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Enzymes are proteins that function as biocatalysts. They accelerate biochemical reactions, often allowing them to run millions of times faster, and are thus essential for almost all the processes occurring in a cell. Like all catalysts, enzymes achieve this acceleration by lowering the activation energy needed for a particular chemical change, while remaining unchanged in the process. So they can be used over and over again.
One characteristic feature of enzymes is their high specificity. They bind tightly to a particular substrate molecule with their active site and selectively reduce the energy needed to start one out of several alternative reaction paths. As a consequence, the set of enzymes in a cell determines the metabolic pathways occurring in this cell and an organism's success basically depends on its cells' ability to synthesise many different types of enzymes.
Until quite recently it was thought that evolutionary convergence amongst enzymes would be uncommon, but it is now clear that there are hundreds of examples in the various classes of enzyme that play key roles in biochemistry and metabolism. Only 11 of the 22 natural amino acids are found in enzymes and particular combinations are significantly more common than others, which limits the number of solutions for an enzyme. There are basically two different types of enzymatic convergence, mechanistic (i.e. repeated evolution of the same active site) and transformational (i.e. the same chemical reaction is catalysed by unrelated enzymes), although overlaps are not infrequent.
One of the most remarkable enzymatic convergences involves carbonic anhydrase, which plays a central role in photosynthesis, respiration and other vital processes and is thus one of the key enzymes in life. Based on its critical functions it seems fair to assume that it would have been one of the first enzymes to evolve but the opposite is actually true, with independent evolution of different carbonic anhydrase types in different groups. To date it appears that this enzyme has evolved independently six times, five employing zinc and one other using the related element cadmium.
Alcohol dehydrogenases are a group of enzymes found in many groups of organisms, where they often serve to detoxify alcohol. They may well represent an example of molecular convergence, as several mammals and even insects (e.g. fruit flies) consume alcohol that must be broken down by such enzymes.
Lysozyme, a family of enzymes that can damage bacterial cell walls by cleaving the linkages between their components, shows interesting molecular convergences in terms of substitution of amino acids at key sites. Not only does lysozyme have an antibacterial function and can be found in tears, saliva, egg white or human milk, but it has also been recruited several times to assist in the breakdown of plant material that is generally difficult to digest. Foregut fermentation employing lysozyme activity can be found in ruminants, monkeys and some birds (e.g. the hoatzin), and this too is a fine example of evolutionary convergence.