The Ultimate Guide to Enzymes - Part 2
Advanced Classification, Structural Types, and Biological Roles
B. Carbohydrate Hydrolyzing Enzymes (Carbohydrases)
Carbohydrases are a type of enzyme that catalyze the hydrolysis of carbohydrates into simpler sugars.
There are several different types of carbohydrases, including:
- Amylases: These enzymes hydrolyze starch and glycogen into glucose.
- Lactases: These enzymes hydrolyze lactose into glucose and galactose.
- Sucrase: This enzyme hydrolyzes sucrose into glucose and fructose.
- Maltases: These enzymes hydrolyze maltose into glucose.
- Isomaltases: These enzymes hydrolyze isomaltose into glucose.
- Glucoamylases: These enzymes hydrolyze glucosyl units from the non-reducing ends of oligosaccharides and polysaccharides.
C. Lipids Hydrolyzing Enzymes
Lipids are a diverse group of biomolecules that includes fats, oils, waxes, and other related compounds. There are several enzymes that can hydrolyze lipids, breaking them down into simpler molecules.
a. Lipases
Lipases are a type of enzyme that catalyze the hydrolysis of lipids, such as triglycerides, into fatty acids and glycerol.
There are several different types of lipases, including:
- Pancreatic lipases: These enzymes are produced by the pancreas and are responsible for the digestion of fats in the small intestine.
- Hormone-sensitive lipases: These enzymes are activated by hormones such as adrenaline and are involved in the breakdown of fats in adipose tissue.
- Lipoprotein lipases: These enzymes hydrolyze lipids in lipoproteins, such as very low-density lipoproteins (VLDL) and chylomicrons.
- Lecithin-cholesterol acyltransferases (LCAT): These enzymes transfer fatty acids from lecithin to cholesterol, forming cholesterol esters.
- Lipase A: This enzyme hydrolyzes lipids in the outer membrane of bacteria.
- Lipase B: This enzyme hydrolyzes lipids in the inner membrane of bacteria.
b. Cholesterases
Cholesterases are enzymes that catalyze the hydrolysis of cholesterol and related compounds.
There are several different types of cholesterases, including:
- Cholesteryl ester hydrolases: These enzymes hydrolyze cholesteryl esters, such as cholesterol esters, into free cholesterol and a fatty acid.
- HMG-CoA reductases: These enzymes catalyze the reduction of 3-hydroxy-3-methylglutaryl-CoA (HMG-CoA) to mevalonate, a key step in the synthesis of cholesterol.
- LDL receptors: These receptors bind to low-density lipoproteins (LDL) and internalize cholesterol, leading to its degradation.
- ACAT (acyl-CoA:cholesterol acyltransferase): These enzymes transfer an acyl group from an acyl-CoA molecule to cholesterol, forming a cholesteryl ester.
- CETP (cholesteryl ester transfer protein): This protein transfers cholesteryl esters from high-density lipoproteins (HDL) to other lipoproteins, such as VLDL and LDL.
- SR-BI (scavenger receptor class B type I): This receptor binds to HDL and mediates the uptake of cholesterol by cells.
c. Phospholipases (Previously called Lecithinases)
Phospholipases are a type of enzyme that catalyze the hydrolysis of phospholipids, such as lecithin, into fatty acids, glycerol, and a phosphate group.
There are several different types of phospholipases, including:
- Phospholipase A1: This enzyme hydrolyzes the sn-1 position of phospholipids, producing a fatty acid and lysophospholipid.
- Phospholipase A2: This enzyme hydrolyzes the sn-2 position of phospholipids, producing a fatty acid and lysophospholipid.
- Phospholipase B: This enzyme hydrolyzes the sn-1,2 positions of phospholipids, producing diacylglycerol and a phosphate group.
- Phospholipase C: This enzyme hydrolyzes the sn-3 position of phospholipids, producing diacylglycerol and a phosphate group.
- Phospholipase D: This enzyme hydrolyzes the sn-1 position of phospholipids, producing phosphatidic acid and a fatty alcohol.
- Phospholipase E: This enzyme hydrolyzes the sn-2 position of phospholipids, producing a lysophospholipid and a fatty alcohol.
