What is the difference between $S_{0.5}$ values and $K_m$ values in enzyme kinetics?
1 Answer
This expands my comment on the question to an answer.
If an enzyme exhibits Michaelis-Menten kinetics, then it is valid to define a KM and this equates to the substrate concentration when reaction velocity is 0.5 * Vmax.
However, many enzymes do not exhibit Michaelis-Menten kinetics. One example is when the enzyme shows a co-operative response to substrate concentration. In these circumstances the substrate concentration when reaction velocity is 0.5 * Vmax is still a useful parameter, but there is no KM.
See this paper for a discussion of the kinetic properties of the glucokinase of pancreatic β cells, which acts as the glucose sensor. This enzyme shows positive co-operativity with respect to its substrate glucose, and the authors develop a kinetic analysis in terms of various parameters. These include an S0.5 value for glucose, but a KM value for ATP (presumably because the kinetics with respect to ATP are simple Michaelis-Menten).
http://biology.stackexchange.com/questions/5004/s-0-5-vs-k-m-values-in-enzyme-kinetics
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