Enzymes Explained | Mechanism, functions and factors affecting them

Enzymes are proteins specialized to catalyse specific metabolic reactions (speeds up the reaction). They are categorised into two i.e. Intracellular enzymes are used in the cells which synthesise them while extracellular enzymes are produced by other cells and are moved to other parts of the body (eg digestive enzymes). Zymogens are enzymes which are secreted in inactive form and ultimately activated by an agent secreted by other cells eg trysinogen which is activated by enterokinase to give active trypsin. Zymogens secretion may be protective mechanism against membrane lysis by these enzymes.  

Zymases are extracellular enzymes, which are secreted ready for action eg amylase.

Mechanisms of enzyme action

Enzymes speed up the rate of chemical reactions because they lower the energy of activation, the energy that must be supplied in order for molecules to react with one another. Enzymes lower the energy of activation by forming an enzyme-substrate complex allowing products of the enzyme reaction to be formed and released.

Characteristics of Enzymes

Chemically, enzymes are generally globular proteins. (Some RNA molecules called ribozymes usually found in the nuclear region of cells can also act as enzymes.)

Enzymes are catalysts that breakdown or synthesize more complex chemical compounds. They allow chemical reactions to occur fast enough to support life. Enzymes speed up the rate of chemical reactions which are needed to provide nutrients to the body. Anything that an enzyme normally combines with is called a substrate. Enzymes are very efficient and can typically catalyze between 1 and 10,000 molecules of substrate per second.

Enzymes are only present in small amounts in the cell since they are not altered during their reactions.

Enzymes are highly specific for their substrate. Generally there is one specific enzyme for each specific chemical reaction.

Enzymes are heat labile.

They contain 16% weight as nitrogen 

They can be precipitated by protein precipitating reagents ( ammonium sulfate or trichloroacetic acid)

FUNCTIONS OF ENZYMES

The most important function of enzymes is that they aid in digestion. Digestion is the turning of food eaten into energy compounds in the body. For example there are enzymes in the saliva ( enzyme amylase or ptyalin) , pancreas intestines and stomach. They break down fats, carbohydrates, proteins and vitamins . Enzymes use these nutrients for growth and cell repair.

Nerve function. In the nervous system, the enzymes stretches from intracellular level to extracellular level hence maintaining metabolism, cell survival and proliferation and enabling intercellular communication and neuronal trophic support.

Building muscle. Digestive enzymes like lipases, proteases, lactase, cellulase ensures proper absorption of proteins and other nutrients to the body for muscle gain.

Breathing. Enzymes play a vital role in respiratory system in that the transport electrons from one molecule to another. They are also key in both aerobic and anaerobic respiration in the body of the living organisms.

Removing toxins from the body. The two major enzymes that assists in detoxification include cytochrome p450s and flavin-containing monooxygenases ( FMOs). They attack foreign toxins, destroy them and splits them out in form that they can eliminate them from the body.

Enzyme specificity

Group specificity-act on molecules with certain functional group eg methyl, aminoacid

Absolute specificity- the enzyme catalyses only one reaction.

Linkage specificity – the enzyme will act on a particular type of chemical bond regardless of the rest of the molecular structure.

Stereochemical specificity– the enzyme will act on a particular steric or optical isomer.

ENZYME NOMENCLATURE

Enzymes are classified according to the reactions they catalyse. In addition to its trivial name (eg maltase – acting on maltose) each has a symmetric name, with the suffix -ase and four digit enzyme commission number (EC,), The fourth figure is the individual number of the enzyme. The second and third the type of reaction within that class in terms of the chemical groups involved. The first of which indicates the class to which the enzyme belongs, 

In general all enzymes catalyzing the same reactions have the same EC number. The enzyme commission (EC) numbers divide enzymes into six main groups according to the type of reaction catalysed. They include;

OXIDOREDUCTASES – Oxidoreductases which involve redox reactions that is, catalyse electron transfer reactions where one substrate is oxidised while the other is reduced. The hydrogen or oxygen atoms or electrons are transferred between molecules. This extensive class includes the:- Dehydrogenases (hydride transfer), Oxidases (electron transfer to molecular oxygen), Oxygenases (oxygen transfer from molecular oxygen), Peroxidises (electron transfer to peroxide).

TRANSFERASES – Catalyse the transfer of an atom or group of atoms (e.g. acyl-, alkyl- and glycosyl-), between two molecules, but excluding such transfers as are classified in the other groups (e.g. oxidoreductases and hydrolases). Such groups transferred include acyl, phosphoryl, glyceryl, amino, carbon or other groups from one substrate to another. Examples of these enzymes includes; Phosphorylases, MethyPhosphorylase, Transaminases, FormyltrTransaminase, Acyltransferases, etc. 

For example: aspartate aminotransferase (EC 2.6.1.1, systematic name, L-aspartate:2-Oxoglutarate, aminotransferase; also called glutamic-oxaloacetic transaminase or simply GOT).

HYDROLYSES – They catalyse hydrolysis of their substrates by adding constituents of water across the bonds they cleave.. This is presently the most commonly encountered class of enzymes within the field of enzyme technology and includes the:- Esterases, GlycosEsterase, Lipases, Proteases.

 For example: chymosin (EC 3.4.23.4, ).Hydrolases cleave ester, amide, peptide bonds etc 

And include all the digestive enzymes such as esterases, peptidases, amidases etc.

