But, in nature, this is not the case.
What do you mean 'in nature', are you claiming that racemases are synthetic? these enzymes occur in nature, but I have no problem agreeing that they would not have been present when life first arose, if that is what you were getting at.
Threre are specific control mechanisms modulating cellular activity. What happens if there is no control? This is when we develop some diseases known as Metabolic disorders. For example, if Phenyl alanine is not metabolized, then it accumulates. The clinical condition is known as Phenylketonuria.
I don't see what this has to do with chirality.
Random chemical reactions cannot and should not occur. If they did, then there will be chaos and severe metabolic disorders.
Well actually they occur a lot, and there are a number of systems in the cell to cope with them when they do. The entire point of many enzymes is to catalyse reactions which might otherwise occur randomly but only rarely.
In the brain, Phenylalanine is converted into hydroxy phenylalanine by the enzyme phenyl alanine hydroxylase. L-Phenylalanine is the substrate. But, D-Phenylalanine is an inhibitor of this enzyme.
Which is interesting but one D-amino acid acting as an inhibitor of a specific enzyme is by no means clear evidence that D-amino acids are general inhibitors of enzyme activity.
TTFN,
WK