Introduction
Enzymes
They are proteins that act as a catalyst in living organisms, regulating the rate which chemical reactions proceed without it being altered.
Characteristics of enzymes
They are proteins in nature.
Speed up chemical reactions.
They’re required in minute quantities.
They are highly specific in their actions.
They are affected by temperature and pH.
Some require co-enzymes.
Some catalyze reversible reactions e.g. transferases
They can be inhibited by inhibitors
Mode of action of enzymes
An enzyme attracts substrates to its active site,catalyzes the chemical reaction by products are formed and then allows the products to dissociate i.e separate from the enzyme surface. The combination formed by an enzyme and its substrates is called the enzyme-substrate complex.
Factors affecting enzyme action
Concentration of substrate: increase in substrate concentration gradually increases the speed of enzyme reaction within the limited range of substrate concentration.
Temperature: increase in temperature increases the speed of enzyme reaction up to the optimum and then it declines. Optimum temperature for most enzymes is 40-45 degrees Celsius. Higher temperature above optimum denatures enzymes while lower temperatures inactivate enzymes.
pH: each enzyme has an optimum pH at which the speed of catalyzing is at maximum. Most enzymes of higher organisms show optimum activity around the neutral pH of 6.8-7.
Product concentration: accumulation of by-products decreases the speed of enzyme action. The byproduct combine with the active site to form a loose complex hence inhibits the enzyme.
Effect of activators: some enzymes require certain inorganic metallic cations for their optimum activity e.g magnesium, zinc, copper and sodium.
Time: the longer the enzyme will be incubated with its substrate, the higher the amount of product that will be formed. While the shorter the time of enzyme incubation with its substrate, the smaller the amount of product formed.
Transferases
These are enzymes that catalyze the transfer of functional groups from one molecule to another. This makes these enzymes ideal catalysts for polymerization reactions. In nature, they are responsible for the synthesis of many important macromolecules
Types of transferases
Transketolase
Transketolase is an important enzyme in the non-oxidative branch of the pentose phosphate pathway (PPP), a pathway responsible for generating reducing equivalents, which is essential for energy transduction and for generating ribose for nucleic acid synthesis. Transketolase also links the Pentose Phosphate Pathway to glycolysis, allowing a cell to adapt to a variety of energy needs, depending on its environment. Abnormal transketolase expression or activity have been implicated in a number of diseases where thiamin availability is low, including Wernicke-Korsakoff's Syndrome and alcoholism. Yet, the precise mechanism by which this enzyme is involved in the pathophysiology of these disorders remains controversial.
Riboflavin synthase
Riboflavin synthase is an enzyme that catalyzes the final reaction of riboflavin biosynthesis. In its working mechanism, no cofactors are needed for catalysis. Additionally, the formation of riboflavin from 6,7-dimethyl-8-ribityllumazine can occur in boiling aqueous solution in the absence riboflavin synthase.
Phosphotransferase
Phosphotransferases are a category of enzymes that catalyze phosphorylation reactions. The Phosphotransferase catalyzes uptake of certain sugars, coupling membrane transport of its substrates with their phosphorylation. In addition to its transport function, the PTS is an important component of the signaling machinery that controls chemotaxis to its sugar substrates.
The general form of the reactions they catalyze is:
A-P + B {\displaystyle \rightleftharpoons }B-P + A
Where P is a phosphate group and A and B are the donating and accepting molecules, respectively.
The phosphotransferase works in a complex system a complex translocation group system present in many bacteria where it transports sugars such as glucose, mannose, and mannitol into the cell. The first step of this reaction is phosphorylation of the substrate via phosphotransferase during transport. In the case of glucose, the product of this phosphorylation is glucose-6-phosphate (Glc-6P). Due to the negative charge of the phosphate, this Glc-6P can no longer freely leave the cell. This is the first reaction of glycolysis, which degrades the sugar to pyruvate.
Peptidyltransferases
It is a primary enzymatic function of the ribosome, which forms peptide bonds between adjacent amino acids using transfer RNAs during the translation process of protein biosynthesis. The substrates for the peptidyl transferase reaction are two tRNA molecules, one bearing the growing peptide chain and the other bearing the amino acid that will be added to the chain.
Peptidyl transferase speeds up the reaction by lowering its energy of activation. It does this by providing proper orientation for the reaction to occur. The peptidyl transferase provides proximity, meaning that it brings thing closer together, but it does not provide an alternate mechanism. Instead, it provides proper substrate orientation, increasing the probability that the existing mechanism will occur.
Kinase
A kinase is an enzyme that catalyzes the transfer of phosphate groups from high-energy, phosphate-donating molecules to specific substrates. This process is known as phosphorylation, where the high-energy ATP molecule donates a phosphate group to the substrate molecule.
It works in form of Protein kinases (PTKs) which are enzymes that regulate the biological activity of proteins by phosphorylation of specific amino acids with ATP as the source of phosphate, thereby inducing a conformational change from an inactive to an active form of the protein.
Polymerase
A polymerase is an enzyme that synthesizes long chains of polymers or nucleic acids. DNA polymerase and RNA polymerase are used to assemble DNA and RNA molecules, respectively, by copying a DNA template strand using base-pairing interactions or RNA by half ladder replication. DNA polymerase I and II are primarily required for DNA repair while DNA poly III is for the synthesis of DNA.
Transaldolase
Transaldolase is the enzyme of the non-oxidative phase of the pentose phosphate pathway that catalyzes the transfer of a dihydroxyacetone group. Unlike transketolase, which uses thiamine pyrophosphate to do the transferring, the transaldolase forms a Schiff base between the catalytic lysine residue and the substrate molecule.
Methyl transferase
Methyltransferases are enzymes that transfer methyl groups to their substrates resulting in substrate methylation.
Its function mainly in genetics is for Methylation, as well as other epigenetic modifications, affects transcription, gene stability, and parental imprinting. It directly impacts chromatin structure and can modulate gene transcription, or even completely silence or activate genes, without mutation to the gene itself. Overall, it responds to mutations in DNA, signaling to the cell to fix them or to initiate cell death so that these mutations cannot contribute to cancer.
Sulfotransferase
These are transferase enzymes that catalyze the transfer of a sulfo group from a donor molecule to an acceptor alcohol or amine. The sulfotransferases comprise cytosolic and Golgi-resident enzymes; Golgi-resident enzymes represent fertile territory for identifying pharmaceutical targets. Structure-based sequence alignments indicate that the structural fold, and the Phosphoadenosine Phosphosulphate-binding site (PAPS), are conserved between the two classes.
Acyltransferases
Acyltransferases are a special group of lipases with highly enhanced affinity for other nucleophiles than water compared to classical lipases. In the presence of a suitable acceptor, they catalyze acyltransfer reactions such as alcoholysis, aminolysis, or perhydrolysis much faster than hydrolysis even in aqueous media with a high thermodynamic activity of water. This allows kinetically controlled reactions where the donor ester is fully converted into the ester product before any significant release of free fatty acids.
References
Rehm BHA, Steinbuchel A (2013) Introduction to Biological Macromolecules. Oxford, USA.
Doi Y (2011) Microbial Polysters. Wiley, New York
Watson, James D. (2013) Molecular Biology. Upper Saddle River.
Boyce S, Tipton KF (2015) Enzyme classification and Nomenclature. Life sciences.