Life is an intricate meshwork involving a perfect coordination of a vast majority of chemical reactions. This is due to the presence of some catalysts synthesized inside the body of the organism. The term ‘enzyme’ was coined by Friedrich Wilhen Kuhne (1878) to designate these biological catalysts. The name ‘enzyme’ (en – in, zyme – yeast) literally means ‘in yeast’. The name of enzyme usually ends in – ase. Example: Cytochrome dehydrogenase. The study of enzyme is called Enzymology.
Enzymes are proteins or large biomolecules that can catalyze certain biochemical reactions for metabolic process within the cell. The substances that can speed up a chemical reaction without being permanently altered itself are called catalysts. Enzymes accelerate the rate of chemical reactions. The molecule upon which enzyme may act are called substrate and the enzyme convert the substrate into different molecules known as products. The enzyme serves as biological catalyst (Table 4.3).
Tyrosinases are syn- thesized by Agaricus bisporus, which is involved in melano-genesis (pigmentation of skin and hair).
Table 4.3: Enzyme Classification Based on Type of Chemical Reaction
• are highly substrate specific
• are reused at several times
• synthesized within the cells are determined by genes
• speed up the chemical reaction
• decrease the activation energy needed to start
• act as a biocatalyst
Proteins have four levels of structure (i) primary (sequence of amino acids), (ii) secondary (regular coils or pleats linked by peptide bonds), (iii) tertiary overall three dimensional structure of a polypeptide linked by disulphide bonds) and (iv) quatenary structure (two or more polypeptides chains). Like all proteins, enzymes are composed of one or more long chain of inter connected amino acids.
Low level of catalase plays a major role in greying process of human hair.
Enzymes are generally globular proteins that range in molecular weight from about 10,000 to several million. Each enzyme possesses a unique sequence of amino acid that causes it to fold into a characteristic three dimensional shape with a specific surface configuration. This enables it to find the correct substrate from large number of diverse molecules in the cell.
A molecule acted upon by an enzyme is called a substrate. Enzymes are specific and act on specific substrates and each enzyme catalyzes only one reaction. Enzyme consists of a protein portion, named apoenzyme and a non protein component, named cofactor (Figure 4.11).
The region of an enzyme where substrate molecules bind and undergo a chemical reaction is its active site. Each active site is specially designed in response to their substrate; as a result most enzymes have specificity and can only react with particular substances. After the formation of enzyme substrate complex (Figure 4.12), forces exerted on the substrate by the enzyme cause it to react and become the product of the intended reaction.
Example: Sucrase catalyses the hydrolysis of sucrose to glucose and fructose.
Apoenzyme is the inactive form of the enzyme which gets activated after binding with a cofactor. Coenzymes are small organic molecules that can be loosely bound to an apoenzyme and they transport chemical group from one enzyme to another.
Cofactor is a chemical compound or metallic ion that is required for enzyme activity. Example: NAD+ is derived from vitamin B. Some cofactors are metal ions including iron (Fe), copper (Cu), magnesium (Mg), manganese (Mn), Zinc (Zn), calcium (Ca) and cobalt. If the cofactor is tightly or firmly attached to the apoenzyme it is called a prosthetic group. The prosthetic group may be organic [such as vitamin, sugar, and lipid] or inorganic [such as metal ion] but is not composed of amino acids.
The complete enzyme consisting of the apoenzyme and its cofactor is called the holoenzyme
Many microbes synthesize and excrete large quantities of enzymes into the surrounding medium. Using this feature of these tiny organisms many enzymes like Amylase, Cellulase, Catalase, Protease, and Lipase are produced commercially.
Microbial enzymes are extensively used in food processing, preservation, washing powder preparation, leather industry, and paper industry and in scientific research (Table 4.4).
Table 4.4: Industrial application of microbial enzymes
Idoenella sakaiensis is a bacterium capable of breaking down PET plastics. The bacterium first uses PETase to break down the PET plastic. This has potential importance in the recycling process of PET plastics.
Lipase is used in the determination of triglyceride and blood cholesterol level. Lipase producing microorganism have been found in industrial wastes, vegetable oil processing factories, diary plants and soil contaminated with oil.
Inhibitors: An enzyme inhibitor is a molecule that binds to an enzyme and decreases its activity (Flowchart 4.1). This adverse affect of inhibitors on the rate of enzymatically catalyzed reactions are called inhibition.
An irreversible inhibitor inactivates an enzyme by binding covalently to a particular group at the active site. A reversible inhibitor inactivates an enzyme by non covalent, more easily reversible interactions. Competitive inhibitor is any compound that bears a structural resemblance to a particular substrate for binding at the active site of an enzyme. Non competitive inhibitors do not compete with the substrate for the enzyme’s active site; instead, they interact with another part of the enzyme. Uncompetitive inhibitors bind only to the enzyme substrate complex without binding to the free enzyme (Figure 4.13).
Administration of the enzyme DNase I to the lungs of cystic fibrosis patients decrease the viscosity of the mucus and the breathing is made easier.
In Feedback inhibition, the final product allosterically inhibits the enzyme that catalyses the first stage in the series of reactions. This process is used to regulate the synthesis of amino acids (Flowchart 4.2). Example: Threonine deaminase is the first enzyme in the conversion of Threonine to Isoleucine. Isoleucine inhibits Threonine deaminase through feedback inhibition.
Microbial enzymes are
• helpful to save energy and prevent pollution
• highly specific
• be immobilized and reused
• inexpensive and more stable
• easily extracted and purified
• genetically manipulated to yield higher quality