Enzymes
Enzymes are organic catalysts that catalyze biochemical reactions without being utilizing themselves
Enzymes occur in a colloidal state and are often produced in the active form called proenzymes or zymogens. These are converted to active enzymes in the presence of specific factors like pH, substrate, etc. With few exceptions, all known enzymes are proteins but all proteins are not enzymes Ribozyme, ribonuclease-P, and peptidyl transferase are three non-protein enzymes. Some nucleic acids behave like enzymes called ribozymes.
Classification and Nomenclature of Enzymes
According to the modern system of enzyme classification, introduced by the International Union of Biochemistry and Molecular Biology (IUBMB) in 1961, enzymes are classified into six classes, based on the type of reaction catalyzed> Oxidoreductases/dehydrogenases: These enzymes catalyze the oxidation-reduction reactions by transfer of electrons. These enzymes are of three types-(a) Oxidases, eg, cytochrome oxidase, (b) dehydrogenases, eg. succinate S oxidized + S'reduced dehydrogenase (c) reductases, eg, nitrate reductase.
Transferases: These enzymes catalyze the transfer of a specific group (G) other than hydrogen from one substrate (S) to another (S”). The group transfer does not occur in the n a free state
Hydrolases: These enzymes catalyze the splitting of larger molecules into smaller ones by addition of water (hydrolysis of bonds), Le, with the help of hydrogen and hydroxyl groups Usually bonds formed by dehydration are ester, ether glycosidic, C-C, C-halide, or P-N bonds are catalyzed by these enzymes e.g., amylase, sucrase, lactase
Lyases: These enzymes catalyze the breakdown of specific covalent bonds and removal of groups without hydrolysis producing double bonds or removal of double bonds by adding groups.
Isomerases: These enzymes catalyze the change of a substrate into a related isometric form by rearrangement of molecules. These enzymes too are of three types: Isomerases (change aldose to ketose group or vice-versa), Epimerases (change in position of one constituent or carbon group). Mutases (shift the position of side group like Glucose-6-phosphate Glucose-1-phosphate).
Ligases: These enzymes catalyze the joining of two substrate molecules by utilizing the energy from the hydrolysis of ATP. eg enzymes which catalyze joining of C-Q, C-S, C-N, P-0, etc. bonds
Characteristics of Enzymes
Proteinaceous nature: Enzymes are usually globular proteins except for two RNA enzymes
Efficiency: The efficiency of an enzyme depends upon the number of active sites present over an enzyme. The number of substrate molecules changed per minute by an enzyme is called the turnover number. The higher the turnover number the more efficient an enzyme.
Enzyme specificity: Enzymes are highly specific in their action. Different enzymes act on the same substrate but give rise to different products. Similarly, an enzyme may act on different substrates to produce different end products.
Chain of reactions: Enzymatic reactions are not isolated, they occur in quick succession or a team of enzymes (complex) work together to perform multistep reactions.
Colloidal nature: Enzymes are hydrophilic and form hydrosol in a free state.
Reversibility: Almost all enzymatic reactions are reversible. However, reversibility is dependent upon energy requirements, availability of reactants, the concentration of end products, and pH
Thermolabile: Enzymes are heat sensitive or thermolabile and operate between 25-35°C (optimum temperature range). They become inactive at freezing temperatures and denature at 50-55°C.
Enzyme-substrate complex: The active sites of enzymes have crevices or conformations that fit in the substrates in a complementary fashion forming a complex known as enzyme-substrate complex. The latter is changed into products. As soon as the products are released, the active sites became free to perform another catalytic act.
Chemical Reaction
The rate of a physical or chemical process refers to the amount of product formed per unit of time. It can be expressed
The rate can also be called velocity if the direction is specified Rates of physical and chemical processes are info by temperature among other factors. A general rule of thumb is that the rate doubles or decreases by half for every 1 change in either direction Catalysed reactions proceed at rates vastly higher than that of uncatalyzed ones. When e catalyzed reactions are observed, the rate would be vastly higher than the same but uncatalyzed reaction.
In the absence of an enzyme this reaction is very slow, with about 200 molecules of HCO, being formed in an hour How by using the enzyme present within the cytoplasm called carbonic anhydrase, the reaction speeds dramatically with about 600,000 molecules being formed every second. The enzyme has accelerated the reaction rate by about 10 million time.
