Enzyme Types, Properties, and Action

Enzymes are biological catalysts that accelerate biochemical reactions in living organisms without being consumed in the process. They are crucial for vital processes such as digestion, respiration, metabolism, and DNA replication.

Introduction

• History - Buchner discovered and isolated the enzyme zymase from yeast cells
• While Kuhne coined the term enzyme.
• J.B. Sumner purified and crystallised the urease enzyme from the Canavalia/Jack bean/Lobia plant.

Characteristics of Enzymes

• Almost all enzymes are protein. Though, there are some nucleic acids that behave like enzymes. These are called ribozymes .
• Enzymes are colloidal substances, which are macromolecules of amino acids and are synthesised by ribosomes under genetic control.

Enzyme can be depicted by a line diagram. An enzyme like any protein has –

1. Primary structure: Amino acid sequence of the protein. It lacks active sites.
2. Secondary structure: It is a helical structure which also lack active sites.
3. Tertiary structure: In this structure backbone of the protein chain folds upon itself, the chain criss-crosses itself and hence many crevices or pockets are made such pockets represent active sites. The catalytic structure of most of the enzymes is tertiary and globular.
4. Quaternary structure: Represented by isoenzymes and active sites are present.

• Active site: An active site of an enzyme is a crevice or pocket into which the substrate fits. Thus, enzymes through their active site catalyse reactions at a high rate.
• Enzymes are very specific to their substrate or reactions. They are required in very small amount to catalyse a reaction.

Catalytic power of an enzyme depends upon –

(a) Turnover number - It is the number of substrate molecules converted into products per unit time by a molecule of enzyme. Thus, catalytic power is directly proportional to turnover number. Carbonic anhydrase enzyme is considered the fastest enzyme.

(b) Km constant - Michaelis and Menten coined this. It is the concentration of substrate at which the rate of reaction attains half of its maximum velocity.

$K_m=\frac{1}{2}{V}_{\text{max}}$

• Enzymes increase the reaction rate several times by lowering the activation energy.
• The catalytic power of an enzyme remains the same even outside the living system.
• Enzymes, when not in use, exist in an inactive form called a zymogen or proenzyme. Pepsinogen is an inactive form of pepsin; similarly, trypsinogen is an inactive form of trypsin.

Activation Energy 

• The difference in average energy content of Substrate ‘S’ from that of this transition state is called activation energy.
• Enzymes eventually bring down this energy barrier, making the transition of ‘S’ to ‘P’ easier.
• All changes (ES complex, EP complex) that occurred during the transition state are transient and unstable.

Nature of Enzyme Action

• Each enzyme(E) has a substrate (S) binding site in its molecular structure so that a highly reactive enzyme-substrate complex (ES) is produced. 
• This complex is short-lived and dissociates into its product(s), P, and the unchanged enzyme, with the enzyme-product complex (EP) forming as an intermediate. 
• The formation of the ES complex is essential for catalysis.

Enzyme+ Substrate ⥦ ESC→EPC→Enzyme+ Product

1.0Mechanism of Enzyme Action

The catalytic cycle of an enzyme action can be described in the following steps:

1. First, the substrate binds to the active site of the enzyme, fitting into the active site.

2. The binding of the substrate induces the enzyme to alter its shape, fitting more tightly around the substrate.

3. The active site of the enzyme, now in close proximity of the substrate breaks or form the chemical bonds of the substrate and the new enzyme- product complex is formed.

4. The enzyme releases the products of the reaction and the free enzyme is ready to bind to another molecule of the substrate and run through the catalytic cycle once again.

2.0Types of Enzymes

The International Union of Biochemistry and Molecular Biology (IUBMB) has classified enzymes into six major classes based on the type of reaction they catalyse.

(i) Oxidoreductases/Dehydrogenases: Enzymes which catalyse oxidoreduction (oxidation-reduction) between two substrates, i.e. S and S,' e.g. Cytochrome c oxidase, Dehydrogenase, etc.

S reduced + S' oxidised ⎯→ S oxidised + S' reduced

(ii) Transferases: Enzymes catalysing a transfer of a group, G (other than hydrogen), between a pair of substrates S and S'. e.g. Transaminase, Hexokinase, etc.

S – G + S' ⎯→ S + S ' –G

(iii) Hydrolases: Enzymes catalysing the hydrolysis of ester, ether, peptide, glycosidic, C–C, C–halide or P–N bonds with the addition of water. e.g. Proteases, Lipases, Carbohydrases, etc.

(iv) Lyases: Enzymes that catalyse the removal of a group from substrate by mechanisms other than hydrolysis and leaving double bonds. e.g. Aldolase.

(v) Isomerases: Includes all enzymes catalysing the interconversion of optical, geometric or positional isomers. e.g. Hexoisomerase, Mutase, etc.

(VI) Ligases/synthase: Enzymes catalysing the linking together of two compounds. Such enzymes

catalyse the joining of C–O, C–S, C–N, P–O, etc. bonds. e.g. Citrate synthase, DNA ligase, etc.

Factors Affecting Enzyme Activity

The activity of an enzyme can be affected by a change in the conditions which can alter the tertiary

structure of the protein. These include :

Temperature 

• Enzymes generally function in a narrow range of temperature.
• Each enzyme shows its highest activity at a particular temperature called the optimum temperature. 
• Activity declines both below and above the optimum value.  
• Low temperature preserves the enzyme in a temporarily inactive state, whereas high temperature destroys enzymatic activity because proteins are denatured by heat.
• A general rule of thumb is that the rate doubles or halves for every 10°C change in either direction. Thus, the Q10 value for enzymatic activities is 2.

pH

• Enzymes generally function over a very narrow pH range. 
• Each enzyme shows its highest activity at a particular pH called the optimum pH.
•  Activity declines both below and above the optimum value.

Change in substrate concentration

• With increasing substrate concentration, the velocity of the enzymatic reaction initially increases. 
• The reaction ultimately reaches a maximum velocity (Vmax). Any further rise in the concentration of the substrate does not exceed this velocity. 
• Reason: The enzyme molecules are fewer than the substrate molecules, and after saturation of these enzyme molecules, there are no free enzyme molecules available to bind with the additional substrate molecules.

Inhibitor

• The activity of an enzyme is also sensitive to the presence of specific chemicals that bind to the enzyme. 
• When the binding of the chemical shuts off (inhibits) enzyme activity, the process is called inhibition and the chemical is called an inhibitor.