D. Deaminases or Aminohydrolases
Deaminases are a type of enzyme that catalyze the removal of an amino group from a molecule.
There are several different types of deaminases, including:
- Adenine deaminases: These enzymes remove the amino group from adenine, producing hypoxanthine.
- Cytosine deaminases: These enzymes remove the amino group from cytosine, producing uracil.
- Histidine deaminases: These enzymes remove the amino group from histidine, producing urocanic acid.
- Xanthine deaminases: These enzymes remove the amino group from xanthine, producing hypoxanthine.
- Tyrosine deaminases: These enzymes remove the amino group from tyrosine, producing p-cresol.
- Glutamine deaminases: These enzymes remove the amino group from glutamine, producing glutamate.
E. Deamidases or Amidohydrolases
Deamidases are enzymes that catalyze the removal of an amide group from a molecule. They are involved in a wide range of important biochemical reactions, including the metabolism of proteins and the synthesis of compounds such as nucleotides and purines.
There are several different types of deamidases, including:
- Glutaminyl cyclase: This enzyme removes an amide group from the side chain of glutamine, producing an imine group and ammonia.
- Peptide deformylase: This enzyme removes an amide group from the N-terminal of a peptide, producing a formyl group and the amino acid N-formylmethionine.
- Carboxypeptidase E: This enzyme removes an amide group from the C-terminal of a peptide, producing a carboxyl group and the amino acid C-terminal valine.
- Glutamate carboxypeptidase: This enzyme removes an amide group from the C-terminal of a peptide, producing a carboxyl group and the amino acid C-terminal glutamate.
- Dipeptidyl peptidase IV: This enzyme removes an amide group from the side chain of dipeptides, producing two individual amino acids.
- Asparagine deamidase: This enzyme removes an amide group from asparagine, producing aspartate.
F. Other Hydrolyzing Enzymes
There are two groups under this category
i. Phosphatases
Phosphatases are enzymes that catalyze the hydrolysis of phosphate esters, such as ATP, into a free phosphate group and the molecule to which it was attached.
They are of the following types:
- Phosphomonoesterases: Split one phosphate group of a monoester. E.g Acid Phosphatases and alkaline phosphatases i.e. (a). Acid phosphatases: These enzymes hydrolyze phosphate esters at acid pH. (b). Alkaline phosphatases: These enzymes hydrolyze phosphate esters at alkaline pH.
- Phosphodiesterases: Split off one phosphate group from diesters.
- Phosphorylase: Add the phosphate group into the substrate e.g. Glycogen Phosphorylase.
- Pyrophosphatases: Remove pyrophosphate (two phosphatase) from the substrate e.g. ATPase.
- Nucleases or Polynucleotidase: Decompose nucleic acids (DNA and RNA).
- Nucleotidase: Hydrolyze mononucleotides to nucleosides.
ii. Miscellaneous
- Cholinesterases: Hydrolyze acetylcholine and other related substrate.
- Sulfatases: Hydrolyze sulfate esters.
4. Ligases
Ligases are enzymes that catalyze the formation of a covalent bond between two molecules by transferring a group from one molecule to the other. They are involved in biochemical reactions, including the synthesis of nucleic acids and proteins, the repair of DNA, and the metabolism of carbohydrates and lipids.
There are several different types of ligases, including:
- Nucleotidyl transferases: These enzymes transfer nucleotides to a nucleic acid template, synthesizing DNA or RNA.
- Adenylate cyclases: These enzymes transfer a phosphate group from ATP to an acceptor molecule, forming cyclic AMP (cAMP).
- Phosphotransferases: These enzymes transfer a phosphate group from a nucleoside triphosphate to an acceptor molecule.
- Transferases: These enzymes transfer a group, such as an amino group or a sugar residue, from one molecule to another.
- Synthetases: These enzymes synthesize a molecule, such as a protein or a nucleic acid, by joining smaller units together.
5. Lyases
Lyases are enzymes that catalyze the cleavage of a chemical bond by a mechanism other than hydrolysis or oxidation-reduction.
There are several different types of lyases, including:
- Aldolases: These enzymes cleave aldoses or ketoses into two molecules by removing a carbon atom from the middle of the molecule.
