Enzymes, Classification, Enzyme Activity, Enzyme Specificity and Factors Affecting Enzyme Activity
Enzymes
Enzymes are proteins that function as biological catalysts which catalyse the biochemical reactions both in vivo as well as in vitro, accelerating the reactions that are crucial for life processes without being consumed in the process.
These are highly specific to substrates and have great catalytic power, i.e., they enhance the rate of reaction tremendously without being changed. All enzymes are proteins with exception of some small group of catalytic RNA molecules called ribozymes.
Like proteins, the molecular weight of enzymes ranges from about 2000 to more than one million Dalton. Enzymatic activity of proteinaceous enzymes may be affected depending on the conformational structure as well as its denaturation. There are many enzymes which require cofactors for their catalytic activity. The cofactor may be a complex organic molecule called coenzyme (Table:1) or it may be a metal ion such as Fe2+, Mn2+, Zn2+, Mg2+ (Table:2). An enzyme plus its cofactor is called holoenzyme. In such cases, the protein component in cofactor requiring enzyme is called apoenzyme.
Coenzymes take part in catalysis transiently and are carriers of specific functional groups. Most of the coenzymes are derived from vitamins (organic nutrients required in small amounts in diet).
| Table:1: Some coenzymes and their precursor vitamins and their role | ||
|---|---|---|
| Coenzyme | Precursor Vitamin | Role in Catalytic Reaction |
| Biocytin | Biotin (vitamin B7) | Transfer of CO₂ |
| Coenzyme B12 (5'-adenosylcobalamin) | Vitamin B12 | Transfer of an alkyl group |
| Flavin adenine dinucleotide (FAD) | Riboflavin (vitamin B2) | Transfer of electrons |
| Coenzyme A | Pantothenic acid (vitamin B5) | Transfer of acyl and alkyl group |
| Nicotinamide adenine dinucleotide (NAD) | Niacin (vitamin B3) | Transfer of hydride (:H⁻) |
| Pyridoxal phosphate | Pyridoxine (vitamin B6) | Transfer of amino group |
| Thiamine pyrophosphate | Thiamine (vitamin B1) | Transfer of aldehydes |
| Tetrahydrofolate | Folic acid (vitamin B9) | Transfer of one carbon group |
| Table:2: Metal ions that serve as cofactors for enzymes | ||
|---|---|---|
| Metal Ion | Enzyme Name | |
| Fe2+ or Fe3+ | Catalase, Peroxidase, Cytochrome oxidase | |
| Cu2+ | Cytochrome oxidase | |
| Mg2+ | DNA polymerase | |
| Mn2+ | Arginase | |
| K+ | Pyruvate kinase | |
| Mo2+ | Nitrogenase, Nitrate reductase | |
| Zn2+ | Carbonic anhydrase, Alcohol dehydrogenase | |
| Ni2+ | Urease | |
When a coenzyme or metal ion is tightly bound through covalent bond with the enzyme protein, it is called a prosthetic group.
Classification of Enzymes
According to International Union of Biochemistry (I.U.B.), all enzymes are classified into 6 major classes, but recently, another class of enzymes namely translocase has been added (Table:3).
| Table:3: Classification of enzymes adopted by I.U.B. | ||
|---|---|---|
| Class No. | Class Name | Type of Reaction Catalyzed |
| 1 | Oxidoreductases | Oxidation-reduction reactions (transfer of electrons) |
| 2 | Transferases | Transfer of groups |
| 3 | Hydrolases | Hydrolytic reactions (transfer of functional groups to water) |
| 4 | Lyases | Addition or removal of groups to form double bonds |
| 5 | Isomerases | Transfer of groups within molecules to yield isomeric forms |
| 6 | Ligases | Condensation of two molecules coupled through ATP hydrolysis |
| 7 | Translocases | Transfer of ion/molecules across the membrane |
Enzyme Specificity
The enzymes are highly specific in action. In fact, the properties that make enzymes such a strong catalyst are their specificity of substrate binding and their ideal arrangement of catalytic groups. Various types of enzyme specificity are: group specificity, absolute specificity, stereospecificity, and geometrical specificity. When enzymes act on several different closely related substrates then it is called group specificity. When enzymes act only on one particular substrate, it is called absolute specificity.
Stereochemical or optical specificity occurs when substrate exists in two stereochemical forms (chemically identical but different arrangement of atoms in three-dimensional space) then only one of the isomers will undergo reaction by particular enzyme. For example, D-amino acid oxidase catalyses oxidation of the D-amino acids to keto acids. In geometrical specificity, enzymes are specific towards cis and trans forms. For example, fumarase catalyses the interconversion of fumarate and malate.
Enzyme Activity
The rate at which an enzyme catalyzes a chemical reaction is called enzyme activity. Enzyme activity is typically measured by assaying how fast a substrate is converted to a product under defined conditions.
Factors Affecting Enzyme Activity
Rate of enzyme catalysed reactions is influenced by changing the environmental conditions. The important factors that influence the velocity of enzyme catalysed reactions are temperature, pH, substrate concentration, Enzyme concentration and modulators.
Temperature
The rate of an enzyme catalysed reaction increases with the increase in temperature up to a maximum and then falls. When a graph is plotted between temperature versus enzyme activity, a bell-shaped curve is obtained (Shown in Figure). The temperature at which the maximum rate of reaction occurs is called the enzyme’s optimum temperature.
The optimum temperature is different for different enzymes; but for most of the enzymes it is between 40-45°C. Majority of enzymes in the human body have an optimum temperature of around 37°C (98.6°F) and are denatured or degraded at extreme temperatures. However, few enzymes like Taq DNA polymerase present in thermophilic bacteria, Thermus aquaticus, venom phosphokinase and muscle adenylate kinase are active even at 100°C.
Hydrogen Ion Concentration (pH)
Enzyme activity is also affected by pH. A plot of enzyme activity against pH results in a bell shaped curve (Shown in Figure). Each enzyme has its unique optimum pH at which the rate of reaction is greatest. The optimum pH is the pH at which the activity of a particular enzyme is at maximum. Many enzymes of higher organisms show optimum reaction rate around neutral pH (pH 6-8).
However, there are several exceptions such as pepsin (pH 1-2), acid phosphatases (pH 4-5) and alkaline phosphatases (pH 10-11). Below and above the optimum pH, the enzyme activity is much lowered and at extreme pH, the enzyme becomes totally inactive.
Substrate Concentration
The substrate concentration also influences enzyme activity. As the substrate concentration increases the rate of reaction also increases. This is because the more substrate molecules will interact with enzyme molecules, the more products will be formed.
However, after a certain concentration, further increase in substrate concentration will have no effect on the rate of reaction, since the substrate concentration will no longer be the limiting factor (Shown in Figure). At this stage, enzyme molecules become saturated and work at their maximum possible rate.
Enzyme Concentration
At a constant substrate concentration, the velocity of an enzyme catalyzed reaction increases proportionately with the increase in the concentration of the enzyme. This property is utilized in determining the level of serum enzymes for the diagnosis of diseases. On plotting the velocity of the enzymatic reaction with the enzyme concentration, a straight line is obtained.
Sources: NCERT