Mada za sehemu hiiCytologyMada 9
Enzymes are simple or compound organic proteins which act as organic catalysts, catalyzing reactions in living tissues.
ENZYME: Greek word "en" means in and "zyme" means yeast cell.
They are biocatalysts found in living things.
- Catalysts accelerate chemical reactions although a catalyst is a participant in a reaction and undergoes physical change during the reaction. It reverts to its original state when the reaction is complete.
- Enzymes are protein catalysts for chemical reactions in biological systems. Most chemical reactions of living cells would occur very slowly, were it not for catalyzing enzymes as illustrated below.
Energy diagram under catalyst action showing progress of the reaction.
Fig. reduction of necessary activation energy by enzymes.
NB: As seen in the above graph, the activation energy (Ea) necessary to initiate the reaction is much less in the presence of the catalyst than in its absence.
It is this lowering of activation energy barrier by enzyme catalysts that makes possible most of the chemical reactions in life.
By contrast to non-protein catalysts (e.g. H+, OH-, or metal ions), each enzyme catalyzes a small number of reactions, frequently only one, and thus enzymes are reaction-specific catalysts.
Most inorganic catalysts are relatively non-specific. For example, platinum, often used to catalyze the formation of water from hydrogen gas and oxygen gas, will catalyze almost any reaction in which H2 is one of the reactants.
- They generally work faster than inorganic catalysts and greatly lower the activation energy.
- Enzymes are not consumed by the reaction they catalyze — i.e. a given molecule of an enzyme can be used indefinitely if the conditions are kept suitable.
- Enzymes can work in either direction — i.e. catalyze reversible reactions. This is due to the fact that metabolic reactions are reversible and the direction of the reaction depends on the relative amount of substrates and products present.
- Enzymes are denatured by excess heat (temperature) by virtue of their proteinaceous nature.
- Enzymes are sensitive to pH. Every enzyme has its own range of pH at which it functions effectively.
- Enzymes are specific in the action they catalyze. Normally a given enzyme will catalyze only one reaction or one type of reaction.
- Enzymes react in only small amounts. A very small amount of catalyst will catalyze a very large amount of reactants.
- They are colloidal in nature and thus provide large surface area for reaction to take place.
- Enzyme activity can be accelerated or inhibited. The accelerators are called activators e.g. Cu, Zn, Co, Cl, Ca, while the inhibitors are for example DDT, Pb, and Hg etc.
Hypothesis explains the nature and mode of enzyme activity.
- The lock and key theory (hypothesis) by Fischer. In this model, the three-dimensional configuration of the enzyme represents the lock (the active site) into which a particular substrate (key) will fit. The active site is presumed to be rigid.
- The induced fit model by Koshland. Originally little more than an attractive hypothesis, this model has now received considerable experimental support. An essential feature is the flexibility of the region of the active site. In this model, the substrate induces a conformational change in an enzyme, just like the shape of a glove is affected by a hand wearing it.
Enzymes are either composed of:
- Protein alone — simple enzymes.
- Protein and other non-protein molecule (i.e. conjugated enzymes).
The protein part of an enzyme is called apoenzyme. The non-protein part is called coenzyme.
The protein part of an enzyme is made up of enzyme protein (zymoprotein).
The two components (the apoenzyme and coenzymes) make up the active enzymes called holoenzymes.
Holoenzymes = coenzyme + apoenzyme.
Prosthetic groups are usually metallic ions such as Co, Mg, Ni, Cu, Zn (mineral salts). This is also a non-protein part. The well-known coenzymes are those which function as hydrogen carriers in oxidation-reduction in energy metabolism. For instance, coenzyme NAD, NADP, Q, and Coenzyme A. Coenzyme A is involved in transfer of an acetyl group.
These are substances which increase the activity of the holoenzymes. Their absence may retard the catalytic activity of the enzymes or prevent them from acting.
Activators are usually inorganic ions e.g. Ca2+ for thrombokinase, Cl- for ptyalin, Mg2+ for phosphatase.
Coenzymes and activators are needed by the enzymes for proper activities.
- Temperature. Over a limited range of temperature, the velocity of enzyme-catalyzed reactions increases as the temperature rises. The exact ratio by which the velocity changes for a 10°C temperature rise is the Q10 or temperature coefficient.
The velocity of many biological reactions roughly doubles with a 10°C rise in temperature (Q10 = 2) and is halved if the temperature is decreased by 10°C. Many physiological processes e.g. the rate of contraction of an exercised heart consequently exhibit Q10 of about 2.
When the rate of enzyme-catalyzed reaction is measured at several temperatures, the result shows in the figure below is typical. There is an optimal temperature at which the reaction is most rapid. Above this temperature, the rate decreases sharply due to heat denaturation of the enzyme, and below this the energy content of enzymes is too low to make them participate in their reaction.
Fig. enzyme activity as a function of temperature.
- pH. Although temperature sensitivity varies somewhat from one enzyme to another, the curve shown here may be taken as applying to an average enzyme.
Its activity rises steadily with temperature (approximately) doubling for each 10°C increase until thermal denaturation causes a sudden sharp decline, beginning between 40°C and 45°C. The enzyme becomes completely ineffective/inactive at temperatures above 60°C, presumably because its three-dimensional configuration has been severely disrupted.
Denaturation of a protein enzyme by heat is the loss of its biological activity. This can be done also by heat, acid, or high salt concentration.
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