This entry represents a multi-helical structural domain consisting of two structural repeats (duplication) of a 3-helical motif. This domain can be found in both eukaryotic and prokaryotic haem oxygenases [1, 2], in TENA/THI-4 proteins that lack the haem-binding site [3], and in coenzyme PQQ (pyrrolo-quinoline-quinone) biosynthesis protein C (PqqC) [4].

Haem oxygenase (1.14.99.3) (HO) is the microsomal enzyme that, in animals, carries out the oxidation of haem, cleaving the haem ring at the alpha-methene bridge to form biliverdin and carbon monoxide. Biliverdin is subsequently converted to bilirubin by biliverdin reductase. In mammals there are three isozymes of haem oxygenase: HO-1 to HO-3. The first two isozymes differ in their tissue expression and their inducibility: HO-1 is highly inducible by its substrate haem and by various non-haem substances, while HO-2 is non-inducible. Haem oxygenase is also present in certain bacteria, where it is involved in the acquisition of iron from the host haem.

The THI-4 protein is involved in thiamine biosynthesis, while TENA is one of a number of proteins that enhance the expression of extracellular enzymes, such as alkaline protease, neutral protease and levansucrase.

Coenzyme PQQ (pyrrolo-quinoline-quinone) biosynthesis protein C (PqqC; 1.3.3.11) is required for the synthesis of PQQ, where PQQ is a prosthetic group found in several bacterial enzymes, including methanol dehydrogenase of methylotrophs and the glucose dehydrogenase of a number of bacteria.

1. Crystal structure of human heme oxygenase-1 in a complex with biliverdin. Biochemistry 43, 3793-801
2. Crystal structure of heme oxygenase from the gram-negative pathogen Neisseria meningitidis and a comparison with mammalian heme oxygenase-1. Biochemistry 40, 11552-8
3. The 2.35 A structure of the TenA homolog from Pyrococcus furiosus supports an enzymatic function in thiamine metabolism. Acta Crystallogr. D Biol. Crystallogr. 61, 589-98
4. Quinone biogenesis: Structure and mechanism of PqqC, the final catalyst in the production of pyrroloquinoline quinone. Proc. Natl. Acad. Sci. U.S.A. 101, 7913-8

This entry represents a multi-helical structural domain consisting of two structural repeats (dup
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