Assimilatory pathways

Two major pathways are known for the reduction of sulfate. One is the assimilatory pathway, which reduces sulfate to the extent necessary for satisfying the nutritional requirements of the organism. In this pathway, which has been extensively studied in yeast by Robbins and Lip-mann (S68) and Bandurski and his colleagues 369, 370), sulfate is first activated in the presence of ATP by the enzyme ATP-sulfurylase to form adenosine 5 -phosphosulfate (APS). Then in a second reaction, APS is phosphorylated in the 3 position by ATP to form 3 -phosphoadenosine 5 -phosphosulfate (PAPS)... 

The initial six-electron oxidation of sulfide to sulfite is catalysed by a soluble, dis-similatory sulfite reductase that contains siroheme and at least one iron-sulfur center as prosthetic groups [83-85]. While similar enzymes in plants and in non-pho-totrophic bacteria usually function to reduce sulfite to sulfide in assimilatory pathways, the enzyme in photolithoautotrophically grown C. vinosum appears to function in the reverse direction, with the electrons from sulfide oxidation being delivered to an as yet unidentified acceptor. Evidence is also available for the... 

In this chapter, our goal is to review the practical aspects of enzyme activity measurements and their interpretation and to illustrate how such measurements have advanced our knowledge of marine N cycHng. While our main focus will be the assimilatory pathways found in marine photoautotrophs, we wiU also consider degradation pathways and secondary metabolism, and will point out other groups of marine organisms where enzyme activity has proven useful in examining aspects of the N cycle. 

All of the nitrite reductases just described convert NO2" to NO in a dissimilatory manner for energy extraction other nitrite reductases in assimilatory pathways convert N02 directly to NH4+ for synthetic purposes. Most of these are nonheme flavoproteins, but one has been found in Achromobacter fischeri (422) with properties similar to the dissimilatory reductases two hemes c per molecule of weight 95,000. The analogy... 

For the assimilatory pathway it was proposed (Meyer and Schultze, 1894) that the reduction of nitrite to ammonia proceeded by three steps, each step involving the transfer of two electrons with the production of hyponitrite and hydroxylamine as intermediates. Although belief in this three step reductive pathway was maintained for 60 years, current evidence shows that nitrite is reduced to ammonia by the enzyme nitrite reductase according to this scheme. 

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