2.12 The Role of Enzymes
Epun Dissanayake
MBBS (UG) | Microsoft Specialist | GitHub Developer | Google Developer
The role of Enzymes in regulating metabolic reactions
An enzyme is a macromolecule, which acts as a biological catalyst. Enzymes are produced in living cells.
General characteristics of an enzyme:
1.????Most of the enzymes are globular proteins.
2.????Enzymes are biological catalysts. They lower the activation energy of the reaction they catalyze (increases the rate of reaction).
3.????Most enzymes are heat liable/ sensitive
4.????Their presence does not alter the nature or properties of the end products of any reaction.
5.? Enzymes are highly specific to the substrate (substrate specific)
6.????Most enzyme-catalyzed reactions are reversible.
7.????The rate of enzyme activity is affected by pH, temperature and substrate concentrations.
8.????They are not being used up during the reaction.
9.????Enzymes possess active sites where the reaction takes place.
10. Some enzymes need non-proteinous components to catalyse the reaction which is known as cofactors.
The mechanisms of enzyme action
The reactant and enzyme act on are referred to as the substrate. The enzyme binds to its substrate forming an enzyme-substrate complex. While enzyme and substrate form their complex, the catalytic action of the enzyme converts the substrate to the product.
Enzyme + substrate???Enzyme-substrate complex ? Enzyme?+ Product
The reaction catalyzed by each enzyme is very specific. The specificity of the enzyme results from its shape. The substrate binds to a specific region of the enzyme. This region is called the active site. The active site is formed by only a few amino acids. Other amino acids are needed to maintain the shape of the enzyme molecule. The shape of the active site is complementary to the shape of the specific substrate of the enzyme, and hence important in the substrate specificity of the enzyme. The shape of the active site of an enzyme is not always fully complementary to its substrate. As enzymes are not rigid structures, the interactions between the substrate and active site may slightly change the shape of the active site, so that the substrate and the active site become complementary to each other. This is called the induced fit mechanism. The tight fit not only An increase brings the substrate molecules and the active site close to each other but also ensures the correct orientation of the molecules to help the reaction proceed and catalyze the conversion of substrate to product. Thereafter, the product departs from the active site of the enzyme. The enzyme is then free to take another substrate molecule into its active site.
Cofactors
Non-proteinous components which are essential for the catalytic activities of certain enzymes are called cofactors.
These cofactors bind to the enzymes in two ways. Some tightly bind and remain permanently and others loosely bind temporarily. Loosely bound cofactors are reversible under certain circumstances.
Organic co-factors are called co-enzymes. e.g. derivatives of vitamins e.g. NAD, FAD and biotin
Inorganic co-factors – e.g. Zn2+, Fe2+, Cu2+
Factors affecting the rate of enzymatic reactions
Temperature
An increase in temperature increases molecular motion. Therefore the speed of the moving molecules of both enzymes as well as the substrate will be accelerated. This will enhance the colliding probability for both enzyme active sites and substrate molecules. More collisions between the enzyme active sites and substrate molecules generate greater chances for the reaction to occur. This can continue up to a certain point, after which there is a rapid decline in enzyme activity. This point is referred to as the optimum temperature. This may vary from organism to organism.
For e.g. most human enzymes have an optimum temperature around the body temperature (35?C-40?C). The optimum temperature of bacteria in hot springs is about 70?C.
When the temperature increases beyond the optimum temperature, the hydrogen bonds, ionic and other weak chemical bonds of enzyme active sites may be disrupted. This will result in a change in the shape of the active site of the enzyme which will alter the complementary nature of the active site of enzyme molecules. Therefore, the complementary binding of enzyme active sites and substrate molecules will be prevented. The above event is called as denaturation of enzyme molecules.
Therefore the rate of enzyme-catalyzed reaction will start to decline when the temperature increases beyond the optimum temperature and stops entirely at a certain temperature, although the rate of collision will keep on increasing.
pH
Enzymes function most efficiently within a certain pH range despite maintaining the temperature of the environment constant.
