Desaturases fused to their electron donor

Abstract

Fatty acid desaturations in the carboxy‐terminal segment from C1—C10 are catalyzed in many, but not in all cases, by desaturase enzymes which are fused to their electron donor cytochrome b5. Several of these enzymes (“front‐end desaturases”) from a wide variety of organisms have been cloned and functionally expressed for proof of regio‐, stereo‐ and chain length‐selectivity. In most cases the actual status of the substrate chain, whether coenzyme A thioester or component of a membrane lipid, is not known. The cytochrome b5 domain is located N‐terminally, internally or C‐terminally. Compared to the free cytochrome b5 , the fused domains show a significant reduction of acidic amino acid residues on the surface of the four helices enclosing the heme group. It is discussed how this may contribute to hydrophobic domain pairing required for interdomain electron transport. This is in contrast to the mode of interaction of free cytochrome b5 with its partners, which is governed by electrostatic charge pairing. A look at crystallized or computer‐simulated models involving fused or free cytochrome b5 helps to outline the problems encountered by optimizing the docking of partners and the exchange of electrons between domains of different degrees of mobility.

Pathways of Lipid Metabolism in Marine Algae, Co-Expression Network, Bottlenecks and Candidate Genes for Enhanced Production of EPA and DHA in Species of Chromista

“…Two types of desaturases can be distinguished; (1) The front-end desaturase contains an N -terminal cytochrom b5-domain and inserts the new double bond between the FA carboxyl group and a possible existing double bond. (2) The ω6/ω3 desaturase inserts a new double bond between the FA methyl end and a pre-existing double bond [114]. An example for an identified front-end desaturase in P. tricornutum is the high substrate specific plastidial-located Δ12 desaturase, PtFAD6, synthesizes palmitoleic acid (POA, 16:1 n -7) to hexadecadienoic acid (HAD, 16:2 n -4), indicated in Figure 5 [105].…”

Accumulation of High-Value Lipids in Single-Cell Microorganisms: A Mechanistic Approach and Future Perspectives

“…8 The second group comprises the less common ω6/ω3 desaturases capable of inserting a new double bond between the FA methyl end and a pre-existing double bond. 8,112 …”

Isolation of a Δ5 Desaturase Gene from Euglena gracilis and Functional Dissection of Its HPGG and HDASH Motifs

“…The HPGG motif is expected to be on the cytochrome b 5 surface, in contact with the heme through van der Waals interactions [27]. The conserved HPGG motif was thought to be essential in maintaining cytochrome b 5 electron transfer function, with the histidine serving as a heme axial ligand [50].…”

“…Compared to free cytochrome b 5 , while there are several conserved acidic amino acids, there is a characteristic reduction in the number of aspartate and glutamate residues in the vicinity of the HPGG motif of cytochrome b 5 domain of desaturases. This reduced number of acidic residues around the heme pocket is thought to contribute to stabilizing nonpolar intermolecular interactions between the cytochrome b 5 and desaturase domains [27, 54]. Substitutions involving aspartate or glutamate residues in the HPGG motif may affect the interface geometry of electron donor/acceptor docking that is exhibited in desaturase activity due to altered electron transfer.…”

“…The active site of desaturases has been characterized as a diiron cluster that is bound to the enzyme by three regions of highly conserved histidine-rich (His-rich) motifs [27, 30]. These three His-rich motifs H(X) 3–4 H, H(X) 2–3 HH, and H/Q(X) 2–3 HH are conserved among all front-end desaturases, and the eight histidine residues of these three motifs are essential for catalytic activity [33].…”