A protein with catalytic properties due to its power of specific activation.
Chemical reactions need an initial input of energy = THE ACTIVATION ENERGY
During this part of the reaction the molecules are said to be in a transition state.
One way to reach the transition state rapidly is to increase the temperature.
But biological systems are very sensitive to temperature changes.
Enzymes can increase the rate of reactions without increasing the temperature. They do this by lowering the activation energy. They create a new reaction pathway “a short cut”.
Enzyme controlled reactions proceed 108 to 1011 times faster than corresponding non-enzymic reactions.
The substrate of an enzyme are the reactants that are activated by the enzyme.
An additional non-protein molecule that is needed by some enzymes to help the reaction.
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Tightly bound cofactors are called prosthetic groups.
Cofactors that are bound and released easily are called coenzymes.
Many vitamins are coenzymes.
Inhibitors are chemicals that reduce the rate of enzymic reactions. The are usually specific and they work at low concentrations.
They block the enzyme but they do not usually destroy it.
Many drugs and poisons are inhibitors of enzymes in the nervous system
FACTORS THAT AFFECT ENZYME ACTIVITY
The effect of pH
Extreme pH levels will produce denaturation.
The structure of the enzyme is changed.
The active site is distorted and the substrate molecules will no longer fit in it.
At pH values slightly different from the enzyme’s optimum value, small changes in the charges of the enzyme and it’s substrate molecules will occur. This change in ionisation will affect the binding of the substrate with the active site.
Note: The pH at which the enzyme molecule’s charges are equal is called it’s isoelectric point. This corresponds to its optimum pH. On an electrophoresis gel the enzyme would not move as it has no overall charge at this pH.
The effect of temperature
Q10 (the temperature coefficient) = the increase in reaction rate with a 10°C rise in temperature.
For chemical reactions the Q10 = 2 to 3
(that is the rate of the reaction doubles or triples with every 10°C rise in temperature)
Enzyme-controlled reactions follow this rule as they are chemical reactions.
BUT at high temperatures proteins denature.
The optimum temperature for an enzyme controlled reaction will be a balance between the Q10 and denaturation.
For most enzymes the optimum temperature is about 30°C.
Many are a lot lower, cold water fish will die at 30°C because their enzymes denature.
A few bacteria have enzymes that can withstand very high temperatures up to 100°C.
Most enzymes however are fully denatured at 70°C.
Irreversible inhibitors: These combine with the functional groups of the amino acids in the active site, irreversibly.
Examples: nerve gases and pesticides, containing organophosphorus, combine with serine residues in the enzyme acetylcholine esterase.
Reversible inhibitors: These can be washed out of the solution of enzyme by dialysis. There are two categories.
Applications of inhibitors
Negative feedback See end point or end product inhibition
Poisons snake bite, plant alkaloids and nerve gases.
Medicine antibiotics, sulphonamides, sedatives and stimulants