[Lisa_JO]

An enzyme-catalyzed reaction proceeds with a maximum rate of
0.34 mM/min. How long will it take for the concentration of substrate to
decrease from 9.8 mM to 5.0 mM?

So I solved the question by following the zero order reaction formula and solved for t " [A]=[A]0-Kt" and I got the right answer which is 14. However, I have a question about why this reaction is solved as a zero order reaction is it because the concentration dropped ?! although I know that zero order reaction has a rate that is independent of the concentration of the reactant(s). Thank you for the help

[Borek]

Think about mechanism of the

enzyme-catalyzed reaction

Hint: assume concentration of the enzyme is much lower than the concentration of the substrate.

[clarkstill]

If a crowd of schoolkids are in a lunch queue, the rate at which people are served doesn't depend on the size of crowd, it just depends on how quickly the dinnerladies can serve each dish.

In the same way, a zero order enzymatic reaction implies the enzyme is working as fast as it can, and the amount of substrate available isnt the limiting factor.

[Babcock_Hall]

Good question.  Under certain conditions, zero-order behavior is a very good approximation.  Start with the Michaelis-Menten equation; set the initial substrate concentration to 98*K_M and calculate the ratio of the velocity to V_max.  Then see how much v/V_max changes when substrate decreases to 50*K_M.  My example is hypothetical, in that we do not know the value of the Michaelis constant for this enzyme and substrate.  However, it illustrates a point.

[sgojja]

It is so because we are interested in the substrate (the thing that is undergoing the reaction). since, the concentration of substrate is (as in most cases) very very high. it only depends on enzyme, how fast it can convert (popularly known as, turn over number).
alternate-analogy_look at this reaction more as a surface reaction. all enzyme does is provide an active site (surface) since surface is a limiting factor here. the rate in independent of the concentration of substrate. however, at very low pressure in case of surfaces, rate of reaction also changes.
thus enzymatic reactions are first order when substrate concentration is much much higher than that of enzyme.
best

[Babcock_Hall]

@OP, Under most in vitro conditions the key issue is what is the concentration of substrate relative to its K_M.  The ratio of [Substrate] to [Enzyme] may be very high even when [Substrate] << K_M, and the reaction follows apparent first-order kinetics under these conditions.  When [substrate] >> K_M, then the reaction follows apparent zero-order kinetics.  See if you can determine the apparent first order and zero order rate constants, respectively, from the Michaelis-Menten equation.