pts) The imidazole ring of histidine is often an important group in catalysis, d

Question

pts) The imidazole ring of histidine is often an important group in catalysis, due to the fact its pKa is near physiological pH that a) what is the role of the imidazole ring in the reaction sequence below? NO H,C NO2 CH3COOH protein protein protein 2 b) what would you expect to happen to the activity of this enzyme at pH 5? Why ? c) for 2nd order reactions, the rate increases as the concentration of reactants increases. One way enzymes help to accelerate reactions is by bringing the reactants close together. This concept has been examined in model systems such as the one below. When each reactant is at a concentration of 1 M, the rate for reaction (2) is 20x faster than for reaction (1). Explain why H20 + CH,COOH + HO NO2 (1) H2O HN -OH + HO- -No, (2)

Explanation / Answer

a) From the mechanism of the reaction given, it can be observed that the basic nitrogen of imidazole ring of histidine abstracts a proton from water, thus releasing hydroxide ions in the medium and it is these ions which tend to attack the carbonyl carbon and bring about the reaction to the final product. Therefore, the role of imidazole here can be deduced here is a base whose action on water releases the hydroxide species responsible for the hydrolysis reaction given.

b) When the enzyme is put at a pH of 5, compared to the physiological pH of nearly 7.4, there will be a very high concentration of protons already present in the medium which will tend to be attracted towards the basic nitrogen in the imidazole present in the active site of the enzyme. This will result in the imidazole not acting as a base as it would have already been protonated by those in the medium, leaving all the water molecules as such. This inability to abstract protons from water will now result in the shortage of hydroxide ions which implies drastic decrease of enzymatic activity.

c) As mentioned before, the role of imidazole in this reaction is as a base as it abstracts protons from the medium, releasing hydroxide ions which bring about the reaction. In reaction (1), the imidazole is free while in the reaction (2), it is made to be in the vicinity of the substrate by linking the basic site covalently near the substrate. When the imidazole is free, all the hydroxide ions released might not attack the particular substrate's carbonyl carbon and bring about the desired reaction and instead drift off and attack more vulnerable sites after imidazole's N abstracts a protons from water. However, if the basic site is the vicinity of the substrate, the moment the basic N abstracts a proton, the hydroxide ion formed, which tends to seek out electron-deficient sites will immediately attack the substrate in the desired manner as it is closest to the molecule. This as we know will speed up the reaction as rate of reaction will be nearly equal to the rate of proton abstraction, which itself is quite high.

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