Why only L amino acids?

gc genecutl at mendel.berkeley.edu
Sun Jul 10 18:34:09 EST 1994


In article <jmack-070794234545 at jmack.pipeline.com>, jmack at pipeline.com
(John Mack) wrote:

> Yes. We know L-amino acids fold into right-handed helices. (And vice
> versa.) 
> 
> Therefore, look for forces (coriolis? asymmetric surface catalysts?) that
> favor right-handed helices. 
> 
> If right-handed helices are favored in one hemisphere due to the coriolis
> force, then that may be the hemisphere where these were first incorporated
> into successful, Darwinian replicating, nano-machines which eventually
> became living things.
> 
> Thus, it is just fortuitous that life began in the hemisphere that favored
> right-handed helices and also L-amino acids. After all, life would be just
> the same with D-amino acids and left-handed helices.
> 
> Maybe the L and D nanomachines existed simultaneously on Earth and
> dominated their respective hemispheres! But, for some unknown reason (a
> comet striking Earth?) the D nanomachines never developed into living
> things. The L machines did. The living beings invaded the entire Earth
> afterward and chaged conditions such that D nanomachines could not compete
> and never became truly alive!

But on a more serious note, if we accept that organisms could have first
evolved with either exclusively D or exclusively L amino acids, the
existence of an exclusively L-a.a. world is easy to understand even in the
absence of selection pressure one way or the other, given the often
neglected power of genetic drift.

Why, though, should there have been organisms with exclusively one
chiralty in the first place?  Sure, if you wanted to construct a right
handed a-helix, you'd have fewer amino acids to choose from, but to make
up for that you'd probably have novel secondary structures that aren't
possible with one chiralty of amino acids alone.



-- 
--gc

I know you like rock 'n' roll. Have you considered overdosing
on heroin, like many of your pop-star favorites?  --Life in Hell



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