Question

Superoxide dismutase is a very unique protein. It is the only protein that can catalyze faster than the rate of diffusion, which is usually a limiting rate for proteins since you cannot catalyze something faster than the amount of time it takes for your substrate to arrive at the active site. However, superoxide dismutase is able to do this. If you look at a diagram of superoxide dismutase, you will find a large cluster of basic amino acids surrounding the active site where the substrate binds. First, state the purpose of superoxide dismutase and what its substrate is. Then discuss how you think superoxide dismutase is able to have a kcat/Km that is faster than the diffusion limit.

Answer 1

Substrate of superoxide dismutase(SOD) is the superoxide anion radical (O₂ -) which has a negative charge on it,along with unpaired electron.

Function of superoxide dismutase(SOD)-  These are a group of enzymes that catalyses the dismutation of the superoxide anion radical (O₂ -).Under varied circumstances it can be used as a reducing agent and is converted to molecular oxygen (O₂) and in some other reciprocal condition it may function as an oxidant and connert it to H₂O₂ and thus provide protection against ROS (reactive oxygen species).

The superoxide dismutase tends to have faster kcat/ km than the diffusion limit- These enzymes tend to operate with kinetics in a much faster rate than the diffusion limits. Several mechanism would be responsible for such behaviour of the enzymes - (i) some proteins can draw the substrate in and pre-orient them using dipolar electric fields,which will result in acceleration of reaction, (ii) some suggest a quantum -mechanical tunelling -where a proton or electron tunnel through activation barriers,(iii)strong electrostatic field gradients situated near the active site of superoxide dismutase(SOD) may be responsible for high rate of association and thus higher kcat/km value.