Baptist Vine

I have read about Urey and Millers amino acid producing experiments.

Concerning Urey and Miller, I remember reading one 'creationist' book by the Dutch biochemist A.E. Wilder Smith, wherein Smith writes concerning Urey and Miller that although they may have created amino acids in the lab, the mixture they created was recemic - Urey and Millers pool of amino acids contained both left and right handed molecules.

Smith went on to write that the kind of amino acids found in living systems including the human body were all of one type only, exhibiting one type of chirality (reflecting the plane of plane polarized light either left or right). I forget what rotation was claimed for living systems, but it was claimed that living systems required either all left or all right handedness, not both.

The argument being made was essentially that a mixture of both right and left handed amino acids was insufficient for a living system.

Can anyone here provide information on the following:

Were Urey and Miller's amino acids recemic or not, and is this even significant?

To what extent is chirality a requirement or not of human biochemistry, ie is it true that our amino acids are all leavo or dextro rotary, and if so, are there any other types of molecules important to human biochemistry that exhibit a preference for solely one kind of chirality?

Coragyps

Hmmm. Timing is everything! I just finished a post elsewhere on this same subject. I once knew quite a bit about chirality (= handedness) in general, but I've forgotton some and haven't kept up, particularly as it relates to abiogenesis. The only emphasis in the oil field on handedness is "Righty tighty, lefty loosey" so I'm a little out of the loop.
That said, I can give you OK answers, but I'm just starting to dig for the current best answers to the questions that are crowding up right behind the specific ones you asked. They were certainly racemic, meaning an equal mix of right- and left-handed molecules. Chirality is a little like the old creationist canard that "life can only come from life" - there really must be some handedness somewhere around in order to form a sample of amino acid with one handedness predominating. This source of handedness could possibly be many things: polarized light, magnetic fields + earth's rotation, the whirlpool in some cosmic bathtub , or naturally occurring crystals of minerals such as quartz, if you can somehow pick the lefties from the righties. Let me read up on this some more before I speculate myself into a hole, though.
Chirality is an absolute requirement for life like we know it: proteins wouldn't fold reliably, enzymes wouldn't work - on and on. All of earthern life's amino acids are the "L" form, and the "D" forms don't get biosynthesized, AFAIK. (L=levo = left, D = dextro = right. As if it weren't confusing enough without the Latin.) I saw a calculation once on just how confused it would be if you had two choices, D and L, for each of the twenty standard amino acids in a protein - it doesn't look real workable with 2^20 additional complications in manufacturing a short little peptide.
All the "naturally occurring" sugars on earth are of the D form, though this doesn't necessarily mean that they are "opposite" in any sense to the amino acids. The names have to do with how solutions of the compounds rotate polarized light, and I don't know (today) how they relate to the absolute configurations of the various classes of biomolecules. The names were assigned long before the real shapes were known, anyway.

Starches, cellulose, and lots of the odd things on the surfaces of cells are made of sugars strung together into polymers, so they're all chiral, too. DNA and RNA contain sugars, and I'm sure they wouldn't coil the right direction (or even coil at all) without that underlying handedness.
Fats aren't chiral - my belly is quite symmetric, thank you.

lpetrich

I've considered possible sources for biomolecule chirality and I find them almost impossibly weak. I suspect that it was a matter of choosing one handedness or the other, because having one handedness makes it easier to get a good shape for self-reproducing. Consider DNA's familiar double-helix structure. It might be possible to do self-reproduction with racemic DNA, but the structure would look much more complicated, with the helicity reversing with each handedness reversal.

So it might be as accidental as screw threads, though screw-thread handedness may have been determined by what handedness is easiest to tighten. Since we have a handedness asymmetry that is usually right-handedness, that could impose an asymmetry on the design of screw threads.

In particular, if you use a wrench to tighten a right-handed nut that is in front of you, you will pull inward with the screw-handedness hand and push outward with the other hand. Pulling is more precise than pushing, which is an important consideration when tightening fasteners. And most of us being northpaws means that the right hand would be the most suitable. And the right hand pulling means right-handed screw threads.

It would be interesting to see if similar biomechanical considerations apply to turning screwdrivers -- which direction is easier with the right hand?

A possibly better example is which side of the road to drive on. Nobody knows for sure why some places have selected the right side and some the left side -- but it's clear that one side or the other has had to be selected.

Coragyps

That's easy. The Brits, Singaporans, and Indonesians are out to kill my left-looking pedestrian ass, that's why they chose the wrong side!

Camaban

CURSES!

Our evil plan has been discovered!

That was why Australia got so well started and became such a great tourist destination. we wanted to attract yanks to our soil to be run down by motorists!

Eventually, all Americans will have come here and been run down, allowing Mother England to reclaim America

muhuwahahahahahahahahaha!