Structural Biochemistry/Degradation of Odd-Chain and Unsaturated Fatty Acids

Unsaturated and Odd-Chain Fatty Acids Require Additional Steps for Degradation
Most fatty acids have an even number of carbon atoms due to the process by which they were synthesized, thus the beta-oxidation pathway can successfully complete the degradation of these molecules. However, there are more steps required for fatty acid chains that are not as simple, i.e. those that are unsaturated or which have an odd number of carbons.

To oxidize unsaturated fatty acids, an isomerase and a reductase are required
Unsaturated fatty acids, while prominent in our diets, are more complicated to metabolize than saturated ones. In addition to the reactions required for the degradation of saturated fatty acids, the degradation of unsaturated fatty acids calls for two supplementary enzymes: an isomerase and a reductase.

For example, let’s look at the oxidation of the unsaturated fatty acid palmitoleate (pictured), which has 16 carbons with one double bond between C9 and C10. Just like saturated fatty acids, this unsaturated fatty acid is first activated then transported across the inner membrane of the mitochondria.

The now palmitoleoyl CoA proceeds to undergo 3 degradation cycles carried out by the same enzymes that oxidize saturated fatty acids. A problem arises however when the cis-Δ3-enoyl CoA is formed in the third round of these degradations: cis-Δ3-enoyl CoA is not a substrate for acyl CoA dehydrogenase. As seen in the picture, there is a double bond between C3 and C4 which prevents a double bond from forming between C2 and C3. This obstacle in degradation is overcome by shifting the position and configuration of the cis-Δ3 double bond to a trans-Δ2 double bond; this new reaction is facilitated by cis-Δ3-Enoyl CoA isomerase. Now that the double bond is between C2 and C3, the rest of the reactions relevant to saturated fatty acid oxidation can be done on trans-Δ2-enoyl CoA.

Excess polyunsaturated fatty acids (ones with more than one double bond) are degraded via beta-oxidation and are important to humans as precursors for signal molecules. There is another obstacle to be overcome when dealing with polyunsaturated fatty acids, however, which can be discerned by looking at the oxidation of the 18-carbon polyunsaturated fatty acid linoleate (pictured). Linoleate has cis-Δ9 and cis-Δ12 double bonds; when the cis-Δ3 double bond is formed after 3 rounds of beta-oxidation, it is converted into a trans-Δ2 double bond by the same isomerase mentioned in the palmitoleate degradation. After another round of beta-oxidation, the acyl CoA produced contains a cis-Δ4 double bond. When this species is dehydrogenated by acyl CoA dehydrogenase it yields a 2,4-dienoyl intermediate. This intermediate is not a substrate for the next enzyme in the beta-oxidation pathway, so 2,4-dienol CoA reductase is employed to convert the intermediate into trans-Δ3-enoyl CoA. 2,4-dienol CoA reductase does this by using NADPH to reduce the 2,4-dienoyl intermediate to trans-Δ3-enoyl CoA. cis-Δ3-enoyl CoA isomerase can then convert the trans-Δ3 into the trans-Δ2 form, which is an acceptable intermediate in the beta-oxidation pathway.

To sum up: odd-numbered double bonds are taken care of by the isomerase while even-numbered double bonds are handled by the isomerase and the reductase together.

In the final thiolysis step, odd-chain fatty acids yield propionyl CoA
Fatty acids with an odd number of carbons are a minor species and are oxidized in the same way as fatty acids with an even number of carbons. The difference is that when the odd-numbered fatty acid is oxidized, it produces propionyl CoA and acetyl CoA in the final round of degradation rather than two molecules of acetyl CoA. The activated 3 carbon unit in propionyl CoA, once converted into succinyl CoA, enters the citric acid cycle. The pathway that takes propionyl CoA to succinyl CoA requires vitamin B12 for a certain rearrangement. The conversion of propionyl CoA to succinyl CoA is pictured. The carboxylation reaction is catalyzed by propionyl CoA carboxylase, which is a biotin enzyme with a catalytic mechanism analogous to that of pyruvate carboxylase.

