HMG-CoA cycle

The acetyl-CoA molecules are the immediate sources of the ketone bodies. The actual formation of acetoacetate proceeds via the 3-hydroxy-3-methylglu-taryl-CoA (HMG-CoA) cycle with HMG-CoA synthetase (HMGS) and HMG-CoA lyase (HMGL, see Sect. 6.8) as the key enzymes. Extrahepatic tissues are able to utilize the ketone bodies and require for this the action of succinyl-CoA 3-oxoacid CoA transferase (SCOT) and 3-oxothiolase (see Sect. 7.4). 

Ketone body synthesis occurs only in the mitochondrial matrix. The reactions responsible for the formation of ketone bodies are shown in Figure 24.28. The first reaction—the condensation of two molecules of acetyl-CoA to form acetoacetyl-CoA—is catalyzed by thiolase, which is also known as acetoacetyl-CoA thiolase or acetyl-CoA acetyltransferase. This is the same enzyme that carries out the thiolase reaction in /3-oxidation, but here it runs in reverse. The second reaction adds another molecule of acetyl-CoA to give (i-hydroxy-(i-methyl-glutaryl-CoA, commonly abbreviated HMG-CoA. These two mitochondrial matrix reactions are analogous to the first two steps in cholesterol biosynthesis, a cytosolic process, as we shall see in Chapter 25. HMG-CoA is converted to acetoacetate and acetyl-CoA by the action of HMG-CoA lyase in a mixed aldol-Claisen ester cleavage reaction. This reaction is mechanistically similar to the reverse of the citrate synthase reaction in the TCA cycle. A membrane-bound enzyme, /3-hydroxybutyrate dehydrogenase, then can reduce acetoacetate to /3-hydroxybutyrate. 

FIGURE 25.33 HMG-CoA reductase activity is modulated by a cycle of phosphorylation and dephosphorylation. 

HMG-CoA synthase is not snccinylated, i.e. the interconversion cycle involving snccinylation does not occur (Appendix 7.5). 

The two-step reduction of HMG-CoA to mevalonate (Fig. 22-1, step a)n 15 is highly controlled, a major factor in regulating cholesterol synthesis in the human liver.121617 The N-terminal portion of the 97-kDa 888-residue mammalian FlMG-CoA reductase is thought to be embedded in membranes of the ER, while the C-terminal portion is exposed in the cytoplasm.16 Tire enzyme is sensitive to feedback inhibition by cholesterol (see Section D, 2). The regulatory mechanisms include a phosphorylation-dephosphorylation cycle and control of both the rates of synthesis and of proteolytic degradation of this key en-... 

Cholesteryl esters that are internalized via the LDL receptor are hydrolyzed to produce cholesterol and an acyl chain. Cholesterol, in (urn, activates the enzyme acyl-CoA cholesterol acyl-transferase (ACAT) which re-esterifies cholesterol. In an apparently futile cycle, the cholesteryl esters are hydrolyzed by cholesteryl ester hydrolase. The cholesterol moiety has several fates it may leave the cell and bind to an acceptor such as high-density lipoprotein (HDL), it may be converted to steroid hormones, or it may be reesterified by ACAT. When the cellular cholesterol concentration falls, the activity of HMG-CoA reductase is increased, as is the number of LDL receptors, which results in an increase of cellular cholesterol, due both to de novo synthesis and to the uptake of cholesterol-rich lipoproteins in the circulation. An increase in cellular cholesterol results in the rapid decline in the mRNA levels for both HMG-CoA reductase and the LDL receptor. This coordinated regulation is brought about by the presence of an eight nucleotide sequence on the genes which code for both proteins this is termed the sterol regulatory element-1. 

This is done in a two stage process involving first the addition of a third acetyl CoA by the second enzyme HMG-CoA synthase (2) via an aldol condensation (note the similarity to the formation of citrate in the TCA cycle, including driving the reaction by the hydrolysis of a thiol ester bond). 

B. This compound is acetoacetate, which is synthesized in the liver when blood insulin levels are low. HMG CoA synthetase is the key regulatory enzyme for synthesis, not oxidation. Acetoacetate is transported to tissues, such as muscle, where it is activated in the mitochondrion by succinyl CoA (not ATP), cleaved to 2 acetyl CoA, and oxidized via the TCA cycle, which requires the vitamin thiamine as thiamine pyrophosphate, a cofactor for a-ketoglutarate dehydrogenase. Biotin is not required. 

Figure 7-7. Degradation of amino acids. (A) Amino acids that produce pyruvate or intermediates of the TCA cycle. (B) Amino acids that produce acetyl CoA or ketone bodies. OAA = oxaloacetate HMG CoA = hydroxy-methylglutaryl CoA.

Assmus B, Urbich C, Aicher A, Hofmann WK, Haendeler J, Rossig L, Spyridopoulos I, Zeiher AM, Dimmeler S (2003) HMG-CoA reductase inhibitors reduce senescence and increase proliferation of endothelial progenitor cells via regulation of cell cycle regulatory genes. Circ Res 92 1049-1055... 

Hepatic yd-oxidation, without oxidation of acetyl-CoA through the TCA cycle, produces a substantial amount of energy. At such a time, liver is actively engaged in gluconeogenesis so that mitochondrial oxaloacetate is depleted, TCA cycle activity is depressed, and acetyl-CoA levels rise. The last reaction in -oxidation is conversion of acetoacetyl-CoA to acetyl-CoA, with an equilibrium in favor of high levels of acetoacetyl-CoA. Thus, acetyl-CoA and acetoacetyl-CoA accumulate and form HMG-CoA cleavage of this last compound yields acetoacetate, which is reduced to jd-hydroxybutyrate. Acetone results from nonenzymatic decarboxylation of acetoacetate. Ketone body formation occurs exclusively in liver (see Chapter 18) and is prominent in starvation and diabetes owing to the... 

In addition to these newer agents, there are currently three fibrate-based combinations with HMG-CoA reductase inhibitors in clinical trials. In phase III, Sciele has fenofibrate/pravastatin combination and AstraZeneca and Abbott have rosuvastatin/choline fenofibrate (ABT335), while in phase II, Life Cycle Pharma have atorvastatin / fenofibrate. 

Many key metabolic enzymes are modulated by phosphorylation-dephosphorylation and it has long been known that HMG-CoA reductase catalytic activity is inhibited by phosphorylation (Z.H. Beg, 1973). Rodent HMG-CoA reductase is phosphorylated on Ser 871 by an AMP-activated protein kinase that uses ATP as a phosphate donor (P.R. Clarke, 1990). However, examination of HMG-CoA reductase activity in rat liver showed that phosphorylation-dephosphorylation could not account for the long-term regulation that occurred with diurnal light cycling, fasting, or cholesterol-supplemented diet (M.S. Brown, 1979). Approximately 75-90% of HMG-CoA reductase enzyme was found to be phosphorylated (inactive) under all physiological conditions. This reservoir of inactive enzyme may allow cells to respond transiently to short-term cholesterol needs. 

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