Sulfur Assimilation

Sulfur is an essential element in biological systems, contributing to synthesizing key biomolecules, including amino acids such as cysteine and methionine, and cofactors such as coenzyme A and biotin. Microorganisms primarily assimilate sulfur as sulfate (SO₄²⁻) from the environment, which must undergo a series of biochemical transformations before it can be incorporated into cellular components. As sulfate is highly oxidized, it must undergo assimilatory sulfate reduction to become biologically useful. Some microorganisms can utilize sulfur-containing amino acids (cysteine, methionine) or reduced sulfur compounds (thiosulfate, elemental sulfur) as alternative sulfur sources. These compounds bypass the need for sulfate activation and reduction, allowing for more energy-efficient sulfur assimilation.

Sulfate Activation and Reduction Pathway

The process begins with the activation of sulfate by ATP sulfurylase, an enzyme that catalyzes the formation of adenosine-5′-phosphosulfate (APS). APS serves as a key intermediate in sulfate metabolism, either undergoing further phosphorylation to form phosphoadenosine-5′-phosphosulfate (PAPS) or proceeding directly to reduction in some bacteria. PAPS is crucial not only for sulfate assimilation but also for sulfation reactions in cellular processes.

The reduction of PAPS to sulfite (SO₃²⁻) occurs via PAPS reductase, followed by the conversion of sulfite to hydrogen sulfide (H₂S) through sulfite reductase. These sequential reductions convert sulfur into a biologically accessible form for organic molecules. Reducing equivalents, such as NADPH, donate electrons to facilitate the conversion in these reactions.

Incorporation of Hydrogen Sulfide into Cysteine

The final step of sulfur assimilation involves the incorporation of hydrogen sulfide into amino acid metabolism. In fungi, H₂S reacts with serine to form cysteine. In bacteria and some archaea, H₂S combines with either O-acetylserine or O-phosphoserine to form cysteine through enzymatic pathways. O-acetylserine is more commonly used in Escherichia coli and related bacteria, whereas O-phosphoserine-dependent pathways are found in certain archaea, such as Methanocaldococcus jannaschii.

Cysteine is a key precursor for methionine biosynthesis and the production of sulfur-containing cofactors such as coenzyme A and biotin. Additionally, sulfur metabolism plays a role in redox homeostasis, particularly through glutathione synthesis, which protects cells from oxidative stress.