Alanine (abbreviated as Ala or A; encoded by the codons GCU, GCC, GCA, and GCG) is an α-amino acid that is used in the biosynthesis of proteins. It contains an α-amino group (which is in the protonated −NH3+ form under biological conditions), an α-carboxylic acid group (which is in the deprotonated −COO− form under biological conditions), and a side chain methyl group, classifying it as a nonpolar (at physiological pH), aliphatic amino acid. It is non-essential in humans, meaning the body can synthesize it. The L-isomer (left-handed) of alanine is one of the 20 amino acids encoded by the human genetic code. L-Alanine is second only to leucine in rate of occurrence, accounting for 7.8% of the primary structure in a sample of 1,150 proteins. The right-handed form, D-Alanine occurs in bacterial cell walls and in some peptide antibiotics.