Vitamin B12 contains a cobalt atom and a corrin ring
Cobalamin enzymes catalyze 3 types of reactions: intramolecular rearrangements, methylations, and the reduction of ribonucleotides to deoxyribonucleotides. The two reactions in mammals that require coenzyme B12 are: (1) the conversion of L-methylmalonyl CoA into succinyl CoA and (2) methylation of homocysteine to form methionine. Reaction (2) is particularly important as Met is necessary for the generation of coenxymes that play a role in the synthesis of purines and thymine. The basic structure of a cobalamin is pictured: the core consists of a corrin ring with a central cobalt atom.

The rearrangement in the formation of succinyl CoA is catalyzed by methylmalonyl CoA (mechanism)
During this rearragement, two groups attached to adjacent carbon atoms are excahnged, the process being catalyzed by coenzyme B12. The first step in these rearrangements is the carbon-cobalt bond of 5’-deoxyadenosyl being cleaved (a homolytic cleavage reaction). This creates the Co2+ coenzyme form and a radical of 5’-deoxyadenosyl (pictured). The highly reactive radical serves its purpose by abstracting a hydrogen atom from the substrate, forming 5’-deoxyadenosine and a radical substrate, which spontaneously rearranges (the carbonyl CoA group travels to the position previously occupied by the neighboring carbon atom’s hydrogen). This produces a different radical, which abstracts an H atom from the methyl group of 5’-deoxyadenosine thus completing the rearrangement. To sum up, coenzyme B12’s function in these intramolecular migrations is to be a source of free radicals for the abstraction of H atoms.

Peroxisomes are also sites for fatty acid oxidation
While the majority of fatty acid oxidation takes place in the mitochondria, some oxidation can occur in peroxisomes (a small membrane-bound organelle found in most eukaryotes). One of the main roles of oxidation that takes place in the peroxisomes is the oxidation of fatty acids down to octanoyl CoA (a better substrate for beta-oxidation in mitochondria). The difference between beta-oxidation in the peroxisomes versus the mitochondria is found in the initial dehydrogenation reaction. Instead of capturing high-energy electrons as FADH2 for the electron transport chain as in mitochondrial oxidation, in peroxisomal oxidation the flavoprotein acyl CoA dehydrogenase transfers electrons from the substrate to FADH2 then to oxygen, yielding hydrogen peroxide. In order to degrade H2O2 into water and oxygen there is a large concentration of the enzyme catalase within the peroxisomes.

When fat breakdown predominates, ketone bodies are formed
If the fat and carbohydrate degradation in a cell are balanced properly the acetyl CoA from fatty acid oxidation will enter the citric acid cycle. For entry into the citric acid cycle to be granted to the acetyl CoA, it must combine with the oxaloacetate; the concentration of available oxaloacetate, normally formed from pyruvate (the product of glycolysis of glucose), is dependent on the supply of carbohydrate.

When oxaloacetate isn’t readily available (as with those who suffer form diabetes), acetyl CoA is diverted to the formation of D-3-hydroxybutyrate and acetoacetate, which are often called ketone bodies.

Acetyl CoA forms into acetoacetate in 3 steps (pictured), and the first step is catalyzed by thiolase. The overall reaction is 2Acetyl CoA + H2O --> acetoacetate + 2CoA + H+

D-3-hydroxybutyrate is formed when acetoacetate in the matrix of the mitochondria is reduced by D-3-hydroxybutyrate dehydrogenase. Acetoacetate undergoes an additional slow, spontaneous decarboxylation to acetone because it is a beta-ketoacid.

In some tissues ketone bodies are used as a major fuel source
D-3-hydroxybutyrate is formed when acetoacetate in the matrix of the mitochondria is reduced by D-3-hydroxybutyrate dehydrogenase. Acetoacetate undergoes an additional slow, spontaneous decarboxylation to acetone because it is a beta-ketoacid.

The conversion of acetoacetate into acetyl CoA occurs in 2 steps. First the transfer of CoA from succinyl CoA to acetoacetate activates the molecule. This step is catalyzed by a CoA transferase. In the second step, thiolase cleaves acetoacetyl CoA to yield two molecules of acetyl CoA, which can then enter the citric acid cycle. When 3-hydroxybutyrate is reacted to generate acetyl CoA, an additional step is required: it must be initially oxidized by NAD+ to produce acetoacetate.

To sum up, we can look at ketone bodes as water-soluble, easily transportable forms of acetyl units. Having high levels of acetoacetate is a sign of abundant levels of acetyl units; this leads to a decrease in the rate of lipolysis.