Introduction to Enzymes:
For an organism to sustain, several enzymatic reactions are essential to occur. The majority of chemical processes move too slowly to support life on their own. They require catalysts to speed up the rate of enzymatic reactions. Enzymes are biocatalysts that greatly accelerate the rates of these chemical reactions, without them undergoing any change. This property of enzymes leads to commercial usage of enzymes. In ancient days, enzymes were used in manufacture of foods and beverages. Enzymes are widely used in health sector presently. Some human diseases are linked with the aberrant activities of one or a few enzymes.
Enzymes are mostly proteinous except for a class of RNA modifying catalysts known as ribozymes. Ribozymes are enzymes made of ribonucleic acid that catalyze reactions on the phosphodiester bond of other RNAs.
Holoenzymes, Cofactors, and Coenzymes:
The tertiary structure and unique conformation of an enzyme are crucial to its catalytic activity.
A holoenzyme is the active form of an enzyme, composed of an apoenzyme (the protein component) together with its coenzyme.
Cofactors are an additional non-protein molecule that is needed by some enzymes to help the reaction. Tightly bound cofactors are called prosthetic groups. Cofactors that are bound and released easily are called coenzymes. Many vitamins are coenzymes.
Examples of coenzymes include NAD, FAD, and Coenzyme A. Examples of cofactors include coenzymes like NAD and FAD, and metal ions such as zinc, magnesium, and iron
The reactants that the enzyme activates are known as the substrate. Enzymes are specific to their substrates. The specificity is determined by the active site. The active site (or active centre) of an enzyme is where substrate(s) binds and participates in the catalysis.
Basic Characteristics of Enzymes:
Enzymes are mostly proteinous and they follow the physical and chemical reactions of proteins
- Enzymes are sensitive and labile to heat
- Enzymes are water soluble
Classification of Enzymes:
Currently enzymes are grouped into seven functional classes by the International Union of Biochemists and Molecular Biology (IUBMB). According to the IUBMB system, the first four numbers of each enzyme’s name are followed by EC (enzyme class). The class is represented by the first digit, the subclass by the second digit, the sub-subclass or subgroup by the third numeral, and the specific enzyme is provided by the fourth digit.
Seven major classes of enzymes are
- Oxidoreductases: Enzymes involved in oxidation-reduction reactions.
- Transferase: Enzymes that catalyze the transfer of functional groups
- Hydrolases: Enzymes that cause the hydrolysis of different substances.
- Lyases: Enzymes that catalyze the breakdown of molecules through elimination reactions, forming new double bonds or ring structures without hydrolysis or oxidation
- lsomerases: Enzymes involved in isomerization reactions.
- Ligases: Enzymes catalyzing the synthetic reactions
- Translocases: Class of enzymes (EC 7) that catalyze the passage of ions or molecules across biological membranes or inside cellular compartments
Group 1: Oxidoreductases
- Reaction carried out: Transfer of electrons
- General reaction: A⁻ + B → A + B⁻
- Example: Alcohol dehydrogenase
Group 2: Transferases
- Reaction carried out: Transfer of C-, N-, or P-containing groups
- General reaction: A–B + C → A + B–C
- Example: Hexokinase
Group 3: Hydrolases
- Reaction carried out: Bond cleavage by addition of water
- General reaction: A–B + H₂O → A–H + B–OH
- Example: Trypsin
Group 4: Lyases
- Reaction carried out: Cleavage of C–C, C–S, and C–N bonds often to form a double bond
- General reaction:
A—B A=B + C–D
| →
C D
- Example: Pyruvate decarboxylase
Group 5: Isomerases
- Reaction carried out: Formation of optical or geometric isomers
- General reaction:
A—B A—B
| → |
C D D C
- Example: Maleate isomerase
Group 6: Ligases
- Reaction carried out: Hydrolysis of high-energy phosphates to form new bonds
- General reaction: A + B → A–B
- Example: Pyruvate carboxylase
Group 7: Translocases
These enzymes are currently categorized as a new translocase EC class. Enzymes belonging to the translocase class (EC 7) accelerate the passage of ions or molecules through biological membranes or into cellular compartments without causing chemical changes to the substrates. catalyze the passage of ions or molecules through membranes or the division of those molecules inside membranes.
Example: Transporter
Enzyme Action Hypotheses:
Lock and Key Model
The classical ‘lock-and-key’ model compares an enzyme to a lock and its substrate to a key, emphasizing that only a correctly shaped substrate can fit into the enzyme’s active site. The enzyme’s selectivity for particular substrates is explained by how well the active site complements the transition states of those substrates.
Induced Fit Hypothesis
According to the Induced Fit Hypothesis/ Koshland Model, some proteins can change their shape (conformation). When a substrate combines with an enzyme, it induces a change in the enzyme’s conformation. The active site is then molded into a precise conformation making suitable for substrate to bind to initiate the reaction
Mechanism of Enzyme Action (Michaelis-Menten Model):
The enzyme action theory put forward by Michaelis and Menten is the most acceptable. According to their hypothesis, the enzyme molecule (E) first combines with a substrate molecule (S) to form an enzyme substrate (ES) complex which further dissociates to form product (P) and enzyme (E) back. The ES complex is an intermediate or transient complex and the bonds involved are weak non-covalent bonds, such as H-bonds, Van der Waals forces, hydrophobic interactions.
Normally the molecular size and shape of the substrate molecule is extremely small compared to that of an enzyme molecule. The active site, also known as the catalytic site, is the highly specific location where a substrate can attach to the enzyme molecule.
Salient Features of Active Sites:
- The tertiary structure of the protein, which produces three-dimensional native shape, is what gives rise to the active site.
- Active sites are thought of as pockets, clefts, or fissures that take up a tiny area within a large enzyme molecule.
- The active site typically has a catalytic and a substrate binding site. The latter is for the particular reaction’s catalysis.
- The catalytic site contains the coenzymes or cofactors that some enzymes require.
- Enzymes are specific in their function due to the existence of active sites.
- Amino acids commonly found in the active sites are serine, aspartate, histidine, cysteine, lysine, arginine, glutamate, tyrosine etc.