Pooja kulhade last edited by
For B REASON:
I think the prevalence of L-amino acids in biological systems is a quirk of our evolutionary history; there's no reason D-amino acids wouldn't work chemically, but once early life forms started building proteins out of L-amino acids, L-amino acids became abundant an it became selectively advantageous to use L-amino acids because then you could use the amino acids you obtained by eating someone else.
It's a matter of compatibility; if you use D-amino acids, you have to build them all from scratch, because you're not going to find them in your environment. As long as you use L-amino acids, you don't even need to maintain the de novo synthetic pathways; you can just eat something that has those pathways (hence the 'essential' dietary amino acids).
As for carbohydrates, there are so many more stereoisomers, and organisms generally don't use the monomers they obtain in their diets as building blocks (but rather oxidize them for energy, and build the monomers they need using their own metabolic pathways), that there hasn't been the same selective pressure for standardization.
Reason for d :
If an amino acid has two amino groups and only one carboxyl group (like arginine and lysine), then they are positively charged (++ vs. -) at neutral pH. On the other hand, if amino acids have two carboxyl groups and only one amino group (like glutamic acid and aspartic acid), they are negatively charged at pH7 (-- vs. +). You are not correct that the groups are truly neutral. The carboxyl group is negatively charged in a solution much higher than the pKa of the carboxyl group (which is pH 2.48 for arginine). Same is true for the amino groups (pKb is at pH 11.5 for arginine). So, at pH 9.5, the ratio of the positively charged amino group to a neutral amino group is 100:1 (at the pKb it would be 50:50). At a pH of 4.48, the carboxyl group of arginine would be 1:100 negatively charged, versus neutral. So, at pH7, all of the carboxyl group is negatively charged (COO-) and all of the amino groups (2 of them) are fully positively charged (NH3+). Net charge is plus 1, in the case of arginine at pH7.
Reason for b :
Basically, the reason is that the biosynthetic enzymes which produce amino acids evolved to produce only the L form. It is an advantage to organisms to produce only one stereoisomer since utilizing both forms of the amino acid would require two enzymes, one recognizing the L-form and one recognizing the D-form, for each biochemical process involving the amino acid. Plus, by producing enzymes which are composed of only L-amino acids, these enzymes are able to catalyze reactions with very high stereoselectivity and stereospecificity, which is an essential feature of many biological reactions.
However, as to why all L as opposed to all D, biologists are still debating. Some claim that asymmetric crystals in clay may have lead to an overabundace of L-amino acids in the "primordial soup". Others believe that polarized light may have had an effect on the relative ratios of D- and L- amino acids. There is also the possibility life forms based on D- and L- amino acids evolving separately, then the L-amino acid-based life forms outcompeting the D-amino acid-based lifeforms (by chance or by genetic drift).
Still, there is only the possibility of random chance.
Maybe the first lifeform arbitrarily incorporated L-amino acids only, so life evolved using only L-amino acids. So the ultimate origin of chirality in biological systems still remains unknown.