Why is free ribose not reduced to deoxyribose rather than the reduction occuring on ribonucleotides
Clarification

I assume that question is posed in the knowledge that ribose is an intermediate in the dark reaction of photosynthesis and can be synthesized in the hexose monophosphate shunt in animals. Hence I assume that the question is not why ribose precedes deoxyribose in the metabolism of mammals and other living things. Rather, the question would seem to be why ribose is not converted directly to dexoyribose, but only after ribonucleotides have been formed (in the ribonucleotide reductase reaction).

Possible Answer

Question as to why a metabolic pathway has one form rather than another can sometimes have a chemical explanation, but in this case I suggest answers can only be speculative.

Thus, one possible answer could be:


  There is no need for free deoxyribose in the cell, and that if free
  deoxyribose were produced there would be potential competitive
  inhibition of enzymes that catalyse reactions involving ribose.


If an ‘RNA world’ preceded the modern ‘DNA world’, then one could argue that:


  The simplest way for the shift from an RNA genome to a DNA genome would by the introduction of a
  specific reaction for the reduction of the ribose in nucleotides. The
  fact that the base that differentiates DNA from RNA — thymine —
  is also synthesized as a nucleotide (see my answer to this
  question)
  is consistent with (but does not, of course, prove) this idea.