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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?

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3 Answers

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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.

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Don't most viruses rely on the availability of some amino acids that humans don't need or use? For example it is to my belief that the herpes virus relies on the availability of a amino acid that is not used by humans and is commonly found in chocolate? – Gabriel Fair Aug 14 '12 at 13:54
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Its pretty rare to require a nonstandard aa at all and a fast growing virus would be strongly inhibited by such a requirement. The viral proteins I know of don't require a special amino acid. I'm sure there are exceptions, but they are rare I believe. – shigeta Aug 14 '12 at 14:41
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many D-amino acids are not used at all, although some have rather elaborate metabolism, both synthesis and utilization: D-alanine for example.

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This sounds interesting. Is there any chance you could add some details (and/or links) to your answer? – Ilmari Karonen May 6 '12 at 16:50
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You may also be interested in D-amino-acid oxidase (EC 1.4.3.3), 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 oxidase (EC 1.4.3.3) catalyzes the reaction which results in the following transformation

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.

Additional Reference

  • 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]
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