RESUMO
The AF0174-AF0176 gene cluster in Archaeoglobus fulgidus encodes a putative oxyanion reductase of the D-type (Type II) family of molybdo-enzymes. Sequence analysis reveals that the catalytic subunit AF0176 shares low identity (31-32%) and similarity (41-42%) to both NarG and SerA, the catalytic components of the respiratory nitrate and selenate reductases respectively. Consequently, predicting the oxyanion substrate selectivity of AF0176 has proved difficult based solely on sequence alignments. In the present study, we have modelled both AF0176 and SerA on the recently determined X-ray structure of the NAR (nitrate reductase) from Escherichia coli and have identified a number of key amino acid residues, conserved in all known NAR sequences, including AF0176, that we speculate may enhance selectivity towards trigonal planar (NO(3)(-)) rather than tetrahedral (SeO(4)(2-) and ClO(4)(-)) substrates.
Assuntos
Archaeoglobus fulgidus/enzimologia , Compostos Organometálicos/metabolismo , Oxirredutases , Conformação Proteica , Pteridinas/metabolismo , Sequência de Aminoácidos , Proteínas Arqueais/química , Proteínas Arqueais/genética , Proteínas Arqueais/metabolismo , Archaeoglobus fulgidus/genética , Sítios de Ligação , Coenzimas , Metaloproteínas , Modelos Moleculares , Dados de Sequência Molecular , Cofatores de Molibdênio , Família Multigênica , Oxirredutases/química , Oxirredutases/genética , Oxirredutases/metabolismo , Alinhamento de Sequência , Especificidade por Substrato , Tungstênio/químicaRESUMO
A number of biochemically distinct systems have been characterized for the microbial reduction of the oxyanions, selenate (SeO(4)(2-)) and nitrate (NO(3)(-)). Two classes of molybdenum-dependent nitrate reductase catalyse the respiratory-linked reduction of nitrate (NO(3)(-)) to nitrite (NO(2)(-)). The main respiratory nitrate reductase (NAR) is membrane-anchored, with its active site facing the cytoplasmic compartment. The other enzyme (NAP) is water-soluble and located in the periplasm. In recent years, our understanding of each of these enzyme systems has increased significantly. The crystal structures of both NAR and NAP have now been solved and they provide new insight into the structure, function and evolution of these respiratory complexes. In contrast, our understanding of microbial selenate (SeO(4)(2-)) reduction and respiration is at an early stage; however, similarities to the nitrate reductase systems are emerging. This review will consider some of the common themes and variations between the different classes of nitrate and selenate reductases.
