BIOCHEMISTRY - L. Stryer - 1984

VOLUME 2

PART II GENERATION AND STORAGE OF METABOLIC ENERGY

CHAPTER 17. FATTY ACID METABOLISM

17.3. Triacylglycerols (Triglycerides) Are Highly Concentrated Energy Reserves

Triacylglycerols (triglycerides), owing to their reduced and anhydrous state, serve as highly concentrated stores of metabolic energy. The energy yield from the Complete oxidation of Fatty acids is about 9 kcal/g, whereas for CARBOHYDRATES and Proteins this value is approximately 4 kcal/g. This large difference in energy yield is explained by the fact that Fatty acids are significantly more reduced compounds. Furthermore, triacylglycerols are highly nonpolar and are therefore stored in an almost anhydrous form, whereas proteins and carbohydrates are much more polar and correspondingly more heavily hydrated. Indeed, one gram of dry Glycogen binds about two grams of Water. Consequently, The amount of energy stored in one gram of nearly anhydrous fat is more than six times greater than that stored in one gram of hydrated glycogen. This explains why triacylglycerols rather than glycogen were selected during evolution as the primary energy source. In a 70-kg human, fuel reserves are normally distributed as follows: 100,000 kcal in triacylglycerols, 25,000 kcal in proteins (predominantly in Muscle), 600 kcal in glycogen, and 40 kcal in glucose. Triacylglycerols account for 11 kg of the total body weight. If this same amount of energy were stored as glycogen, the total body weight would have to be 55 kg greater.

In mammals, the primary site of triacylglycerol storage is the Cytoplasm of adipose Cells. Triacylglycerol droplets coalesce to form large globules that may occupy most of the cellular volume. Adipose cells are specialized for the synthesis and storage of triacylglycerols, as well as for their mobilization as fuel molecules capable of being transported via the Blood to other Tissues.

17.4. Triacylglycerols Are Hydrolyzed by Cyclic AMP-Regulated Lipases

The first step in the utilization of fat as an energy source is the Hydrolysis of triacylglycerol by lipases. Lipase activity in fat cells is regulated by Hormones. Epinephrine, norepinephrine, Glucagon, and adrenocorticotropic hormone stimulate the adenylate cyclase of adipose cells. An elevated level of cyclic adenosine monophosphate (cyclic AMP) subsequently leads to The stimulation of protein kinase, which activates lipase via phosphorylation. Thus, epinephrine, norepinephrine, glucagon, and adrenocorticotropic hormone induce lipolysis. Cyclic AMP plays The Role of a second messenger in activating lipolysis in fat cells, analogous to its role in activating glycogen breakdown (Chapter 16). In contrast to these hormones, Insulin inhibits lipolysis.

Glycerol formed during lipolysis is phosphorylated and oxidized to dihydroxyacetone phosphate, which in turn is isomerized to glyceraldehyde 3-phosphate. The latter serves as an intermediate in both Glycolysis and Gluconeogenesis. Consequently, in the Liver, which contains the appropriate Enzymes, glycerol can be converted into Pyruvate or glucose. The reverse process is also possible—the reduction of dihydroxyacetone phosphate to glycerol 3-phosphate, which is then hydrolyzed by a phosphatase to yield glycerol. Thus, glycerol and glycolytic intermediates are readily interconvertible.

Class="center">Fig. 17.2. Scanning electron micrograph of an adipocyte

17.5. Fatty Acids Are Degraded by Sequential Removal of Two-Carbon Units

In 1904, Franz Knoop made a decisive contribution to elucidating The Mechanism of Fatty acid oxidation. He fed dogs straight-chain fatty acids in which the ω-carbon atom was attached to a phenyl group. Knoop established that feeding dogs phenylbutyrate resulted in The excretion of a phenylacetic acid derivative in the urine, whereas feeding phenylpropionate yielded a benzoic acid derivative. Thus, feeding a fatty acid with an even number of carbon atoms was accompanied by The formation of phenylacetic acid, whereas feeding an acid containing an odd number of carbon atoms led to the formation of benzoic acid (Fig. 17.3). Based on these data, Knoop concluded that fatty acids are degraded by oxidation at the β-carbon atom. These experiments represent a milestone in The Development of biochemistry because they were the first to utilize a synthetic labeled compound to study a biochemical mechanism. Deuterium and radioactive isotopes began to be used in biochemistry several decades later.

17.6. Fatty Acid Oxidation Is Preceded by Attachment to Coenzyme A

In 1949, Eugene Kennedy and Albert Lehninger discovered that fatty acid oxidation takes place in the Cell/35.html">Mitochondria. Subsequent work showed that the penetration of fatty acids into the mitochondrial matrix is preceded by their activation. Adenosine triphosphate (ATP) stimulates the formation of a thioester bond between the carboxyl group of the Fatty acid and the sulfhydryl group of CoA. This activation reaction occurs in the mitochondrial membrane, where it is catalyzed by acyl-CoA synthetase (also called fatty acid thiokinase).

Paul Berg established that fatty acid activation proceeds in two steps. First, the fatty acid reacts with ATP to form an acyl adenylate. In this mixed anhydride, the carboxyl group of the fatty acid is linked to the phosphoryl group of AMP. The other two phosphoryl groups of the ATP substrate are released as pyrophosphate. Next, the sulfhydryl group of CoA acts on the enzyme-bound acyl adenylate to yield acyl-CoA and AMP. Both of these partial reactions are readily reversible. The Equilibrium Constant for the overall reaction is close to 1.

R - СОО-+ СоА + ATR ⇄ Ацил-СоА + АМР + РРi.

Fig. 17.3. Knoop's experiment demonstrating that fatty acids are degraded by the Cleavage of two-carbon units

During these transformations, one high-energy bond is broken (the bond between PP_i and AMP) and one is formed (the thioester bond in acyl-CoA). How is this reaction driven? The answer to this question lies in the fact that pyrophosphate is rapidly hydrolyzed by the action of pyrophosphatase.

R - СОО^- + СоА + АТР + Н₂O → Ацил-СоА + АМР + 2Pi + 2Н⁺.

As a result, the entire reaction becomes irreversible because two high-energy bonds are consumed while only one is formed. We see here yet another example of a recurring motif in biochemistry: many biosynthetic reactions are rendered irreversible by the hydrolysis of inorganic pyrophosphate.

Another recurring motif concerns the activation reaction. The existence of an enzyme-bound acyl adenylate intermediate is not unique to acyl-CoA synthesis. Acyl adenylates are frequently formed during the activation of carboxyl groups in biochemical reactions. For example, Amino Acids are activated for Protein Synthesis via precisely this type of mechanism.