Where did I say that? I think you are misremembering.
In the lab the experimenter sets things up to maximize his success. He puts his gene on an inducible plasmid, for example, then grows the cells carrying his gene, carefully selected to allow him to detect the phenotype, under controlled conditions in a monoculture. Assuming the experimenter is selecting from a random library he has made, he uses conditions he has optimized for the selection to identify the best candidates, then if he is smart he rescreens them in freshly transformed cells. Do you get what I am saying?
Most random newly emerging enzyme activities are weak. They are only useful to the cell, or even detectable, unless strongly overexpressed. The only reason that weak activity is not inactivated by the cell to reduce the cost of expression is because we, the human investigators, apply massive amounts of selection for the function.
Now I have said it is easy to recruit promiscuous enzymes. Harder to recruit from shared reactions but no overlapping promiscuous activity (trpF HisA). They were successful, probably because of their active site location. BioF and Kbl are similar in some ways but one releases CO2 and the other doesn’t. And the active site is constrained. All things I learned as we were going along, BTW. So it didn’t work for Kbl and BioF, even though overexpressed in a very sensitive assay…
The best lab example I know is this: Evolutionary Potential of (!/R)8-Barrels: Functional Promiscuity Produced by Single Substitutions in the Enolase Superfamily†Biochemistry (2003) 42:8387
In this study they found a single mutation that could convert an enzyme from one catalytic activity to a completely new one. One mutation. That’s the goal. For evolution to explain enzyme diversity you need to be able to convert to new functions (get new chemistry) with just a few (two or three max) mutations. The authors of this paper say:
Functional interconversion of homologous enzymes that
catalyze different chemical reactions has been accomplished
in Vitro with as few as four to eight substitutions (13 , 14 ).
However, the natural acquisition of a new reaction with this
number of substitutions is unlikely; during random accumulation
of the required mutations, deleterious mutations
are also expected to occur. In other words, the probability
of evolving a new enzymatic function that requires specific>
multiple mutations is low.
They succeeded in converting both the substrate preference and the reaction chemistry with a single mutation. But it was done on a multicopy plasmid that was overexpressed, so it is not at all clear it would have worked apart from the lab setting. And it was one of the beta barrel enzymes.
If you want to find an evolutionary story that will stump me you will have to go to experiments in the wild. Copley’s papers are a good place to start. But I can tell you that all the ones I have seen are usually a shift in substrate preference with a shared chemistry. Swamidass and friends cited one of them in their review. That is a much better place to argue your case for evolution. Because it really did evolve unaided by human engineering or environmental control.
There are other places where it has worked, usually involving several mutations an
There are other places where it has worked, usually involving several mutations and a