I would like to put in my two bits as well. After all I have to teach this
stuff too.
Present life forms are locked into chemistry that requires one isomer has to
be chosen rather than both by living organisms.
1. A good catalyst such as an enzyme built of protein, RNA or
anything else will have to interact at multiple points with the substrate in a
stereospecific manner. If either the substrate or product is potentially an
optical isomer, only one will be involved. If you want the second isomer,
another enzyme is required.
2. A protein which contains some L amino acids and some D amino acids will be
sterically different from an all L or all D protein, and therefore at best a
different catalyst. Ditto if the enzyme is made of RNA-all one optical
isomer for each monomer
3. For a double helix to form e.g in primordial RNA life, it is imperitive
that all one optical isomer be used. Ditto for a helix of any sort or beta
sheet in proteins.
4. Many amino acids are made from other amino acids by modification of the
side chain.
5. Many of the same enzymes or homologous enzymes are used for introducing
asymmetry, e.g. at the keto acid to amino acid step. This allows a living
organism to minimise the acquisition and mantainance costs for another amino
acid,
If a life form based on D amino acids did arise early, it would have to be an
independent event. The chances of two separate lifeforms independently arising
having equal selective advantage would have to be near enough to zero.
Under these circumstances to ask why L rather than D seems a non question!?
Chris Driver
Deakin University
Australia
Chris Driver, Ph D
School of Biology and Chemistry, Rusden Campus
Deakin University
662 Blackburn Rd
Clayton, VIC, 3168
AUSTRALIA