Nature of Enzyme Action
Each enzyme [E) has a substrate (5) binding site in its molecule to produce a highly reactive enzyme-substrate complex (ES) This complex is short-lived and dissociates into its products) P and the unchanged enzyme with an intermediate formation of the enzyme-product complex (EP). The formation of the ES complex is essential for catalysis.
The catalytic cycle of enzyme action can be described as follows:
(i)First, the substrate binds to the active site of the enzyme, fitting into the active site
(ii) The binding of the substrate induces the enzyme to alter its shape, fitting more tightly around the substrate
(iii) The active site of the enzyme, now close to the substrate breaks the chemical bonds of the substrate, and the new enzyme-product complex is formed.
(iv) 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,
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 Optimum temperature for enzyme activity is 30°C-40°C in animals and 20°C-30°C in plants. High temperature (above 45°C) denatures enzymes due to degradation of linkages in its polypeptide chain whereas low temperature inactivates them due to reduction in speed of molecular movement. Most intracellular enzymes function near-neutral pH with the exception of several digestive enzymes which work either zin acidic range of pH or alkaline.
Low temperature preserves the enzyme in a temporarily inactive state whereas high temperature destroys enzymatic activity because proteins are denatured by heat. With the increase in substrate concentration, the velocity of the enzymatic reaction rises at first. The reaction ultimately reaches a maximum velocity (V) which is not exceeded by any further time in the concentration of the substrate. This is because the enzyme molecules are fewer than the substrate molecules and after saturation of these molecules, there are no free enzyme molecules to bind with the additional substrate molecule.
Michaelis-Menten equation: To determine the effect of substrate concentration in enzymatic reaction Leonor Michaelis and Maud Menten (1913) proposed a mathematical model and derived a relationship that is mathematically expressed as
Km = Michaelis - Menten constant i.e., the substrate concentration to produce half maximum velocity. V = Velocity of reaction. Vmax= Maximum velocity, [S] = Substrate concentration.
Co-factors
Co-factors are called non-protein constituents bound to the enzyme to make the enzyme catalytically active. In these instances, the protein portion of the enzymes is called the
apoenzyme. Three kinds of co-factors may be identified: prosthetic groups,co-enzymes, and metal ions.
Prosthetic groups: These are organic compounds and are distinguished from other co-factors in that they are tightly bound to the apoenzyme. For example, in peroxidase and catalase, which catalyze the breakdown of hydrogen peroxide to water and oxygen, haem is the prosthetic group and it is a part of the active site of the enzyme.
Co-enzymes: These are also organic compounds but their association with the apoenzyme is only transient, usually occurring during the course of catalysis. Furthermore, co-enzymes serve as co-factors in some different enzyme-catalyzed reactions. The essential chemical components of many coenzymes are vitamins, e.g., co-enzyme nicotinamide adenine dinucleotide (NAD), and NADP contain the vitamin niacin.
Metal ions: Several enzymes require metal ions for their activity which form coordination bonds with side chains at the active site and at the same time form one or more coordination bonds with the substrate, e.g., zinc is a co-factor for the proteolytic enzyme carboxypeptidase.
Inhibition of Enzyme Action
The activity of an enzyme is also sensitive to the presence of specific chemicals that bind to the enzyme. When the binding inhibitor forms a complex with In e enzyme at a site other than the a
active site. of the chemical shuts off enzyme activity, the process is called inhibition and the chemical is called inhibitor
Reversible inhibition is temporary and is overcome by increased concentration of substrate, dilution, and dialysis. enzyme activity, the process is called inhibition, and the chemical is called an inhibitor.
Irreversible inhibition is permanent, in which the inhibitor combines with a specific functional group of the enzyme through a covalent bond. Based on competitiveness enzyme inhibition can be divided into - competitive inhibition, non-competitive inhibition, and.
Competitive inhibition: Reversible occurs due to substrate or increase enzyme analogue.
Non-competitive inhibition: Inhibitor forms a complex with the enzyme at a site other than the
active site.
Allosteric inhibition: Inhibitor binds to the enzyme at a specific site other than the active site and changes the structure of the active site to affect substrate binding.
Allosteric modulation or feedback inhibition is an enzyme regulatory mechanism where a product or intermediates of a reaction functions as a temporary allosteric inhibitor, (which combines with a regulatory site) if its concentration crosses the threshold value. The end product inhibitor functions as a negative modulator and the enzyme inactivated are called an allosteric enzyme. Michaelis- Menten or K constant is not applicable in allosteric enzymes.