2,4 Dienoyl-CoA reductase
|2,4-dienoyl CoA reductase 1, mitochondrial|
|PDB||1w6u (RCSB PDB PDBe PDBj)|
|Locus||Chr. 8 q21.3|
This gene encodes an accessory enzyme that participates in the beta oxidation and metabolism of polyunsaturated fatty enoyl-CoA esters. Specifically, it catalyzes the reduction of 2,4 Dienoyl-CoA thioesters of varying length by NADPH cofactor to 3-trans-enoyl-CoA of equivalent length. Unlike the breakdown of saturated fat, cis and trans polyunsaturated fatty acid degradation requires three additional enyzmes to generate a product compatible with the standard beta oxidation pathway. DECR is the second such enzyme (The others being Enoyl CoA isomerase and Dienoyl CoA isomerase) and is the rate limiting step in this auxiliary flow. DECR is capable of reducing both 2-trans,4-cis-dienoyl-CoA and 2-trans,4-trans-dienoyl-CoA thioesters, as well as double bonds at odd carbon positions, with equal efficiency. At this time, there is no clear explanation for this of lack of stereo-specificity.
2,4 Dienoyl-CoA thioester reduction by NADPH to 3-Enoyl CoA occurs by a two-step sequential mechanism via an enolate intermediate. DECR binds NADPH and the fatty acid thioester and positions them for specific hydride transfer to the Cδ on the hydrocarbon chain. The electrons from the Cγ-Cδ double bond move over to the Cβ-Cγ position, and those from the Cα-Cβ form an enolate. In the final step, a proton is abstracted from the water to the Cα and the thioester is reformed, resulting in a single Cβ-Cγ trans double bond. Since the final proton comes from water, the pH has a significant effect on the catalytic rate with the enzyme demonstrating maximal activity at ~6.0. A decrease in activity at pH < 6.0 can be explained by de-protonation of titratable residues that affect protein folding or substrate binding. Mutant proteins with modifications at key acidic amino acids (E154, E227, E276, D300, D117) show order of magnitude increases in Km and/or decreases in Vmax.
2,4 Dienoyl-CoA Reductase from Escherichia Coli (E. Coli) shares very similar kinetic properties to that of eukaryotes, but differs significantly in both structure and mechanism. In addition to NADPH, E. Coli DECR requires a set of FAD, FMN and iron-sulfur cluster molecules to complete the electron transfer. A further distinction is E. Coli DECR produces the final 2-trans-enoyl-CoA without the need for Enoyl CoA Isomerase. The active site contains accurately positioned Tyr199 that donates a proton to the Cγ after hydride attack at the Cδ, completing the reduction in a single concerted step. Surprisingly, mutation of the Tyr199 does not eliminate enzyme activity but instead changes the product to 3-trans-enoyl-CoA. The current explanation is that Glu164, an acidic residue in the active site, acts as a proton donor to Cα when Tyr199 is not present.
Eukaryotic DECR exists in both the mitochondria (mDECR) and the peroxisome (pDECR, coded by gene DECR2). The enzymes from each organelle are homologous and part of the short-chain dehydrogenase/reductase SDR[disambiguation needed] super-family. mDECR is 124kD consisting of 335 amino acids before posttranslational modification. The secondary structure shares many of the motifs of SDR, including a Rossman fold for strong NADPH binding. The protein exists as a homotetramer in physiological environment, but has been shown to also form monomers and dimers in solution.
Crystallization of mDECR shows the enzyme provides a network of hydrogen bonds from key residues in the active site to NADPH and the 2,4-dienoyl-CoA which positions the hydride at 3.4 Å to the Cδ, compared with 4.0 Å to the Cβ (not shown). The enolate intermediate discussed earlier is stabilized by residues additional hydrogen bonds to Tyr199 and Asn148. Lys214 and Ser210 (conserved residues in all SDR enzymes) are thought to increase the pKa of Tyr199 and stabilize the transition state. Additionally, at one end of the active site there is a flexible loop that provides sufficient room for long carbon chains. This likely gives the enzyme flexibility to process fatty acid chains of various lengths. Substrate length for mDECR catalysis is thought to be limited at 20 carbons, at which the [very long chain fatty acid] is first partially oxidized by pDECR in the peroxisome.
In Knock-out mice studies, DECR1-/- subjects accumulate significant concentrations of mono and polyunsaturated fatty acids in the liver during fasting (such as oleic acid, palmitoleic acid, linoleic acid, and linolenic acid). Mutant subjects were also found to have poor tolerance to cold, decrease in diurnal activity, and an overall reduction in adaptation to metabolic stressors.
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