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Two structurally and kinetically distinct forms of Wolinella succinogenes nitrite reductase.

ABSTRACT: It is shown that the oxidized form of the hexa-haem nitrite reductase of Wolinella succinogenes exists in two structurally and functionally distinct forms, termed 'resting' and 'redox-cycled'. The nitrite reductase as initially isolated, termed 'resting', has five low-spin ferrihaem groups and one high-spin ferrihaem group. The reduction of these haem groups by Na2S2O4 occurs in two kinetically and spectrally distinct phases. In the slower phase the haem groups are reduced by dithionite with a limiting rate of 4 s-1. If the enzyme is re-oxidized after reduction with dithionite or with methyl viologen, the resulting ferric form, termed 'redox-cycled', possesses only low-spin haem centres and a rate of reduction in the slower phase that is no longer limited. In the resting form of the enzyme the high-spin ferrihaem group is weakly exchange-coupled to a low-spin haem group. It is proposed that in the redox-cycled form the exchange coupling occurs between two low-spin ferric haem groups. This change in spin state allows a more rapid rate of electron transfer to the coupled pair.

SUBMITTER: Blackmore RS

PROVIDER: S-EPMC1149542 | biostudies-other | 1990 Oct

REPOSITORIES: biostudies-other

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Project description:C(4)-dicarboxylate transport is a prerequisite for anaerobic respiration with fumarate in Wolinella succinogenes, since the substrate site of fumarate reductase is oriented towards the cytoplasmic side of the membrane. W. succinogenes was found to transport C(4)-dicarboxylates (fumarate, succinate, malate, and aspartate) across the cytoplasmic membrane by antiport and uniport mechanisms. The electrogenic uniport resulted in dicarboxylate accumulation driven by anaerobic respiration. The molar ratio of internal to external dicarboxylate concentration was up to 10(3). The dicarboxylate antiport was either electrogenic or electroneutral. The electroneutral antiport required the presence of internal Na(+), whereas the electrogenic antiport also operated in the absence of Na(+). In the absence of Na(+), no electrochemical proton potential (delta p) was measured across the membrane of cells catalyzing fumarate respiration. This suggests that the proton potential generated by fumarate respiration is dissipated by the concomitant electrogenic dicarboxylate antiport. Three gene loci (dcuA, dcuB, and dctPQM) encoding putative C(4)-dicarboxylate transporters were identified on the genome of W. succinogenes. The predicted gene products of dcuA and dcuB are similar to the Dcu transporters that are involved in the fumarate respiration of Escherichia coli with external C(4)-dicarboxylates. The genes dctP, -Q, and -M probably encode a binding-protein-dependent secondary uptake transporter for dicarboxylates. A mutant (DcuA(-) DcuB(-)) of W. succinogenes lacking the intact dcuA and dcuB genes grew by nitrate respiration with succinate as the carbon source but did not grow by fumarate respiration with fumarate, malate, or aspartate as substrates. The DcuA(-), DcuB(-), and DctQM(-) mutants grew by fumarate respiration as well as by nitrate respiration with succinate as the carbon source. Cells of the DcuA(-) DcuB(-) mutant performed fumarate respiration without generating a proton potential even in the presence of Na(+). This explains why the DcuA(-) DcuB(-) mutant does not grow by fumarate respiration. Growth by fumarate respiration appears to depend on the function of the Na(+)-dependent, electroneutral dicarboxylate antiport which is catalyzed exclusively by the Dcu transporters. Dicarboxylate transport via the electrogenic uniport is probably catalyzed by the DctPQM transporter and by a fourth, unknown transporter that may also operate as an electrogenic antiporter.

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Two structurally and kinetically distinct forms of Wolinella succinogenes nitrite reductase.

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