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Amazing Respirations
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Collecting Elements
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Sulfur and Phosphate

©2000 Timothy Paustian, University of Wisconsin-Madison

Many other nutrients have to be incorporated into a living cell besides carbon and nitrogen. We cover the more important elements and how microbes in general add these nutrients to a growing cell.

Sulfur assimilation

Sulfur's point of entry to biosynthesis is through cysteine. Sources of sulfur are converted into cysteine and it then serves as the starting compound for most biosynthetic processes. Cysteine can be converted into methionine, glutathione, thiamine, Coenzyme A and many other organic sulfur containing molecules needed by the cell.

The majority of sulfur is needed as amino acids for protein synthesis. In other cases, the sulfur is donated from cysteine to help form cofactors for enzymes. For example; in the synthesis of coenzyme A, part of cysteine, including the sulfur group, is added to the growing coenzyme A molecule.

Organic sources

Cysteine and methionine (sulfur containing amino acids) can serve as a source of sulfur for the cell and in many cases are the preferred form. They are taken up from the outside environment by specific transport proteins and used directly.

Inorganic Sources

Sulfassim picture

Figure 1 - Sulfate Reduction Pathway. BP - Binding protein for sulfate; TP - Transport protein for sulfate

Organic sources of sulfur in the environment are the exception and the most common form of sulfur found in the environment is sulfate ion (SO4-2). Sulfate is transported into the cell by sulfate permease and is reduced to sulfide by the sulfate reduction pathway. Reduction of sulfate requires that it first be activated by attachment to a carrier in the form of ATP. The reaction results in the formation of Adenosine 5'-phosphosulfate (APS), similar to ADP, but with sulfur replacing the terminal phosphate. While on this carrier sulfate is phosphorylated and then reduced to sulfite. At this point the sulfide is released from the carrier adenosine molecule and further reduction to sulfide involves donation of electrons from NADPH by the enzyme NADPH-sulfite reductase.

The synthesis of cysteine from serine is a two step process. In the first step, serine is combined with acetyl-CoA to form O-Acetylserine. Next sulfite reacts with O-Acetylserine to form cysteine. Cysteine can then be used for protein synthesis, to make methionine, coenzymes and other important organic molecules.

SO4-2 + 2 ATP + 2 NADPH + H+ + serine + acetyl-CoAarrow picturecysteine + AMP + ADP + 3 Pi + NADP+

Figure 2 - The overall chemical equation for cysteine synthesis.


Phosphate is a necessary component of DNA, RNA, phospholipids and some proteins. As we have seen they also have an integral role in energy metabolism. The preferred source of phosphate is orthophosphate (Pi), but E. coli, the best understood phosphate metabolism model system, is capable of using numerous phosphate molecules including phosphoenol pyruvate, glyceraldehyde-3-phosphate,. and many other phosphorylated compounds.

Orthophosphate is recognized by a specific transport protein and translocated across the membrane. Once inside Pi finds its way into macromolecules via central metabolism. The most important conduits for orthophosphate are oxidative phosphorylation (ATP synthase), glycolysis (Emden-Meyerhoff-Parnas pathway), and the tricarboxylic acid cycle. Go back and look at these pathways and note where Pi is incorporated into ATP. From here the ATP that is generated serves as the source of phosphate for cellular synthesis of phosphate containing biomolecules.

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