Question

Naturally occurring amino acids have configuration:

• A. D
• B. L
• C. either D or L
• D. neither D nor L

Answer 1

Answer: (C)

either D or L

About $500$ naturally occurring amino acids are known (though only $20$ appear in the genetic code) and can be classified in many ways. These are all $L$-stereoisomers ("left-handed" isomers), although a few $D$-amino acids ("right-handed") occur in bacterial envelopes, as a neuromodulator ($D$-serine), and in some antibiotics.