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Lipoyl synthase is an enzyme that belongs to the radical SAM (S-adenosylmethionine) family. This is a family of sulfurtransferases, which as the name suggests, transfer sulfur-containing groups. Enzymes in this family contain two 4Fe-4S clusters, from which they obtain the sulfur groups that will be transferred onto the corresponding substrates. This particular enzyme participates in lipoic acid metabolism, so it transfers two sulfurs from its 4Fe-4S cluster onto the protein N6-(octanoyl)lysine through radical generation. This enzyme is usually localized to the mitochondria. Two organisms that have been extensively studied with regards to this enzyme are Escherichia coli and Mycobacterium tuberculosis. It is also found in other organisms, such as yeast and plants.

Nomenclature[ edit]
The systematic name of this enzyme class is protein N6-(octanoyl)lysine:sulfur sulfurtransferase. Other names in common use include:


 * LS,
 * LipA,
 * lipoate synthase, and
 * protein 6-N-(octanoyl)lysine:sulfur sulfurtransferase.

Mechanism of lipoyl synthase
Lipoyl synthase uses two sulfurs from its two [4Fe-4S]2+ clusters and attaches them to the 6th and 8th carbon of the protein N6-(octanoyl)lysine substrate, to convert it into protein N6-(lipoyl)lysine. One of the two clusters is located on the radical SAM motif of the enzyme, while the other cluster is attached to three cysteine residues. One of the existing substrates of S-adenosyl-L-methionine, which attaches to the 6th carbon on the enzyme, is converted to L-methionine and 5-deoxy-adenosine-5'-yl radical, which is then reduced to 5-deoxyadenosine attached to a hydrogen atom. This results in one of the [4Fe-4S]2+ clusters being converted into an unstable [3Fe-3S(RS)]-, due to the iron ion being ejected from the cluster, and one of the sulfur atoms attaching to the protein N6-(octanoyl)lysine, creating a 6-thiooctanoyl intermediate. Then, the second S-adenosyl-L-methionine attaches to the enzyme at the 8th carbon of the enzyme, creating a radical which then reacts with the remaining sulfur ion, and the 6-thiooctanoyl intermediate is transformed into the lipoylated product, lipoic acid. Following from the mechanism described, below is the overall reaction for this enzyme:



All in all, the 3 substrates of this enzyme are protein N6-(octanoyl)lysine, sulfur, and S-adenosyl-L-methionine, whereas its 3 products are protein N6-(lipoyl)lysine, L-methionine, and 5'-deoxyadenosine.

Importance of lipoyl synthase
As mentioned before, this enzyme participates in lipoic acid metabolism, where it performs the final step in lipoic acid biosynthesis, so it metabolizes the substrates into lipoic acid. This acid has different functions within different organisms. The lipoic acid generation in yeast cells increases the number of divisions in the cells as well as protects yeast cells from hydrogen peroxide. Lipoic acid is an important co-factor in many enzyme systems, and one of them is the pyruvate dehydrogenase complex. Studies that repressed the function of lipoyl synthase in Arabidopsis thaliana seeds showed that this did not have adverse effects on seed growth and weight, but shortened the generation time as well as the flowering time of the plants. Repression resulted in earlier flowering times, and decreased the generation times between seeds by almost 10%.

Possible side effects
Recently, the effect of overexpression of this enzyme in sunflower plants was found to eventually sequester the amount of SAM present in the plant. SAM is a molecule that is required in other enzymatic complexes found in this plant as well, as well as the overall structure of the plant, so this sequestration may cause a reduction in the fatty acid biosynthesis in the Arabidopsis seeds.