Fatty acid metabolism condensation

Acyl-CoAs are the activated intermediates of fatty acid metabolism formed by the condensation of fatty acids with Coenzyme A. 

Chenoweth believes that an explanation of the above results may lie in the reactions occurring before the entrance of fatty acid metabolites into the citric acid cycle. Activated acetate, i.e. acetyl coenzyme A (AcCoA) is the end-product of fatty acid metabolism prior to its condensation with oxalacetate to form citrate. Possibly fluoro-fatty acids behave like non-fluorinated fatty acids. The end-product before the oxalacetate condensation could be the same for all three fluorinated inhibitors, viz. fluoroacetyl coenzyme A (FAcCoA). Fluorocitrate could then be formed by the condensation of oxalacetate with FAcCoA, thereby blocking the citric acid cycle. The specificity of antagonisms must therefore occur before entrance of the metabolites into the citric acid cycle. 

F Biological Claisen Condensations and Aldol Additions. Fatty Acid Metabolism... 

The most important functions of pantothenic acid are its incorporation in coenzyme A and acyl carrier protein (AGP). Both CoA and AGP/4-phosphopantetheine function metabolically as carriers of acyl groups. Coenzyme A forms high-eneigy thioester bonds with carboxylic acids. The most important coenzyme is acetyl CoA. Acetic acid is produced during the metabolism of fatty acids, amino acids, or carbohydrates. The active acetate group of acetyl CoA can enter the Krebs cycle and is used in the synthesis of fatty acids or cholesterol. AGP is a component of the fatty acid synthase multienzyme complex. This complex catalyzes several reactions of fatty acid synthesis (condensation and reduction). The nature of the fatty acid synthase complex varies considerably among different species (91). 

The mechanism of action of insulin is discussed below. Insulin deficiency interferes with the adequate utilization of glucose. The sugar accumulates in the blood, and hyperglycemia, glucosuria, dehydration, increased diuresis, loss of electrolytes, and polydypsia ensue. By a mechanism that may or may not be related to the primary defect in carbohydrate metabolism observed in diabetes, fatty acid metabolism is also affected. A critical phenomenon is the accumulation of 2-carbon compounds that condense to form 4-car-bon compounds, generating ketosis and acidosis. These symptoms will now be reviewed individually. 

Before leaving the subject of Lipmann s laboratory I should mention the results of two other studies that were key to an understanding of the mechanism of fatty acid metabolism. These studies were carried out in collaboration with Michael Douderoff who spent a few weeks in Lipmann s laboratory while I was there. To determine whether acetoacetate synthesis involved the condensation of two molecules of acetyl CoA or one equivalent each of acetyl-CoA and acetate, Douderoff and I studied the formation of acetoacetate in a coupled enzyme system composed of phosphotransacetylase and acetoacetate synthetase from pigeon liver. We found that the acetoacetate produced from 1-P C] acetyl-P and unlabeled acetate was equally labeled in the carboxyl and carboxyl carbons, whereas acetoacetate was produced from unlabeled acetyl-P and P C] acetate contained no... 

Theoretically, a fall in concentration of oxaloacetate, particularly within the mitochondria, could impair the ability of the citric acid cycle to metabolize acetyl-CoA and divert fatty acid oxidation toward ketogenesis. Such a fall may occur because of an increase in the [NADH]/[NAD+] ratio caused by increased P-oxida-tion affecting the equilibrium between oxaloacetate and malate and decreasing the concentration of oxaloacetate. However, pyruvate carboxylase, which catalyzes the conversion of pyruvate to oxaloacetate, is activated by acetyl-CoA. Consequently, when there are significant amounts of acetyl-CoA, there should be sufficient oxaloacetate to initiate the condensing reaction of the citric acid cycle. 

Two of the three attractant pheromones identified to date are very close structurally to those used in primary metabolism. The biosynthesis of the estolide 5 probably starts from 3-hydroxybutyric acid (4), an intermediate in fatty acid biosynthesis (Fig. 4.3). Condensation of two units furnishes the pheromone 5. The formation of cupilure (3 Fig. 4.2) can be easily explained by two methylations from ubiquitous citric acid. Both compounds are unlike any known insect pheromones, whereas the third known attractant pheromone (ketone 1 Fig. 4.1), bears some resemblance to some insect pheromones. A proper comparison of the differences and similarities between insect and arachnid pheromones will require the identification of representative compounds from most of the families of both groups of organisms. 

