what is a good explanation for:
Many enzyme catalyst reaction start out as 1st order and as time passes by it becomes 0 order. Why?
In the initial first order reaction,
rate = k[substrate concentration]
As time passes the enzyme becomes bound with substrate into an enzyme-substrate complex. At this point the rate of product formation depends on the rate of decomposition of the complex and independent of substrate concentration. Now the reaction is zero order, and,
Rate(max) = constant