Sulphite reductase

Figure 10.3 Crystallographic structures of the most important iron-sulphur centres in proteins (a) dimeric centre of ferrodoxin from Spinacia oleracea [54], (b) trimeric centre of ferredoxin from Bacillus schlegelii [55], (c) cubane cluster of nitrogenase reductase from Azotobacter vinelandii [56], (d) nitrogenase octameric cluster from Azotobacter vinelandii [57], (e) nitrogenase octameric cluster from Clostridium pasteurianum [58], (f) MoFe cluster of nitrogenase from Azotobacter vinelandii [59], and (g) active centre of sulphite reductase from Escherichia coli [60]...

The central macrocyclic rings of vitamin [148] and other similar natural products [149] have been constructed by using unique methodologies, e.g. electrochemical oxidation. As a special example of macrocyclization, here photo-cyclization is discussed. Battersby and coworkers [149 b] efficiently synthesized sirohydrochlorin (249), an isobacteriochlorin isolated as the metal-free prosthetic group of sulphite reductase. Photochemical treatment of the seco precursor 247 for 3 to 4 days yielded 24S which was hydrolyzed to 249 (Scheme 83). 

Jiranek, V, Langridge, R, Henschke, P. A. (1996) Determination of sulphite reductase activity and its response to assimilable nitrogen status in a commercial Saccharomyces cerevisiae wine yeast. Journal of Applied Bacteriology, 81, 329-336. 

ZamboneUi, C., MutineUi, P., Pacchetti, G. (1975) Biosynthesis of sulphur amino acids in Saccharomyces cerevisiae. I. Genetic analysis of leaky mutants of sulphite reductase. Archives of Microbiology, 102, 247-251. 

The sequence of the various steps between uroporphyrinogen III and cobyrinic acid has been the subject of much recent work. A major advance in this area was the observation that sirohydrochlorin (77), the iron-free prosthetic group of the enzyme siroheme, could be modified to accommodate its role as a biosynthetic intermediate. Subsequently a dimethyl isobacteriochlorin (factor II) isolated from P. shermanii was shown to be identical with sirohydrochlorin from E. coli sulphite reductase. The complete stereostructure of (77) was elucidated by a series of biosynthetic experiments using [4- C]- and [5- C]-ALA and independently by more classical structural arguments (B-79MI10401, B-79MI10402>. Sirohydrochlorin labelled biosynthetically from [4- C]ALA and [ CHsJmethionine is incorporated into cobyrinic acid by cell-free extracts of F. shermanii without loss or migration of label. 

Eschenmoser went on to demonstrate how it would be possible to generate the simplest chromophore type, the Fe(II) complex of sirohydrochlorin (the cofactor of nitrite and sulphite reductase), from glycine nitrile in a reaction sequence consisting of only five steps. The main conclusion that Eschenmoser comes to is that the arrangement of acetic and propionic acid side chains around the periphery of the pigments of life (i.e. the type III arrangement) corresponds to the thermodynamically favoured structure type. It has to be borne in mind that for this to happen (and presumably for there to be large amounts of porphyrins and other tetrapyrroles available as prebiotic... 

The mechanisms involved are not fully worked out. It appears that all the intermediates remain bound to protein carriers. Evidence has been obtained that the initial reduction step sulphate reductase) involves two reactions, a low molecular weight hydrogen transporting protein, and reduced FAD as the electron donor. The details of the further reduction steps are even more obscure. It has been suggested that the whole 6-electron oxidoreduction is catalysed by a single eniyme [sulphite reductase) and that reduced NADP is the electron donor. 

A large elevation of Hey in body fluids and tissues is found in several genetic enzyme deficiencies, the homocystinurias. These include cystathionine /3-synlhase deficiency [9], the remethylation defects due to deficiency of MTHF reductase [10], methionine synthase and methionine synthase reductase deficiencies, as well as defects of intracellular cobalamin metabolism [11], namely the cblF, cblC and cblD defects. It is noteworthy that low levels of total Hey (tHcy) have been described in sulphite oxidase deficiency [12]. 

