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Abstract:
Whilst widespread in the microbial world, the hybrid cluster protein (HCP)
has been paradoxically a long-time riddle for microbiologists. During three
decades, numerous studies on a few model organisms unravelled its structure
and dissected its metal-containing catalyst, but the physiological function of
the enzyme remained elusive. Recent studies on bacteria point towards a nitric
oxide reductase activity involved in resistance during nitrate and nitrite reduction
as well as host infection. In this study, we isolated and characterised a naturally
highly produced HCP class I from a marine methanogenic archaeon grown on
ammonia. The crystal structures of the enzyme in a reduced and partially oxidised
state, obtained at a resolution of 1.45 and 1.36-Å, respectively, offered a precise
picture of the archaeal enzyme intimacy. There are striking similarities with the
well-studied enzymes from Desulfovibrio species regarding sequence, kinetic
parameters, structure, catalyst conformations, and internal channelling systems.
The close phylogenetic relationship between the enzymes from Methanococcales
and many Bacteria corroborates this similarity. Indeed, Methanococcales HCPs
are closer to these bacterial homologues than to any other archaeal enzymes.
The relatively high constitutive production of HCP in M. thermolithotrophicus, in
the absence of a notable nitric oxide source, questions the physiological function
of the enzyme in these ancient anaerobes.
