Tag Archive | sugar

The L-amino acids and the D-sugars in nature

These lines I found in a book  ” RIGHT HAND , LEFT HAND , The Origins of Asymmetry in Brains, Bodies, Atoms and Cultures ” By Chris Mc Manus (2002). His website is : http://www.righthandlefthand.com . A rather fascinating story : humans are made of L-amino acids and of D-sugars !
So , as I understand it , it would be better to use L-amino acids for supplementation , since D-amino acids would be not be used by the body  .

Amino acids come in 2 different forms called L ( for laevo , or left-handed ) and D ( for dextro , or right-handed ) . Every time an amino acid is added to the growing protein chain , it is an L-amino acid .The genetic code and the entire translation machinery are based only on L-amino acids .As far as we can tell , that is true for every organism on this planet . Such a complete depence on L-amino acids raises many fundamental questions for biology . Amino acids are not the only place where biochemical asymmetry can be found  . The sugars that make up our body , such as glucose , are also found in just one form , this time the D-form . Again , it is a perplexing truth deeply in need of explanation Just found this


Life as we know it builds its proteins primarily from the same 20 amino acids, although there are many other amino acids that might have been utilized. While it is important that the collection of amino acids used in proteins includes a sufficient number of small, large, hydrophilic, hydrophobic, and charged amino acids, the exact identities of the amino acids in each of these classes may not be critical. Moreover, the amino acids utilized for protein synthesis by familiar life are all L-amino acids, and there is no reason to think that D-amino acids could not have been utilized instead.


In chemical processes, left- and right-handed molecules are generally formed in equal amounts, unless chiral reagents, catalysts, or surfaces are present to introduce a bias. Also, some organisms may synthesize the same chiral compound in different enantiomeric forms (e.g., bacterial peptide toxins). For the formation of regular structures when these monomers are assembled, however, it is generally more efficient to have the same handedness in all molecules. Thus, 100 percent of amino acids in terran proteins have the same absolute spatial relationship of the amino group, the acid group, and the side chain distinctive for the amino acid and are all designated “L”. Likewise, all of the sugars in nucleic acids have the opposite spatial relationship and are designated “D”. Enantiomeric excess (also called chirality, or handedness), the predominance of one enantiomer over the other for a given assembly of atoms, is therefore believed to be a universal feature of chemistry characteristic of living systems.

http://wavefunction.fieldofscience.com/2011/08/why-biology-and-chemistry-is-not.html (  Ashutosh (Ash) Jogalekar )

The same principle applies to chemistry. For instance the supreme being would never have been able to tell us why there are only twenty amino acids, why there are alpha amino acids instead of beta or gamma versions (which have extra carbon atoms), why amino acid stereochemistry is L while sugar stereochemistry is D, why there are four DNA bases with their unique structures, why nature chose phosphates (although Frank Westheimer comes close), why a given protein folds into only one unique functional structure, why water is the only solvent known to sustain life, and in general why the myriad small and large molecules of life are what they are. In retrospect of course one could provide several arguments for the existence of these molecules based on stability, function and structure but there is no way to predict these parameters prospectively.

Found this on a forum : 


Question : Why do all species use only L amino acids ? I learned in biochemistry that all living organisms only utilize the L configuration of amino acids, but if we were to somehow inject the D configuration into an organism could it utilize it and make proteins from it?

Answer ( an example ) : No, it could not utilize the D conformation because the D configuration would actually make a completely different product. They are sometimes used in proteins, though. The truth is, we really don’t know why the L is preferred in the body. It is theorized that the L configuration is slightly more stable than the R, and has been naturally selected to be the configuration of choice. It is also theorized that having all the amino acids in one conformation makes it easier for the body to produce proteins, rather than two.

Also this one on the forum : http://www.reddit.com/r/askscience/comments/zuddf/is_the_l_amino_acid_excess_due_to_d_amino_acids/

Question : Do the laws of physics always show complete symmetry between left and right? I’ve read recently that parity is in fact not conserved, and that electrons are ‘left-handed’. Does this mean that L and D amino acids are not true mirror images (since the electrons handedness are not mirrored), thereby have different properties and if so, can this explain why we have an L amino acid excess over D? Can anyone explain this a bit, or add some insight? I find it very interesting, but don’t really understand the physics. Thanks

Answer ( an example ) : Why are amino acids L? (this is my memory don’t know the current popular theory)The reason amino acids exist predominantly in the L configuration biologically has to do with efficiency, and production of proteins. A protein with a random mix of D and L would not be the same as all D or all L; so biologically it makes sense to just use all L or all D so we consistently have functioning proteins and are not wasting lots of energy on junk. The reason it turned out to be L everywhere is because we all evolved from the same ancestor and that was the luck of the draw most likely.

Why is there no particular chemical reason for L? molecular geometry does not keep track of electron spin only the 3 dimensional arrangement of atoms in space. Second it is just as easy to make D amino acids as L amino acids if you were to synthesize them with a catalyst that doesn’t select for stereochemisty, synthetic production of amino acids is almost always a 50/50 mix. Finally I never heard of electrons being left hand in fact electron spin can change and in atoms there are spin pairs.

Also posted by me on : http://discourse.soylent.me/t/soylent-amino-acids-and-training/7411/3