One Step At A Time

Consider again an uphill change from A to B with a steep barrier (tipping point): link

When faced with such a high barrier, it helps to break the uphill slog into steps. Catalysts (enzymes included) employ this strategy, breaking a reaction into one or more steps.*

Linus Pauling, that greatest of American chemists, introduced the profound notion that enzymes work by stabilizing tipping points (transition state energies in the parlance) and thereby speed things up:
 

Some of the acceleration occurs because requisite energy is just smoothly shaved off the peak, but sometimes a transient intermediate, labelled C above, appears. Thus we have a "stepwise" versus a "concerted" process.
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*The 2010 Nobel prize was awarded to three inventors of a family of very similar catalysts.