Ping pong Mechanism

Ping-pong mechanisms

The ping-pong (double displacement) mechanismThe distinguishing feature of these enzymes is that at least one product is released from the enzyme before all of the substrates have bound. This might seem slightly unlikely at a first glance, but it's actually easily explained and quite a common mechanism. Some very familiar enzymes, for instance the serine proteases (trypsin, chymotrypsin, etc.) and the amino transferases work in this way.

Ping-pong reactions are ones in which at least one product is released before all the substrates have bound.

To clarify these distinctions we'll look at each of these mechanisms in turn using a typical bi bi enzyme: A + B => P + Q

We'll start by looking at an ordered sequential reaction, which is perhaps the simplest in kinetic terms.

The ordered sequential mechanismIn these enzymes the substrates bind to the enzyme, and the products are released in a defined sequence. Firstly the two substrates bind to give an enzyme substrate complex: E + A=>EA EA + B =>(EAB)

Notice that the EAB complex is a central complex and enclosed in parentheses.

The enzyme substrate complex is now converted to enzyme product complex: (EAB)<=>(EPQ)

Again this is a central complex.

The products are now released: (EPQ) => EQ + P EQ => E + Q

This is of course just an extension of the usual sequence of reactions which we are used to with single substrate systems.

This is actually quite a simple mechanism, but the sequence of reactions can get quite complex particularly in enzymes with more than two substrates and products. The Cleland plot is a diagramatic summary of the reaction sequence in which the enzyme is represented as a horizontal line and arrows are used to represent the arrival and departure of substrates and products. Transitory complexes are written below the enzyme line.

The above diagram is the Cleland plot for the ordered sequential enzyme that we've been discussing and should be fairly self-explanatory.