# In the simple enzyme-catalyzed reaction below, which of the rate constants would be second-order? – 2017

in the simple enzyme-catalyzed reaction below, which of the rate constants would be second-order?

Zero-order reactions

• Many reactions in biochemistry appear to occur at a rate independent of substrate concentration. These are said to show zero-order behavior.
• In general, this is because the reaction is enzyme-catalyzed, and the rate is determined by the concentration of enzyme, as long as the substrate is in excess, so that the rate is saturated.

Enzyme kinetics is the study of the chemical reactions that are catalysed by enzymes. In enzyme kinetics, the reaction rate is measured and the effects of varying the conditions of the reaction are investigated. Studying an enzyme’s kinetics in this way can reveal the catalytic mechanism of this enzyme, its role in metabolism, how its activity is controlled, and how a drug or an agonist might inhibit the enzyme.

In the simple enzyme-catalyzed reaction below, which of the rate constants would be first order?

k
ES k2

Second-order kinetics
More complicated reactions can also occur: S + R ? P + Q
For these reactions:

!
v = d[P]
dt = d[Q]
dt = d[S]
dt = d[R]
dt
and
v = k[S][R]
Reactions of this type are second-order, and k is a second-order rate constant,
because the rate of the reaction depends on the product of [S] and [R]. If the
reaction involved the collision of two molecules of S, the velocity equation would be:
v = k[S][S] = k[S]2
The order of the reaction comes from the exponent that describes the number of
reactants. Second-order rate constants have units of M-1sec-1.