Around 100 years ago Emil Fischer in his famous paper proposed that
enzyme and substrate can be compared to lock and key. Since that time this
metaphor has been used to describe enzyme action. In this chapter we shall try
to explore whether this metaphor still holds. On the one hand, enzymology has
made enormous progress in these 100 years, but on the other hand the lockand-
key concept has greatly changed. In Figure 1 three types of ‘keys’ are shown.
The classical key, which Emil Fischer had in mind, is pushed into the lock
and turned clockwise in order to open the lock. Thus, the process is chiral. The
key shown in the figure was manufactured in the year 1853, one year after Emil
Fischer was born; thus, probably this type of key was imprinted in the young boy’s
conceptual memory. Today, the keys have become much more refined. Around
the turn of the century the safety lock was invented, which does a kind of proofreading
of the key. At present an entirely new system is being installed; namely the
magnetic card. This card can unlock money sources and hotel rooms. We shall
discuss further these three key concepts and give a few examples of each of them.