For proteins, living systems are specific for L-AAs but for carbohydrates that degree of specificity is not observed…how do we explain it in the context of evolution..?

Also please tell me what is the degree of specificity for D-sugars & L-AAs…i mean when the specificity can be violated naturally, i’m interested in knowing that…
I got that D-Alanine is a bacterial marker [ i guess due to its role in murein formation of bacterial cell wall] but i don’t know if any ‘polypeps’ contain D-AAs , & if yes …upto what extent & all…

Similar querries for L glucose…& alph & beta sugars…

Please help me… i’m searching the web…suggest any good links too…

Thanking in advance,

hrushikesh