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The narrow range of pH in which a particular enzyme-catalyzed reaction takes place is named the pH range. The pH at which the highest rate of reaction occurs is the optimum pH of the enzyme. The alteration in pH above or below the optimum pH may lead to a decline in enzyme activity. This is due to the alteration of chemical bonds involved in the formation of the enzyme-substrate complex. In most enzymes, optimum pH range is 6-8, but there are exceptions. Pepsin works best at pH 2 and the optimum pH for Trypsin is 8.
Substrate concentration
Increasing substrate concentration increases the probability of collision between the enzyme and substrate molecules with the correct orientation. However, the enzyme molecules will be saturated after a particular concentration and therefore there will not be any further increase in the rate of reaction.
Enzyme inhibitors
Specific molecules or ions selectively bind permanently or temporarily to the enzyme molecules and prevent them from forming enzyme-substrate complexes. These substances are called inhibitors.
They are either binding reversibly with weak interactions or binding irreversibly through covalent bonds.
e.g.
A) Competitive inhibitors
Most of these are reversible inhibitors. These chemicals resemble the shape and nature of the substrate. Therefore they compete with the substrate selectively for the active site of certain enzymes. As a result of the above, the number of active sites available for the enzymes may decline and therefore reduces the rate of enzyme-catalyzed reactions.
The above situation may be reversed by increasing the substrate concentration. e.g. Protease inhibitor of drugs against HIV.-change
B) Non-competitive inhibitors
These chemicals do not compete with substrate molecules. They interrupt enzymatic reactions by binding to a part of the enzyme other than the active site. This causes the enzyme molecule to change its shape in such a way that the active site becomes less effective for the formation of the enzyme-substrate complex.
Regulation mechanism of enzymatic activity in cells
Allosteric regulation of enzymes
In many cases, the molecules that naturally regulate enzyme activity in a cell behave like reversible non-competitive inhibitors. Regulatory molecules (either activators or inhibitors) bind to specific regulatory sites elsewhere (other than the active site) of the molecule via non-covalent interactions and affect the shape and function of the enzyme. It may result in either inhibition or stimulation of enzyme activity.
A.)???Allosteric activation and inhibition
Most enzymes regulated by allosteric regulation are made from two or more subunits. Each subunit is composed of a polypeptide chain with its own active site. The entire complex oscillates between two different shapes one catalyzing active and the other inactive. In these two forms, regulatory molecules bind to a regulatory site called an allosteric site, often located where subunits join.
When an activator binds with this regulatory site, stabilizes the shape with functional active sites. Whereas the inhibitor binds with the regulatory area, it stabilizes the inactive form of the enzyme. Subunits of enzymes are arranged in a way through which they transmit the signals quickly to other subunits. Through the interaction of subunits, even a single activator or inhibitor?molecule that binds to one regulatory site will affect the active site of all subunits. e.g. ADP function as an allosteric activator bind to the enzyme and stimulates the production of ATP by catabolism. If the supply of ATP exceeds demand catabolism shows down as ATP bind to the same enzyme as the inhibitor.
B.)??cooperativity
This is another type of allosteric activation. The binding of one substrate molecule can stimulate binding or activity at another active site. Thereby increasing the catalytic activity. e.g. haemoglobin (not an enzyme) is made up of four subunits each with an O2?binding site. The binding of one molecule of O2 to one binding site increases the affinity for O2 of the remaining binding site.
C.)???Feedback inhibition
In feedback inhibition, a metabolic pathway is stopped by the inhibitory binding of its end product of a process to an enzyme. Thereby limiting the production of more end products than required and thus wasting chemical resources.?
Feedback inhibition is an essential process that regulates the end products produced in metabolism.
e.g. ADP function as an allosteric activator and stimulates the production of ATP??during catabolism.
In case ATP supply exceeds demand, catabolism slows down as ATP???molecules function as allosteric inhibitors. The energy needed for all living processes is obtained directly from ATP. ATP is mainly produced by a process called cellular respiration, in living cells.
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