Hi Emma,
For all of these, let's assume an overall reaction of an enzyme reacting with 2 substrates in an overall reaction:
(A) E + S1 + S2 --> E + P1 + P2
There are multiple mechanisms by which this can be achieved:
1.Sequential reactions: both substrates must be present inside the active site of the enzyme to transform S into P.
In other words:
(B) E + S1 + S2 --> S1-E-S2 (a ternary complex incorporating S1, S2 and E) --> E + P1 + P2
Mechanism (B) can happen in two different ways:
1 a) in ordered sequential reactions, the order in which S1 and S2 attach themselves to E to form the ternary complex matters, that is you have to have something like:
(C) E + S1 --> E-S1 +S2 --> E-S1-S2 --> E + P1 + P2 where S1 must bind to E before S2
1 b) in random order sequential reactions, the order in which S1 and S2 attach themselves to E does not matter, so you can have mechanism (C) or (D) occuring as follows (and both occur to some extent)
(C) E + S1 --> E-S1 +S2 --> E-S1-S2 --> E + P1 + P2
(D) E + S2 --> E-S2 +S1 --> E-S2-S1 --> E + P2 + P1
2. Ping pong substrate binding does not involve the formation of an E-S1-S2 complex (ternary complex) like in these sequential mechanisms.
Instead, the two substrates enter the active site at different intervals as follows:
(E) E + S1 + S2 --> E-S1 + S2 --> E* + P1 + S2 --> E*-S2 + P1 --> E + P1 + P2
Where E* is an enzyme temporarily modified by reaction with S1, often you are passing a group from S1 to S2 in this process using the enzyme as an intermediary.
This is a bit complicated and I would be surprised if the MCAT gave you this without the context of a passage.
Please let me know if this is unclear,
Katt