LYASES – They catalyse the removal of groups from substrates by other mechanisms other than hydrolysis leaving double bonds (catalyse the non hydrolytic removal of a group from the substrate). They also catalyse reactions that involve addition across the double bonds such as dehydration and the scission of C-C single bonds as occurs in decarboxylations reactions. Examples of these enzymes include:- Desmolases, DecarbDesmolase, the aldolases, decarboxaldolase, dehydratases, some pectinases etc . NB does not include hydrolases. 

For example: histidine ammonia-lyase (EC 4.3.1.3, or histidase).

ISOMERASES – They catalyse reactions that involve intermolecular rearrangements, that is, the transfer of groups within molecules to yield isomeric forms. Examples include :- Racemase, Epimerases, cis – trans isomerases, Intramolecular transferases.

 For example: xylose isomerase (EC 5.3.1.5, or glucose isomerase).

LIGASES – They are also known as synthetases and form a relatively small group of enzymes which involve the formation of a covalent bond joining two molecules together, coupled with the hydrolysis of a nucleoside triphosphate or similar high energy compounds. They catalyse the formation of C-C, C-N, C-O or C-S bonds in reactions requiring energy from the hydrolysis of these nucleosides triphosphates examples Coenzyme A ligases. 

** Many times the word O.T.H.L.I.L is used to remember six classes

Factors affecting the rate of enzyme catalysed reaction

For a chemical reactions to occur, molecules must collide to form a bond and there is an energy barrier that must be overcome in order for a reaction to occur. For a collision to result in a reaction, the reacting molecules must possess sufficient energy to overcome this energy barrier. Hence anything that raises the kinetic energy reacting molecules lowers the energy barrier for reaction, or increases collision frequency should increase the rate of reaction. .

pH

can have a marked effect on enzyme activity since many of the amino acids in the enzyme bear ionisable groups. Change in pH will modify the degree of ionisation of some or all of these groups and this will affect the ionisation pattern of the enzyme molecules as a whole. Extreme PH will change the degree of ionisation of many groups in the enzyme so that there occurs a conformational change in the enzyme, disturbing the 3 dimensional structure of the enzyme.

This may cause denaturation and irreversible loss of enzyme activity because of the destruction of the secondary tertiary and quaternary structure. Small changes in pH will affect smaller number of ionisable groups but if theses happens to be present in the active site they’ will cause a marked change in enzyme activity. Changes in pH may cause change in the ionisation of the substrate and this could modify the binding of the substrate to the enzyme hence for many enzyme a plot of activity against pH gives a bell shaped curve with a well defined optima usually between pH-5-9.

Temperature.

Temperature rise increases the number of molecules that can react by both elevating their kinetic energy and by increasing their frequency of collision. While raising temperature increases the rate of an enzyme-catalysed reaction, this holds only over a strictly limited range of temperature. The reaction rate initially increases as temperature rises owing to increased kinetic’ energy of the reacting molecules. Excess kinetic energy of the enzyme will break the weak hydrogen and hydrophobic bond that maintain the secondary-tertiary structure leading to enzyme denaturation hence loss of catalytic activity.

Low temperatures will lead to inactivation of the enzyme leading decreased level of catalysis. Enzymes therefore exhibits an optimal’ temperature and for most enzymes the activity will increase from 0-45°C rise by 10oC doubles the activity of the enzyme. For most enzymes the optimum temperature is between 23°C 45°c. Each enzyme has an optimum temperature at which it works best. A higher temperature generally results in an increase in enzyme activity. As the temperature increases, molecular motion increases resulting in more molecular collisions. If, however, the temperature rises above a certain point, the heat will denature the enzyme, causing it to lose its three-dimensional functional shape by denaturing its hydrogen bonds. Cold temperature, on the other hand, slows down enzyme activity by decreasing molecular motion.

Substrate concentration . At a fixed enzyme concentration, the initial rate of reaction will increase with substrate concentration until all enzyme molecules are saturated with the substrate. The reaction will reach a maximum when all enzymes are used. Any further increases in the concentration of substrate will have no effect on the rate of reaction. Uncatalysed reactions do not show this saturation effects. At a constant enzyme concentration and at lower concentrations of substrates, the substrate concentration is the limiting factor. As the substrate concentration increases, the enzyme reaction rate increases. However, at very high substrate concentrations, the enzymes become saturated with substrate and a higher concentration of substrate does not increase the reaction rate.

Enzymes concentration

Rate of chemical reaction is directly proportional to enzyme concentration provided there is availability of the substrate. In this case there are more enzyme molecules onto which the substrate can bind.

 Presence of inhibitors

An inhibitor is a substance, which is able to reduce the activity of an enzyme. Inhibition is likely to occur if the active site bids molecules other than the substrate. If Such inhibitions can be readily displaced by substrate molecules this results in competitive inhibition. Structural analogues of the substrates frequently inhibit in this way, If however, an inhibition’ binds very strongly at the active site it may not be displaced by the substrate.

Inhibition of this kind can involve the formation of a covalent bond between the inhibitions and specific group in the active site, which is virtually irreversible and the compounds are known as irreversible inhibitors. Other inhibitor can bind reversibly to enzymes at a site distinct from the substrate biding site and are not therefore displaced by the substrate. Reducing the inhibitor concentration reverses such inhibition effects.

Salt concentration

Each enzyme has an optimal salt concentration. Changes in the salt concentration may also denature enzymes

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