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New Enzymes by Mutation?

That genetic variations including mutations do occur in populations of organisms is a fact which we can all observe. But the changes which occur are always variations on a pre-existing theme, and they are limited in scope. The horse continues as horse, the fruit fly as fruit fly, E. coli as E. coli, etc. Recalling the enzyme and control systems involved in protein synthesis, consider whether they could have originated by mutations.

The bacterium, E. coli, uses the enzyme a-galactosidase to split the milk sugar, lactose, into its two constituent sugars, galactose and glucose which the bacterium can digest. When glucose is already present, it fits into a special slot in the enzyme and changes the enzyme's shape so that it cannot split lactose and so produce and excess of glucose. When glucose is absent, the enzyme is left free to split more lactose.²⁵

If random mutations were responsible for the design of this system, the lactose-splitting function certainly had to evolve first. Then if glucose were plentiful, the enzyme system would make the cell inefficient by continually producing large quantities of excess glucose from lactose. The most likely change to occur by mutation and be selected would be a single mutation to destroy the lactose-splitting function, a result easily accomplished by any one of many possible mutations in the gene for a-galactosidase.

This would surely happen long before the complex series of mutations could occur to produce the clever and complicated control system described above. then if glucose again became scarce, the enzyme would have to be re-evolved, or the bacterial strain would perish. Upon re-evolving by trial and error mutations, it would also by accident have the control feature incorporated. does this sound like science fiction?

Or we might hypothesize a long period of fluctuating glucose supply with fluctuating lactose-splitting capacity for the enzyme, not total destruction. And in the meantime a lengthy succession of random mutations affecting the backside of the enzyme molecule finally produces a slot into which glucose can fit to deactivate the enzyme. The whole scheme is completely speculative and highly unlikely to the point of incredibility. This is the same type of reasoning which Darwin made popular with The Origin of Species, full of assumed conditions, processes, and imaginations. And when one's speculations have been recorded, the origin of the initial lactose-splitting enzyme remains unexplained, as does also the origin of the fantastically complex set of enzyme systems for the metabolism of the glucose produced the lactose-splitting enzyme.

Indeed, just as it is not possible to deal independently with the origins of an enzyme system and its control mechanisms, it is also not possible to consider any enzyme system or function of DNA or RNA entirely independently of the whole cell and the entire organism. The observed facts are most reasonably understood in terms of created design systems, rather than as products of unplanned, undirected mutations and natural selection.

References

25. Stryer, Lubert, Biochemistry (W.H. Freeman, San Francisco, 1975), pp. 679ff.