Enzymes are biocatalysts. They facilitate biochemical reactions. Enzymes lower activation energy as they facilitate productive collision between reactants. The rate of enzymes-substrate reaction at optimum temperature, and pH, depends on substrate concentration and rate constant Km.
ENZYME LOWERS ACTIVATION ENERGY
Chemical reactions are well-balanced reactions where substrates combine with each other to form products. The product dissociates to form substrate this type of reaction is called reversible reaction. Other types of reactions where products cannot be converted into reactants are called Irreversible reactions. In Irreversible reactions, the conversion of the products back to the reactants is thermodynamically not feasible. For a chemical reaction to occur the substrates have to collide with each other to combine and form the product. The collision depends on the kinetic energy of the substrates. If the kinetic energy is very low the substrates will not productively collide with each other that is, the substrate will unable to form products. The substrates are required to gain that much energy to collide and form products. This is called the Energy barrier appears in the form of activation energy.
Activation energy
The activation energy is the minimum amount of energy that is required for a productive collision of substrates. The activation energy is always higher than the energy which reactants possess and this energy act as an energy barrier for the conversion of reactants into products.
This Energy barrier is overcome by heating or supplying the external energy to the reactants. This increases their kinetic energy, and they now can collide with each other to form products.
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| ENZYME LOWERS ACTIVATION ENERGY |
ENZYME LOWERS ACTIVATION ENERGY
For enzyme-substrate reactions, enzymes transiently bind with reactants and facilitate their collision. This transient binding of enzymes with reactants lowers the requirement of high kinetic energy to convert reactants into products. Enzyme lowers the activation energy required for a chemical reaction. This facilitates the reactions to occur at body temperature.
Further, the enzyme-substrate reactions are multistep reactions where a small amount of energy is released in each step and this energy is stored in the form of bond energy of the products.
ORDER OF ENZYME-SUBSTRATE REACTION
Enzymes substrate reactions can be a first-order reaction or second-order reaction, as the rate of reaction depends upon the concentration of reactants that productively collide with each other. If the rate of reaction doubles if the concentration of reactants is doubled. The rate of reaction doubles when concentration is doubled for one reactant and the other reactant is kept constant, then the rate of reaction will be first-order for the first reactant and second-order for the second reactant. The rate of reaction can be first-order only if the rate of reaction depends on the concentration of the reactant.
As the chemical reactions are always at equilibrium so the rate of the forward reaction is always equal to the rate of backward reaction. The ratio of reaction constant for the forward reaction and backward reactions are always equal to the rate constant of equilibrium.
The enzyme lowers the kinetic energy required for a reaction, but enzymes have no effect on the equilibrium constant Keq, the rate constant of reaction at equilibrium.
What affects enzyme-substrate reaction?
The enzyme-substrate reactions are affected by many factors including temperature, pH, and substrate concentration.
Temperature
For an optimum rate of reaction, the temperature must be optimum. Raising the temperature increases the rate of reaction because it increases the rate of collision of the reactants.
Enzymes are made of protein so, rising temperatures too high will denature the enzyme and this will decrease the rate of reaction.
Human enzymes remain active at a temperature from 45 degrees Celsius to 55 degrees Celsius. Enzymes of the thermophilic organism remain active even at 100 degrees Celsius.
Temperature coefficient Q10 states that every 10-degree rise in temperature doubles the rate of reaction.
Temperature Coefficient applies to cold-blooded animals where their body temperature depends upon the temperature of the environment. It rises and falls with the external environment.
The organisms that have the same body temperature and they do not follow the rise and fall in temperature of the environment, their body temperature varies only in case of fever or hypothermia.
The pH of the medium
Intracellular enzymes require optimum pH for their activity. The concentration of hydrogen ions varies with pH. At a very low concentration of hydrogen ion that is at basic pH and a very high concentration of hydrogen ion that is at acidic pH.
Enzymes are made of proteins, hydrogen ions are required for the formation of a Salt Bridge between the enzyme and substrate. In absence of hydrogen ions, enzymes are unable to form Salt Bridge so, they cannot recognize and transiently bind with the substrate. This will not allow the reaction to occur.
The effect of concentration of substrate on enzyme kinetics
Enzymes transiently bind with the substrate to convert them into products and the rate of reaction for enzyme-substrate reaction directly depends upon the concentration of substrates. If the concentration of enzyme is kept constant then any increase in the concentration of substrate will increase the rate of reaction. However, this will not appear as a straight line graph because enzyme after reaching saturation, there will be no further increase in the rate of reaction.
When substrate concentration is maximum the rate of reaction will now depend upon the dissociation of enzymes from the substrates so that free enzymes are available for binding with the reactants. At a high concentration of reactants, the concentration of enzyme will become a limiting factor
Michaelis Menten and Hill equation
Michaelis- Menten, and Hill give an equation to determine the concentration effect of substrate on the rate of reaction. It was proposed that the rate of reaction is directly proportional to the concentration of substrates.
Enzymes transiently bind with the substrate to convert them into products and the rate of reaction for enzyme-substrate reaction directly depends upon the concentration of substrates. If the concentration of enzyme is kept constant then, any increase in the concentration of substrate will increase the rate of reaction. However, this will not appear as a straight line graph because enzyme after reaching saturation, there will be no further increase in the rate of reaction.
When substrate concentration is maximum the rate of reaction will now depend upon the dissociation of enzymes from the substrates so that free enzymes are available for binding with the reactants. At a high concentration of reactants, the concentration of enzymes will become a limiting factor.
If A and B are reactants, P is the product of a reaction at equilibrium. The initial rate of reaction Vi will be proportional to the concentration of reactants.
(1)If substrate concentration is very low
At a very low concentration of substrate initial velocity is proportional to the concentration of substrate
(2) If substrate concentration is much greater than Km
At very high concentration initial velocity is equal to maximum velocity, at a very high concentration rate of reaction is maximum
(3) If the concentration of substrate equal to Km
Competitive inhibition of enzyme
From the above discussion, it can be concluded that the rate of reaction for an enzyme-substrate reaction depends on the concentration of substrates and Km of the enzyme. Km is specific for an enzyme.This discussion also explains the enzyme inhibition by competitive inhibitor, where an inhibitor competes for the active site of an enzyme. If the concentration of substrate is higher than the inhibitor the enzyme will form the products. if the inhibitor is present in high concentration it will bind with the enzyme. Thus occupy the active site of the enzyme. As the enzyme can not convert the inhibitors into products, the rate of reaction decreases i.e. the reaction is inhibited.
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