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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).
Table 4.3: Enzyme Classification Based on Type of Chemical Reaction
Characteristics of Enzymes
- 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
Structure of Enzymes
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: Industrail application of microbial enzymes
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)
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.
Uses of Microbial Enzymes
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
Biochemical pathway that functions in both anabolism and catabolism are called amphibolic pathways, meaning that they are dual purpose.
The energy of catabolic reactions is used to drive anabolic reactions. The energy for chemical reactions is stored in ATP. The chemical reactions are catalyzed by different enzymes. Enzymes catalyze chemical reactions by lowering the activation energy.
Most of the cells energy is produced from the oxidation of carbohydrates. During respiration organic molecules are oxidized. Energy is generated from the ETC. In aerobic respiration, O2 function as the final election acceptor. In anacrobic respiration, the final electron acceptor is an inorganic molecule NO2-, SO42- other than O2.
Complete oxidation of glucose molecule takes place in 3 sequential reactions.
- Glycolysis occurring in cytoplasm
- Krebs cycle occurring is mito chondrial matrix
ETC (Oxidative Phosphorylation) occurring is inner mitochondrial matrix. In aerobic prokaryotes, 38 ATP molecules can be produced from complete oxidation of a glucose molecule in glycolyins, krebscycle, and ETC. In eucaryotes 36 ATP molecules are produced from complete oxidation of a glucose molecule. In incomplete oxidation of glucose molecules will revolt in fermentation, O2 in anaerobic condition.
Various commercial products are produced from pyruvic acid. Lipid can be catabolised by lipase which hydrolyze lipid into glycerol and fatty acid. Then fetly acids are catabolised by Beta oxidation. Proteins can be catabolised by Deamination and Transamination process into amino acids.
Carbohydrate, Fat, Protons can all be the source of electrons and protons for respiration. Microbial enzymes are extensively used in food processing, preservation, paper industry and in scientific research.