Microbial Enzymes and Their Role in Metabolism
Enzymes as biological catalysts are the key proteins of cellular metabolism. Usually they have a complex protein nature, and demonstrate relatively strict catalytic specificity .
In large parts enzymes predetermine the total behaviour and properties of bacterial cell. They catalyze (speed up) all the scope of cellular chemical reactions. In the absence of an enzyme, the substrate is transformed into a reaction product so slowly that it is even less likely to measure the product’s formation. By contrast, the enzyme converts the substrate into a product in a short time.
Enzyme action leads to chemical changing of substrate molecules. It results from stabilization of substrate transition state by weak but specific binding forces within enzyme-substrate complex that accelerates substrate transformation. Enzyme-substrate binding stresses chemical bonds of the substrate enough to break them down with subsequent formation of new bonds.
Enzymes act in two steps. Initially the substrate binds to a specific location of the enzyme, known as active or catalytic site, to create an enzyme-substrate complex, according to “lock-and-key” or “induced fit” mechanisms. After chemical transformation the products of the reaction are released, making the enzyme active site free to bind to new substrate molecules.
International classification divides enzymes into 6 major classes:
- Hydrolases, which catalyse the cleavage of the links between the carbon, oxygen, nitrogen or sulphur atoms in watery solutions with addition of one molecule of water (esterases, proteases, glycosidases, nucleases, etc.).
- Transferases perform the transfer of certain groups and residues from one molecule to another – intermolecular transfer (aminotransferases, transacylases, transglycosidases, etc.).
- Oxidoreductases catalyse the reactions of oxidation and reduction, resulting in electron transfer from the reductant (electron donor) to the oxidant or electron acceptor (oxidases, oxygenases dehydrogenases, catalases and others).
- Isomerases create substrate isomers resulting from intramolecular substrate rearrangement. They play an important role in bacterial carbohydrate metabolism (phosphohexoisomerase, phosphoglucomutase, racemases, etc.). Also they may cause conformational substrate changing.
- Lyases cleave the bonds in molecules via non-hydrolytic manner with intramolecular double bond formation (e.g., bacterial hyaluronate lyase).
- Ligases or synthetases, which catalyze synthetic reactions, thereby coupling molecules together (DNA ligase, acetyl-CoA synthetase, chelatases, etc.)
Some enzymes are excreted by bacteria into external environment (exoenzymes) to participate in extracellular digestion of nutrients or toxicants, whereas other enzymes work inside the microbial cell (endoenzymes).
The constitutive enzymes of bacteria provide their basic metabolic reactions, being constantly expressed regardless of presence or absence of substrate. This group comprises the essential enzymes of cellular metabolism (ATP synthase, nucleases, proteases, oxidases, lipases, etc.) The synthesis of adaptive enzymes commences only after the appearance of corresponding substrate (beta-galactosidase, alkaline phosphatase, penicillinase, and many other adaptive enzymes).
An important role is played in bacteria by aggression and invasion enzymes, which facilitate microbial spread in the affected host, destroying body tissues (hyaluronidase, collagenase, etc.) or blocking the action of antimicrobial drugs (beta-lactamases for beta-lactam antibiotics), or rendering toxic effects (phospholipase C of C. perfringens or zinc endopeptidase of tetanospasmin toxin of C. tetani).
Overall, there is a tremendous variety of enzymes produced by microorganisms that demonstrate extremely high efficacy. Enzymes of microbial origin are broadly used in industry and agriculture, biotechnology and medicine.
For instance, microbial enzymes (e.g., restriction endonucleases, polymerases, proteases, and many others) are actively used in genetic engineering, antibiotic synthesis, production of polysaccharides and alkaloids, synthesis of steroid hormones and other valuable substances. Enzymes as biological products for medicine have an expanding field of clinical applications including the treatment of certain diseases (e.g., administration of streptococcal streptokinase in myocardial infarction).