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The following points highlight the seven factors affecting the activity of enzymes in plants. The factors are: 1. Temperature 2. Hydrogen Ion Concentration 3. Water 4. Concentration of the Substrate 5. Enzyme Concentration 6. Inhibitors and 7. Accumulation of End-Products.
Factor # 1. Temperature:
Usually the activity of the enzymes is optimum at normal body temperature. At very low temperature (i.e., about 0°C, the activity of the enzymes is minimum. An increase in temperature up to a certain limit increases the enzyme activity, maximum being at about 45°C after which the enzyme activity is retarded (Fig. 10.7). Beyond 60°-70°C usually their activity is permanently stopped due to the denaturation of enzymes.
Factor # 2. Hydrogen Ion Concentration (pH):
Enzymes are active only over a limited range of pH. Some enzymes e.g., trypsin are active in alkaline medium (high pH), diastase in neutral medium, while pepsin shows optimum activity in acidic medium (low pH).
Factor # 3. Water:
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In absence of water the enzyme activity is suppressed so much so that in dry seeds the enzymes are almost inactive.
Proper hydration of the cells is necessary for enzyme activity because:
(i) Water provides medium for enzyme reaction to take place, and
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(ii) In many cases it is one of the reactants.
Factor # 4. Concentration of the Substrate:
Increase in the conc. of the substrate brings about an increase in the activity of the enzyme till all the active sites of the enzyme molecules are saturated with substrate. After this the rate of enzyme reaction becomes steady and addition of the substrate will not have positive effect (Fig. 10.8).
Factor # 5. Enzyme Concentration:
Usually a very small amount of the enzyme can consume large amount of the substrate. Increase in the conc. of the enzyme will increase the rate of reaction catalysed by it provided there is enough conc. of substrate (Fig. 10.9).
It is because:
(i) The increased number of enzyme molecules will have more active sites, and
(ii) At higher conc. of the enzyme the inhibitors will fall short.
Factor # 6. Inhibitors:
Presence of inhibitors in the reaction mixture inhibits the activity of the enzymes partially or completely depending upon the nature of the inhibitors. Inhibitors are less effective when the conc. of the enzyme and substrate is higher. Inhibitors are of two types.
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(i) Competitive Inhibitors:
Such inhibitors have structural similarity with the substrate both of which compete for the same active site of the enzyme. If competitive inhibitor pre-occupies the active site, the substrate molecule will be unable to combine with the enzyme and hence, the enzyme activity will be inhibited. But, this inhibition is of reversible type because removal of the competitive inhibitor restores the activity of the enzyme.
An example of competitive inhibitor is malonic acid which inhibits the activity of the enzyme succinic dehydrogenase which normally catalyses the conversion of succinic acid to fumaric acid. Inhibition of the activity of this enzyme is due to the structural similarity between malonic acid and succinic acid which has been shown diagrammatically in Fig. 10.10.
(ii) Non-Competitive Inhibitors:
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These are usually poisons which do not compete for the active sites but destroy the structure of the enzyme and cause permanent or irreversible inhibition of the activity of the enzyme.
Factor # 7. Accumulation of End-Products:
Accumulation of the end-products retards the enzymic activity mainly because the active sites of the enzymes are crowded by them and substrate molecules will have comparatively lesser chances of combing with the active sites.
