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Multifunctional enzyme type 2 (MFE-2) is a peroxisomal enzyme participating in the breakdown of fatty acids in eukaryotes. Depending on the organism, MFE-2 is composed of two to four functional units, out of which the two enzymatic ones, 2-enoyl-coenzyme A (CoA) hydratase 2 and (3R)-hydroxyacyl-CoA dehydrogenase, are found in the all MFE-2s. These units are responsible for the catalysis of the second and third steps of the peroxisomal β-oxidation of various CoA thioesters of fatty acids and fatty acyl derivatives. Their (R)-stereospecificity and ability to accept a broad range of fatty acid CoA esters as substrates, in addition to the fact that they do not share any sequence similarity with the classical mitochondrial counterparts, make the enzymatic units of MFE-2 structurally very interesting. In this study, the three-dimensional structures of the (3R)-hydroxyacyl-CoA dehydrogenase and 2-enoyl-CoA hydratase 2 units were solved by crystallographic methods. The crystal structure of the (3R)-hydroxyacyl-CoA dehydrogenase unit of rat MFE-2 reveals a dimeric enzyme with an α/β short-chain alcohol dehydrogenase/reductase (SDR) fold. A unique feature of (3R)-hydroxyacyl-CoA dehydrogenase, however, is the separate C-terminal domain, which completes the active site cavity of the adjacent monomer and extends the dimeric interactions. The 2-enoyl-CoA hydratase 2 unit is a dimer with a unique two-domain structure proposed to evolve via gene duplication. The fold consists of two side-by-side arranged repeats of the hot-dog fold motifs, thus being highly reminiscent of the tertiary structures of the (R)-specific 2-enoyl-CoA hydratase of the polyhydroxyalkanoate synthesis pathway and the β-hydroxydecanoyl thiol ester dehydrase of fatty acid synthesis type II, both from prokaryotic sources. The importance of the N-domain in the binding of bulky substrates was shown by the enzyme-product complex structure, which also indicates the active site. For the first time, it was shown that the eukaryotic hydratase 2 uses an Asp/His catalytic dyad in catalysis. Moreover, a novel catalytic mechanism was proposed for (R)-specific hydration/dehydration. The solved structures also provide a molecular basis for understanding the effects of the patient mutations of MFE-2. They also allow disussion of the possible organisation of the three units in full-length MFE-2 of mammals. |
