What is a good metaphor/analogy for allosteric binding?

We know that enzymes should evolve to have a binding constant appropriate for optimizing their normal activity. But what defines normal activity for different enzymes? The two obvious constraints are speed and accuracy.

If we consider three professions, neurosurgeon, barber, and candy vendor, we intuitively appreciate that we cannot expect of each one an equal number of transactions with patients/customers per day. Surgeons need to be very discriminating in what/where they cut. Their speed should be no faster than that speed at which they will make no error. The art of cutting hair is not quite as exact, and barbers can proceed at a moderate speed. Vendors may clearly proceed at faster rates, since they may safely correct occasional errors with no harm to the customer.

A relationship that involves cheating is like allosteric binding. A mistress is the cofactor that binds to the allosteric site. So then, the other cofactor gets removed because of the conformational change.