Function
The first reaction in β-oxidation, or fatty acid metabolism, is the catalyzation of the ester bond (C2 and C3) of the substrate Acyl-CoA.[1] This is accomplished through Acyl-CoA Dehydrogenase (ACDH) and its cofactor FAD.[1] ACDH is classified according to its length of its substrates: short (SCAD), medium (MCAD), very and very long-chain (VLCAD).
Disease
Medium-chain acyl-CoA dehydrogenase deficiency (MCADD) is a disorder that affects fatty acid oxidation and can be characterized by a hypoglycemic crisis during times of increased stress.[2] Expression of MCADD results in a decrease of ketone production and an increase in medium-chain fatty acid concentration. MCADD is a disorder inherited genetically through an autosomal recessive trait, and it is caused by mutations in the medium-chain acyl- CoA dehydrogenase (ACADM) gene. The ACADM gene is located on chromosome 1p31. There are over 90 different ACADM gene mutations known so far, most of which are missense mutations. The disorder can lead to symptoms such as a loss in appetite as well as vomiting and diarrhea. This can result in accumulated concentrations of acylcarnitine, which can be potentially toxic. People who are affected and not diagnosed are at a high risk of dying or experiencing permanent neurological damage during their first metabolic crisis. To prevent such events, immediate care should follow catabolic stress and fasting should be averted. Individuals living with MCADD are asymptomatic up until there is an increased demand for energy followed by a prolonged time of fasting. Newborn screening is now widely implemented through the use of liquid chromatography-tandem mass spectrometry.[2]
Relevance
Fatty Acid Metabolism
Acyl-CoA dehydrogenase is the first enzyme used to metabolize fatty acids.
In the first step, fatty acyl-CoA is converted to trans-Δ2-enoyl-CoA via acyl-CoA dehydrogenase with the help of FAD, releasing FADH2 as a byproduct.
In the second step, trans-Δ2-enoyl-CoA is converted to 3-L-hydroxyacyl-CoA via enoyl-CoA hydratase with the help of H2O.
In the third step, 3-L-hydroxyacyl-CoA is converted to β-ketoacyl-CoA via 3-hydroxyacyl-CoA dehydrogenase with the help of NAD+, releasing NADH + H+ as a byproduct.
In the fourth step, β-ketoacyl-CoA is converted to another fatty acyl-CoA with two less carbons or finishes the cycle as Acetyl-CoA if there are two remaining carbons. This reaction occurs via β-ketohiolase with the help of CoA-SH.
The four steps are repeated until the fatty acid is metabolized to less than or equal to 3 carbons.
Structural highlights
Catalytic Residues
Glu255 is responsible for the catalytic activity of mutant Medium Chain Acyl-CoA Dehydrogenase (MCADH). It can be in “active” or “resting” states. It is mostly in the “resting” state, due to its ability to hydrogen bond with Glu99.[2]
Long Chain Acyl-CoA Dehydrogenase (LCADH) and Isovaleryl-CoA Dehydrogenase (IVDH) have a higher catalytic activity than MCADH because they have serine and glycine in their 99 position, respectively. Neither of these amino acids can form hydrogen bonds, so the Glu255 is in the “active state.” The catalytic residue of LCADH is Glu261; the catalytic residue of IVDH is Glu254.[2]
Glu376 is the amino acid responsible for catalytic activity of the wild type.[2]
Significance of the Positions of Glutamate on the Wild Type (Glu376) and Mutant (Glu255)
The distance between the donor proton and the base that attacks the donor proton affect the catalytic activity of the carboxylate base of the glutamates. The distance between the proton and glutamates carboxylates are more than 4.0 A.[2]
Glu376 would conform so that its carboxylate oxygen lies close to the proton (2.4A).[2]
Glu255 has a smaller catalytic activity because is over 4.0 A away from the proton. It is thought that due to MCADH’s flexibility at Gly376, Glu255’s carboxylate oxygen will be closer to the donor proton.[2]
When both Glu255 and Glu376 are available to the Thr255Glu mutant, Glu376 can act as a catalytic residue and Glu255 adopts the “resting” conformation. This is expected given that the kinetic parameters of the mutant and its optimum substrates (C8- and C10-CoA) are similar to those of the wild type enzyme. The substrates (in both wild type and MLCADH) with alkyl chain lengths that are longer than C12-CoA can form multiple conformers at its ω-end in the active site cavity. However, this is not the case for the Glu/Glu mutant due to the glutamate side chains. This steric hindrance prevents the binding of the long substrate in an orientation that would allow catalysis to occur.[2]
Solvent Accessibility and Oxygen Reactivity.
In the MLCADH, the bore of the active site cavity at its midsection is wider and allows the longer substrate to adopt multiple conformations at its ω-end. This better accommodates the MLCADH for catalysis than the wild type. The wider bore also allows bulkier substrates to bind to the enzyme more easily. Having a larger active site allows more solvent molecules, which leads to larger amounts of molecular oxygen, which is becomes readily available to reduced Flavin. This difference in width (8.5 Å vs 5.1 Å) allows for the oxidation of the reduced Flavin to occur somewhere between 10 to 100 times faster than the wild type. Ligands that lack the carbonyl oxygen are much less effective at protecting the reduced enzyme flavin toward molecular oxygen.[2]
References
- ↑ 1.0 1.1 Berg JM, Tymoczko JL, Stryer L. Biochemistry. 5th edition. New York: W H Freeman; 2002. Chapter 22, Fatty Acid Metabolism. Available from: https://www.ncbi.nlm.nih.gov/books/NBK21173/
- ↑ 2.0 2.1 2.2 2.3 2.4 2.5 2.6 2.7 2.8 2.9 Lee, H.-J. K.; Wang, M.; Paschke, R.; Nandy, A.; Ghisla, S.; Kim, J.-J. P. Crystal Structures of the Wild Type and the Glu376Gly/Thr255Glu Mutant of Human Medium-Chain Acyl-CoA Dehydrogenase: Influence of the Location of the Catalytic Base on Substrate Specificity†. https://pubs.acs.org/doi/pdf/10.1021/bi9607867 (accessed May 5, 2019).
