9desaturases

9desaturases, also known as delta-9 desaturases, are enzymes that introduce a cis double bond at the ninth carbon from the carboxyl end of saturated fatty acyl-CoA substrates. The principal reactions convert stearoyl-CoA (C18:0) to oleoyl-CoA (C18:1) and, to a lesser extent, palmitoyl-CoA (C16:0) to palmitoleoyl-CoA (C16:1). They belong to the desaturase enzyme family and require molecular oxygen and electrons supplied from NADH or NADPH via systems such as cytochrome b5 and its reductase. The active site contains a di-iron center coordinated by conserved histidine motifs essential for oxygen activation and double-bond formation.

Localization and mechanism are characteristic of eukaryotic delta-9 desaturases, which are membrane-bound enzymes located in the endoplasmic reticulum. They operate within the lipid bilayer, using electron transfer from NADH/NADPH to introduce a double bond into the fatty acyl chain and produce monounsaturated fatty acyl-CoAs that enter diverse lipid pathways.

Physiological roles of 9desaturases include regulation of membrane fluidity, lipid signaling, and overall energy metabolism. They influence the composition of triglycerides and phospholipids and thereby affect metabolic health. Altered desaturase activity has been linked to obesity, insulin sensitivity, and hepatic steatosis in animal models and humans. Regulation occurs at transcriptional and post-transcriptional levels, with expression often driven by SREBP-1c and other lipogenic signals; fatty acids such as polyunsaturated fats can suppress expression, while hormonal and nutritional cues modulate enzyme activity.

Diversity exists across species, with multiple isoforms encoded by SCD genes in mammals (notably SCD1) and various related enzymes in other organisms. The enzymes are sometimes broadly referred to as delta-9 desaturases or stearoyl-CoA desaturases (SCDs).