Quinohemoprotein amine dehydrogenase (QHNDH) from Paracoccus denitrificans is a heterotrimer consisting of alpha, beta and gamma chains . The alpha chain has a four-domain structure that includes a dihaem cytochrome c, the beta chain forms a 7-bladed beta-propeller that is part of the enzyme active site, and the gamma chain contains the redox factor cysteine tryptophylquinone (CTQ).
The beta chain of QHNDH structurally resembles the 7-bladed beta propeller of the H chain of the periplasmic quinoprotein methylamine dehydrogenase (MADH), found in methylotrophic bacteria . MADH is a heterotetramer consisting of two heavy (H) chains and two light (L) chains, and contains the redox cofactor tryptophan tryptophylquinone (TTQ). There is no similarity between the quinone-containing chains of MAD and QHNDH.
The beta-propeller structure found in MAD and QHNDH is similar to the YVTN (Tyr-Val-Thr-Asn) repeat that folds into a beta-propeller found in the N-terminal domain of archaeal surface layer proteins, which help protect cells from extreme environments .