9desaturase

9desaturase, more commonly referred to as delta-9 desaturase, denotes a group of enzymes that introduce a cis double bond at the ninth carbon from the carboxyl end of saturated fatty acyl-CoA substrates. In most eukaryotes, the primary reaction converts stearoyl-CoA (C18:0) to oleoyl-CoA (C18:1Δ9) and palmitoyl-CoA (C16:0) to palmitoleoyl-CoA (C16:1Δ9), contributing to the synthesis of monounsaturated fatty acids that influence membrane fluidity and lipid metabolism.

Biochemically, delta-9 desaturases are membrane-bound, non-heme diiron enzymes localized mainly in the endoplasmic reticulum. They require molecular oxygen and electrons supplied via the NADH-dependent cytochrome b5 reductase and cytochrome b5. The catalytic sites feature conserved histidine-rich motifs (for example HXXXH and HXXHH) that coordinate the iron centers essential for desaturation.

In mammals, the principal enzymes are encoded by the SCD gene family, with SCD1 and SCD2 as major isoforms. SCD1 is widely expressed, whereas others have tissue-preferential patterns. Regulation occurs at the transcriptional level by factors such as SREBP-1c and LXR, and is influenced by hormones and nutrients; insulin tends to upregulate SCD expression, while polyunsaturated fatty acids can provide feedback inhibition. In yeast, the delta-9 desaturase activity is carried out by OLE1, a functionally analogous enzyme.

Physiologically, delta-9 desaturase controls the balance of saturated and monounsaturated fatty acids, affecting membrane composition, signaling lipids, and energy storage. Dysregulation has been linked to metabolic disorders including obesity and non-alcoholic fatty liver disease. Because of its central role in lipid metabolism, delta-9 desaturase is a target of research in metabolic disease and biotechnological applications aiming to modify fatty acid profiles in crops and microorganisms.