Garrett R.H., Grisham C.M. - Biochemistry (1999)(2nd ed.)(en)

-Oxidation of Odd-Carbon Fatty Acids Yields Propionyl-CoA

Propionyl-CoA

Fatty acids with odd numbers of carbon atoms are rare in mammals, but fairly common in plants and marine organisms. Humans and animals whose diets include these food sources metabolize odd-carbon fatty acids via the -oxida- tion pathway. The final product of -oxidation in this case is the 3-carbon pro- pionyl-CoA instead of acetyl-CoA. Three specialized enzymes then carry out the reactions that convert propionyl-CoA to succinyl-CoA, a TCA cycle intermediate. (Because propionyl-CoA is a degradation product of methionine, valine, and isoleucine, this sequence of reactions is also important in amino acid catabolism, as we shall see in Chapter 26.) The pathway involves an initial carboxylation at the -carbon of propionyl-CoA to produce D-methylmalonyl- CoA (Figure 24.19). The reaction is catalyzed by a biotin-dependent enzyme, propionyl-CoA carboxylase. The mechanism involves ATP-driven carboxylation of biotin at N1, followed by nucleophilic attack by the -carbanion of propi- onyl-CoA in a stereo-specific manner.

D-Methylmalonyl-CoA, the product of this reaction, is converted to the L-isomer by methylmalonyl-CoA epimerase (Figure 24.19). (This enzyme has often and incorrectly been called “methylmalonyl-CoA racemase.” It is not a racemase because the CoA moiety contains five other asymmetric centers.) The epimerase reaction also appears to involve a carbanion at the -position (Figure 24.20). The reaction is readily reversible and involves a reversible dissociation of the acidic -proton. The L-isomer is the substrate for methylmalonyl-CoA mutase. Methylmalonyl-CoA epimerase is an impressive catalyst. The pKa for the proton that must dissociate to initiate this reaction is approximately 21! If binding of a proton to the -anion is diffusion-limited, with kon 109 M 1 sec 1, then the initial proton dissociation must be rate-limiting, and the rate constant must be

koff Ka kon (10 21 M) (109 M 1 sec 1) 10 12 sec 1

The turnover number of methylmalonyl-CoA epimerase is 100 sec 1, and thus the enzyme enhances the reaction rate by a factor of 1014.

(S)-Methylmalonyl-CoA (R)-Methylmalonyl-CoA

Resonance-stabilized carbanion intermediate

Propionyl-CoA carboxylase

_

OOC O

H3C C C SCoA

H

D-Methylmalonyl-CoA

Methylmalonyl-CoA epimerase

H

L-Methylmalonyl-CoA

Methylmalonyl-CoA mutase

_

OOC CH2

Succinyl-CoA

FIGURE 24.19 ● The conversion of propi- onyl-CoA (formed from -oxidation of oddcarbon fatty acids) to succinyl-CoA is carried out by a trio of enzymes as shown. SuccinylCoA can enter the TCA cycle or be converted to acetyl-CoA.

FIGURE 24.20 ● The methylmalonyl-CoA epimerase mechanism involves a resonancestabilized carbanion at the -position.

792 Chapter 24 ● Fatty Acid Catabolism

CH2 O A

OH OH

The Activation of Vitamin B12

Conversion of inactive vitamin B12 to active 5 -deoxyadenosylcobal- amin is thought to involve three steps (see figure). Two flavoprotein reductases sequentially convert Co3 in cyanocobalamin to the Co2 state and then to the Co state. Co is an extremely powerful nucleophile. It attacks the C-5 carbon of ATP as shown, expelling the triphosphate anion to form 5 -deoxyadenosyl-

cobalamin. Because two electrons from Co are donated to the CoOcarbon bond, the oxidation state of cobalt reverts to Co3 in the active coenzyme. This is one of only two known adenosyl transfers (that is, nucleophilic attack on the ribose 5 -carbon of ATP) in biological systems. (The other is the formation of S-adenosylmethionine—see Chapter 26.)

794 Chapter 24 ● Fatty Acid Catabolism

● The malic enzyme reaction proceeds by oxidation of malate to oxaloacetate, followed by decarboxylation to yield pyruvate.

CH3(CH2)7CH2 C CH2 C SCoA

OH

Continuation of β -oxidation

O

6 CH3C SCoA

● -Oxidation of unsaturated fatty acids. In the case of oleoyl-CoA, three

-oxidation cycles produce three molecules of acetyl-CoA and leave cis- 3-dodecenoyl-CoA. Rearrangement of enoyl-CoA isomerase gives the trans- 2 species, which then proceeds normally through the -oxidation pathway.

tively decarboxylated to pyruvate and CO2 by malic enzyme, as shown in Figure 24.22. Pyruvate can then be transported back to the mitochondrial matrix, where it enters the TCA cycle via pyruvate dehydrogenase. Note that malic enzyme plays an important role in fatty acid synthesis (see Figure 25.1).

24.4 ● -Oxidation of Unsaturated Fatty Acids

An Isomerase and a Reductase Facilitate the

-Oxidation of Unsaturated Fatty Acids

Unsaturated fatty acids are also catabolized by -oxidation, but two additional mitochondrial enzymes—an isomerase and a novel reductase—are required to handle the cis-double bonds of naturally occurring fatty acids. As an example, consider the breakdown of oleic acid, an 18-carbon chain with a double bond at the 9,10-position. The reactions of -oxidation proceed normally through three cycles, producing three molecules of acetyl-CoA and leaving the degradation product cis- 3-dodecenoyl-CoA, shown in Figure 24.23. This intermediate is not a substrate for acyl-CoA dehydrogenase. With a double bond at the 3,4-position, it is not possible to form another double bond at the 2,3- (or ) position. As shown in Figure 24.23, this problem is solved by enoyl-CoA isomerase, an enzyme that rearranges this cis- 3 double bond to a trans- 2 double bond. This latter species can proceed through the normal route of-oxidation.

Degradation of Polyunsaturated Fatty Acids

Requires 2,4-Dienoyl-CoA Reductase

Polyunsaturated fatty acids pose a slightly more complicated situation for the cell. Consider, for example, the case of linoleic acid shown in Figure 24.24. As with oleic acid, -oxidation proceeds through three cycles, and enoyl-CoA isomerase converts the cis- 3 double bond to a trans- 2 double bond to permit one more round of -oxidation. What results this time, however, is a cis- 4 enoyl-CoA, which is converted normally by acyl-CoA dehydrogenase to a trans- 2, cis- 4 species. This, however, is a poor substrate for the enoyl-CoA hydratase. This problem is solved by 2,4-dienoyl-CoA reductase, the product of which depends on the organism. The mammalian form of this enzyme produces a trans- 3 enoyl product, as shown in Figure 24.24, which can be converted by an enoyl-CoA isomerase to the trans- 2 enoyl-CoA, which can then proceed normally through the -oxidation pathway. Escherichia coli possesses a 2,4-dienoyl-CoA reductase that reduces the double bond at the 4,5-position to yield the trans- 2 enoyl-CoA product in a single step.