- Decarboxylases: These enzymes remove a carboxyl group from a molecule, producing a compound with one less carbon atom.
- Desulfurases: These enzymes remove sulfur from a molecule, producing a compound with one less sulfur atom.
- Deaminases: These enzymes remove an amino group from a molecule, producing a compound with one less nitrogen atom.
- Dehydratases: These enzymes remove water from a molecule, producing a compound with one fewer oxygen and hydrogen atoms.
6. Isomerases
Isomerases are enzymes that catalyze the conversion of one isomer into another. Isomers are molecules that have the same molecular formula, but different structural arrangements of their atoms.
There are several different types of isomerases, including:
- Epimerases: These enzymes convert one epimer into another. Epimers are isomers that differ in the configuration of one or more asymmetric carbon atoms.
- Esterases: These enzymes catalyze the conversion of an ester into another isomer. Esters are compounds that are formed from the reaction of a carboxylic acid and an alcohol.
- Racemases: These enzymes catalyze the conversion of a racemic mixture into a pure enantiomer. A racemic mixture is a mixture of equal amounts of two enantiomers, which are isomers that are mirror images of each other.
- Dehydratases: These enzymes remove water from a molecule, producing a compound with one fewer oxygen and hydrogen atoms.
- Epoxidases: These enzymes catalyze the conversion of an epoxide into another isomer. An epoxide is a cyclic compound with three atoms in a ring, one of which is oxygen.
Enzymes Classification | On the basis of their structures
Enzymes can be classified into two main types based on their structure:
A. Simple enzymes
Simple enzymes are made up of a single polypeptide chain and do not have any prosthetic groups (non-protein components that are essential for enzyme function).
Here are a few examples of simple enzymes:
- Lysozyme: Lysozyme is an enzyme that breaks down the cell walls of bacteria. It is found in the tears, saliva, and mucus of humans and other animals, and helps to protect against infections.
- Chymotrypsin: Chymotrypsin is an enzyme that breaks down proteins into smaller peptides. It is found in the pancreas and small intestine, and is activated by the presence of trypsinogen.
- Pepsin: Pepsin is an enzyme that breaks down proteins into smaller peptides. It is found in the stomach and is activated by the presence of hydrochloric acid.
- Carbonic anhydrase: Carbonic anhydrase is an enzyme that catalyzes the reversible reaction between carbon dioxide and water to form bicarbonate and protons. It is found in various tissues and organs, including the pancreas and red blood cells.
B. Complex enzymes
Complex enzymes are made up of two or more polypeptide chains, and may also have one or more prosthetic groups. The polypeptide chains may be held together by covalent bonds or non-covalent interactions.
Here are a few examples of complex enzymes:
- Cytochrome c oxidase: Cytochrome c oxidase is an enzyme that catalyzes the reduction of oxygen to water during cellular respiration. It is made up of 13 subunits, including heme and copper prosthetic groups, and is found in the mitochondria of cells.
- Fumarase: Fumarase is an enzyme that catalyzes the reversible reaction between fumarate and water to form malate. It is made up of four subunits and is found in the mitochondria and cytoplasm of cells.
- Ribonucleotide reductase: Ribonucleotide reductase is an enzyme that synthesizes deoxyribonucleotides, which are needed for DNA synthesis. It is made up of two subunits and is found in the nuclei of cells.
- Pyruvate dehydrogenase: Pyruvate dehydrogenase is an enzyme that converts pyruvate to acetyl-CoA, a key step in the production of energy in cells. It is made up of three subunits and is found in the mitochondria of cells.
Role of Enzymes in Human Body
Some examples of the roles that enzymes play in the human body include:
1. In Digestion
The digestion of food begins in the mouth, where saliva containing the enzyme amylase begins to break down carbohydrates. As the food moves through the digestive system, it is acted upon by various enzymes that help to break it down into smaller, more easily absorbed molecules.
There are several types of enzymes that are involved in the digestion of different types of nutrients:
- Proteases: These enzymes break down proteins into smaller peptides and amino acids. Examples include trypsin, chymotrypsin, and pepsin.
- Lipases: These enzymes break down lipids (fats) into fatty acids and glycerol. Examples include pancreatic lipase and gastric lipase.