Synthesis of most phospholipids starts from glycerol-3-phosphate, which is formed in one step from the central metabolic pathways, and acyl-CoA, which arises in one step from activation of a fatty acid. In two acylation steps the key compound phosphatidic acid is formed. This can be converted to many other lipid compounds as well as CDP-diacylglycerol, which is a key branchpoint intermediate that can be converted to other lipids. Distinct routes to phosphatidylethanolamine and phosphatidylcholine are found in prokaryotes and eukaryotes. The pathway found in eukaryotes starts with transport across the plasma membrane of ethanolamine and/or choline. The modified derivatives of these compounds are directly condensed with diacylglycerol to form the corresponding membrane lipids. Modification of the head-groups or tail-groups on preformed lipids is a common reaction. For example, the ethanolamine of the head-group in phosphatidylethanolamine can be replaced in one step by serine or modified in 3 steps to choline. 

The formation of the poly-P-keto chain could be envisaged as a series of Claisen reactions, the reverse of which are involved in the 3-oxidation sequence for the metabolism of fatty acids (see page 18). Thus, two molecules of acetyl-CoA could participate in a Claisen condensation giving acetoacetyl-CoA, and this reaction could be repeated to generate a poly-P-keto ester of appropriate chain length (Figure 3.1). However, a study of the enzymes involved in fatty acid biosynthesis showed this simple rationalization could not be correct, and a more complex series of... 

In known metabolic states and disorders, the nature of metabolites excreted at abnormal levels has been identified by GC-MS. Examples of this are adipic and suberic acids found in urine from ketotic patients [347], 2-hydroxybutyric acid from patients with lactic acidosis [348], and methylcitric acid (2-hydroxybutan-l,2,3-tricarboxylic acid) [349] in a case of propionic acidemia [350,351]. In the latter instance, the methylcitric acid is thought to be due to the condensation of accumulated propionyl CoA with oxaloacetate [349]. Increased amounts of odd-numbered fatty acids present in the tissues of these patients due to the involvement of the propionyl CoA in fatty acid synthesis, have also been characterised [278]. A deficiency in a-methylacetoacetyl CoA thiolase enzyme in the isoleucine pathway prevents the conversion of a-methylacetoacetyl CoA to propionyl CoA and acetyl CoA [352,353]. The resultant urinary excretion of large amounts of 2-hydroxy-3-methylbutanoic acid (a-methyl-/3-hydroxybutyric acid) and an excess of a-methylacetoacetate and often tiglyl glycine are readily detected and identified by GC-MS. 

The second metabolic pathway which we have chosen to describe is the tricarboxylic acid cycle, often referred to as the Krebs cycle. This represents the biochemical hub of intermediary metabolism, not only in the oxidative catabolism of carbohydrates, lipids, and amino acids in aerobic eukaryotes and prokaryotes, but also as a source of numerous biosynthetic precursors. Pyruvate, formed in the cytosol by glycolysis, is transported into the matrix of the mitochondria where it is converted to acetyl CoA by the multi-enzyme complex, pyruvate dehydrogenase. Acetyl CoA is also produced by the mitochondrial S-oxidation of fatty acids and by the oxidative metabolism of a number of amino acids. The first reaction of the cycle (Figure 5.12) involves the condensation of acetyl Co and oxaloacetate to form citrate (1), a Claisen ester condensation. Citrate is then converted to the more easily oxidised secondary alcohol, isocitrate (2), by the iron-sulfur centre of the enzyme aconitase (described in Chapter 13). This reaction involves successive dehydration of citrate, producing enzyme-bound cis-aconitate, followed by rehydration, to give isocitrate. In this reaction, the enzyme distinguishes between the two external carboxyl groups... 

The biosynthetic reactions involve a series of condensation processes and are distributed between cytosol and microsomes. All of the carbons of cholesterol are derived from acetyl-CoA, 15 from the methyl and 12 from the carboxyl carbon atoms. Acetyl-CoA is derived from mitochondrial oxidation of metabolic fuels (e.g., fatty acids) and transported to cytosol as citrate (Chapter 18) or by activation of acetate (e.g., derived from ethanol oxidation) by cytosolic acetyl-CoA synthase (Chapter 18). All of the reducing equivalents are provided by NADPH. 

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