of carbohydrates. Examples are:
• (a) amylase (digest starch or glycogen into maltose)
• (b) cellulase (digest cellulose into cellobiose, a disaccharide)
• (c) maltase (digest maltose into glucoses)
• (d) sucrase (digest sucrose into glucose and fructose)
• (e) lactase (digest lactose into galactose and glucose)
• 4-Nucleases: These are involved in the breakdown of DNA and RNA. Examples are:
• (a) RNAases (digest RNA into ribonucleotides)
• (b) DNAases (digest DNA into deoxyribo nucleotides)
• (c) ATPases (cause hydrolysis of ATP in muscles etc.)
• Diagnostic uses of enzymes:
• (a) Aldolase: progressive muscular dystrophy, viral hepatitis and advanced cancer of the prostate
• (b) Creatine Phosphokinase: damage to muscle cells
• (c) Gamma-glutamyl Transpeptidase: in assessing liver function
• (d) Lactic Dehydrogenase: in differentiating heart attack, anemia, lung injury, or liver disease
• (e) Lipase: Damage to the pancreas
• Venoms as enzyme inhibitors:
• Snake venom is highly modified saliva that is produced by special glands of certain species of snakes.
• Snake venom is a combination of many toxins (proteins) and different enzymes, use for the purposes like increasing the prey's uptake of toxins.
• Snake venom inhibits cholinesterase to make the prey lose control of its muscles.
• Venom is an inhibitor for an essential enzyme cytochrome oxidase in the cells.
• There are three distinct type of venom that act on the body differently:
• Venom occupies the active site of the enzyme or combining with the iron which may present in the prosthetic group or which may be required as an enzyme activator.

60-Second Revision

• Proteases = proteins; examples: pepsin, trypsin, erepsin
• Lipases = lipids; example: pancreatic lipase
• Carbohydrases = carbohydrates; examples: amylase, maltase, sucrase, lactase
• Nucleases = nucleic acids; examples: RNAase, DNAase
• Diagnostic enzymes: Aldolase, Creatine Phosphokinase, Gamma-glutamyl Transpeptidase, Lactic Dehydrogenase, Lipase
• Venoms: Hemotoxic, Neurotoxic, Cytotoxic
• Warning: Proteases act on proteins; Lipases act on lipids; Carbohydrases act on carbohydrates; Nucleases act on nucleic acids. Pepsin works at pH 2; Trypsin works at pH 8. Cellulase is not produced by humans. Diagnostic enzymes are important in medicine. Venoms can inhibit multiple enzymes. The three types of venoms are hemotoxic, neurotoxic, and cytotoxic.