- Amylases: These enzymes break down carbohydrates (such as starch) into simpler sugars like glucose. Examples include pancreatic amylase and salivary amylase.
- Lactases: These enzymes break down lactose (a sugar found in milk) into glucose and galactose.
2. Energy production
Enzymes such as ATP synthase and pyruvate dehydrogenase play a key role in the production of ATP, the primary source of energy for the body.
One of the main ways that enzymes help to produce energy is through the process of cellular respiration, which occurs in the mitochondria of cells. During cellular respiration, glucose and other nutrients are broken down into energy-rich molecules of ATP (adenosine triphosphate) through a series of chemical reactions.
Some of the enzymes involved in energy production include:
- Pyruvate dehydrogenase: This enzyme converts pyruvate (a product of glucose metabolism) into acetyl-CoA, which is then used in the production of ATP.
- Alpha-ketoglutarate dehydrogenase: This enzyme converts alpha-ketoglutarate (a product of amino acid metabolism) into succinyl-CoA, which is then used in the production of ATP.
- Fumarase: This enzyme converts fumarate into malate, which is then used in the production of ATP.
- ATP synthase: This enzyme converts ADP (adenosine diphosphate) into ATP by adding a phosphate group.
3. Detoxification
Some of the enzymes involved in detoxification include:
- Cytochrome P450: This enzyme family is involved in the metabolism of drugs and toxins, including alcohol, caffeine, and certain pesticides.
- Alcohol dehydrogenase: This enzyme breaks down alcohol into acetaldehyde, which is then further broken down into acetate.
- Glutathione S-transferases: These enzymes help to detoxify harmful substances by conjugating them to glutathione, a small molecule that can be excreted in the urine.
- Lactate dehydrogenase: This enzyme helps to break down lactate, a product of the metabolism of certain toxins, into pyruvate.
4. Hormone regulation
Hormones are chemical signaling molecules that are produced by glands and released into the bloodstream, where they act on specific target cells or tissues. Enzymes help to control the levels and activity of hormones by synthesizing them, breaking them down, or modifying them in some way.
Some of the enzymes involved in hormone regulation include:
- Kinases: These enzymes add a phosphate group to a hormone or other protein, activating or inactivating it.
- Phosphatases: These enzymes remove a phosphate group from a hormone or other protein, activating or inactivating it.
- Aromatases: These enzymes convert androgens (male hormones) into estrogens (female hormones).
- 5'-Deiodinases: These enzymes convert thyroid hormones into their active form.
- Peptidases: These enzymes break down peptide hormones into smaller peptides or amino acids.
5. Blood clotting
Enzymes such as thrombin and fibrinogen help to form blood clots to stop bleeding. Blood clotting is a complex process that involves the activation and interaction of several proteins and enzymes.
Some of the enzymes involved in blood clotting include:
- Thrombin: This enzyme converts fibrinogen (a protein in the blood) into fibrin, a long, fibrous protein that helps to form a blood clot.
- Factor XIII: This enzyme helps to crosslink the fibrin fibers, strengthening the blood clot.
- Plasmin: This enzyme breaks down fibrin and other blood clotting factors, dissolving the blood clot once it is no longer needed.
6. DNA Replication
Enzymes play a crucial role in the process of DNA replication, which is the process by which cells create copies of their genetic material. DNA replication is necessary for the reproduction of cells and the maintenance of genetic information.
Some of the enzymes involved in DNA replication include:
- DNA polymerase: This enzyme synthesizes a new DNA strand by adding nucleotides to the template strand.
- Primase: This enzyme synthesizes a short RNA primer, which provides a starting point for DNA polymerase to begin synthesizing the new DNA strand.
- Helicase: This enzyme unwinds the DNA double helix, separating the two strands.
- Topoisomerase: This enzyme helps to relieve the strain that is generated during DNA replication by making cuts in the DNA strands and rejoining them.
- Ligase: This enzyme seals gaps in the newly synthesized DNA strands.
Enzymes are essential for the proper replication of DNA, and a deficiency or dysfunction of certain enzymes can lead to problems with DNA replication and the maintenance of genetic information.
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