Five proteins containing molybdenum are known nitrate reductase, nit-rogenase, xanthine oxidase, aldehyde oxidase and sulphite oxidase. They also contain iron, and the first four are best classified as multi-enzyme systems. Early studies on xanthine oxidase used a number of important ESR techniques, particularly rapid freeze kinetic methods and isotopic substitution in metalloproteins. This work has been reviewed [38, 39], Nitrogenase is the subject of considerable recent interest since it contains detectable iron-sulphur centres but as there is some disagreement at present concerning the interpretations of the results readers are referred to the original literature [40-42]. 

Heme Cytochrome c Cytochrome c oxidase, cytochrome c peroxidase, cellobiose dehydrogenase, nitrate reductase, sulphite oxidase, theophylline oxidase, cytochrome 62... 

Adams CA, Warnes GM, Nicholas DJD (1971) A sulphite-dependent nitrate reductase from Thiobacillus denitrificans. Biochim Biophys Acta 234 398-406... 

Yamanaka T, Yoshioka T, Kimura K (1981b) Purification of sulphite-cytochrome c reductase of Thiobacillus novellus and reconstitution of its sulphite oxidase system with the purified constituents. Plant Cell Physiol 22 613-622... 

Interestingly, the sirohydrochlorin can chelate iron to give siroheme which is the cofactor for the enz)nnes sulphite and nitrite reductase, used to reduce sulphite to sulphide and nitrite to ammonia, respectively, in certain organisms (e.g. Esherichia coif). Meanwhile, the dihydroisobacteriochlorin undergoes further methylation at the mcse-carbon C-20 to give precorrin-3. The use of the term "precorrin" was introduced by Battersby to denote intermediates on the pathway that precede the formation of the fully formed corrin macrocycle of vitamin B]2- The number suffix denotes how many C-methyl groups have been introduced into uro gen III. 

Another important naturally occurring pteridine is molybdopterin, apparently (9.28), which is the coenzyme of xanthine dehydrogenase, aldehyde oxidase, nitrate reductase, sulphite oxidase and presumably other enzymes that need both molybdenum and iron to function (Johnson and Rajagopolan, 1982). Xanthopterin, a co-lymphokine (p. 182), inhibits proliferation of lymphocytes (Ziegler 1983). 

The sulphate reductase factor which has already been mentioned was the first of these polypeptide dithiol-disulphide cofactors to be recognized . In this case the reduction of PAPS to PAP and sulphite was shown possible with a dithiol reductant such as dihydrolipoate or with NADPH and two protein components. One of the protein factors was not inactivated by heating. Incubation of the two protein fractions with NADPH generated... 

Nitrite has an inhibitory effect on SRB, mainly because (a) nitrite is toxic to SRB, and with their nitrite reductase, the bacteria will produce a detoxifying reaction. The end result is that while the bacteria are still aUve, no growth happens and their sulphate reduction activity will be inhibited, (b) Nitrite can directly affect the enzyme required for reducing sulphite to sulphide see [63]. 

Further work has been reported - with Fe-Mo models for nitro-genase, and a molecular mechanism has been proposed for the action of molybdenum in enzymes. In all reactions catalysed by Mo enzymes, the product and substrate differ by two electrons and two protons (or some multiple thereof). The co-ordination chemistry of Mo suggests that there is a distinct relationship between acid-base and redox properties of Mo complexes, and that a coupled electron-proton transfer (to or from substrate) may be mediated by Mo in enzymes. Each of the molybdenum enzymes (nitrogenase, nitrate reductase, xanthine oxidase, aldehyde oxidase, and sulphite oxidase) is discussed and it is shown that a simple molecular mechanism embodying coupled proton-electron transfer can explain many key experimental observations. 

The details of the reaction are not clear although the fact that hydroxylamine is reduced by nitrite reductase has led to the suggestion that the reaction proceeds in two electron steps and that hyponitrite and hydroxylamine are intermediates. However, these compounds have not been detected in nitrite reducing systems and if they are involved they apparently never leave the enzyme surface. An alternative view is that all six electrons are transferred at one enzyme surface—a most unusual reaction (see p. 174 for discussion of sulphite reduction). 

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