As indicated by this question, most of the amino acids in the human body have the L-chirality. As enzymes also have handedness, what happens to the D-amino acids that end up within the human body? Are they simply excreted? Are there enzymes (perhaps not in humans) that convert a D-amino acid to a L- one?
For the most part they are not used. there are amino acid racemases, which interconvert L- and D- forms of some specific amino acids, which may be used in some particular biosynthetic or metabolic pathways.
In particular I'm thinking of firefly luciferase which uses D-Cysteine as a re-dox reagent to regenerate the luciferin substrate that the light - generating enzyme consumes. see slide 4 here.
Nonribosomal peptide synthases often use D amino acids too.
I don't believe that d- amino acids are really incorporated into proteins though.
You may also be interested in D-amino-acid oxidase (EC 22.214.171.124), a flavoprotein (FAD) highly specific for the D-form of amino acids, which was discovered by Hans Krebs in 1935 (see here), and which has a wide distribution (including in humans).
The enzyme has been very thoroughly investigated, in particular by Massey & co-workers (see here for example)
D-amino acid + H2O + O2 = 2-oxo carboxylate + NH3 + H2O2
2-Oxo-carboxylates are what used to be called α-keto acids. For example, pyruvate is produced from D-Ala.
The product of the enzymatic reaction is the imino-acid which is nonenzymatically hydrolyzed to a-keto acid (see Pollegioni et al., 1994, and references therein)
An excellent review
D-Amino Acid Oxidase: Physiological Role and Applications
by S. V. Khoronenkova & V. I. Tishkov,
Biochemistry (Moscow) is freely available from here
These authors have some intersting things to say about D-Serine, D-Proline and D-Alanine, and much more.
- Pollegioni L, Fukui K, Massey V. (1994) Studies on the kinetic mechanism of pig kidney D-amino acid oxidase by site-directed mutagenesis of tyrosine 224 and tyrosine 228. J. Biol. Chem. 269, 31666-31673. [pdf]