This chapter discusses lipid metabolism. The digestion and absorption of fats pose considerable problems because of their insolubility in water. The fat splitting enzymes or lipases are water-soluble, and can only operate at the interface between lipid droplets and the aqueous phase. For this reason, the degree of dispersion or emulsification is very important in ensuring that fat is adequately digested and absorbed. Lipases are secreted both in the stomach and in the pancreatic juice. The value of gastric lipase is doubtful as its optimum pH is 8–0 whereas the pH of the normal adult stomach is 1.0–2.0. The process of emulsification of other fats is probably initiated by the churning motion of the stomach and is greatly facilitated when the chyme passes into the duodenum, and is mixed with the bile and pancreatic juice. These secretions, which are both alkaline, neutralize the acid from the stomach so that conditions become suitable for the action of pancreatic lipase. Bile contains no enzymes but owes its effects to the presence of the bile salts, sodium glycocholate, and sodium taurocholate, which are powerful detergents. The bile salts are derived from cholic acid, a sterol, which is joined by a peptide linkage either to glycine or to taurine. Lipase is relatively unspecific in its action as it is little affected by either the degree of saturation or the chain length of the fatty acids present in the glyceride. It acts preferentially to remove the fatty acid present in the a-position, so that the first product is an α, β-diglyceride.
The digestion and absorption of fats pose considerable problems because of their insolubility in water. The fat splitting enzymes or lipases are water-soluble and consequently can only operate at the interface between lipid droplets and the aqueous phase. For this reason the degree of dispersion or emulsification is very important in ensuring that fat is adequately digested and absorbed.
Lipases are secreted both in the stomach and in the pancreatic juice. The value of gastric lipase is doubtful since its optimum pH is 8·0 whereas the pH of the normal adult stomach is 1·0−2·0. It may be more effective in infants, where the pH of the stomach contents is much higher and where the fat is mainly milk fat which is already highly emulsified.
The process of emulsification of other fats is probably initiated by the churning motion of the stomach and is greatly facilitated when the chyme passes into the duodenum and is mixed with the bile and pancreatic juice. These secretions, which are both alkaline, neutralize the acid from the stomach so that conditions become suitable for the action of pancreatic lipase. Bile contains no enzymes but owes its effects to the presence of the bile salts, sodium glycocholate and sodium taurocholate, which are powerful detergents. The bile salts are derived from cholic acid, a sterol, which is joined by a peptide linkage either to glycine or to taurine.
Lipase is relatively unspecific in its action since it is little affected by either the degree of saturation or the chain length of the fatty acids present in the glyceride. It acts preferentially to remove the fatty acid present in the α-position, so that the first product is an α,β-diglyceride. The fatty acid in the second α-position is then removed to give a β-monoglyceride. Hydrolysis of the monoglyceride probably only occurs after isomerization, involving transfer of the remaining acyl group to one of the α-positions (Figure 18.1). The first, second and third fatty acids are removed with increasing difficulty, so that monoglycerides are a major product of fat digestion and small amounts of diglyceride also remain. It appears that only about 30−40% of the triglycerides are completely hydrolysed to glycerol and fatty acids.
Although fatty acids, glycerol and monoglycerides are the main products of fat digestion, none of these substances appears in the bloodstream in appreciable amounts. Glycerol is water-soluble and readily absorbed by the intestinal epithelial cells. The long chain fatty acids and their monoglycerides, on the other hand, are only sparingly soluble in water and are not readily absorbed. Their absorption is facilitated by the presence of the bile salts with which they form molecular aggregates or micelles. It is in such micellar dispersions that the fatty acids and monoglycerides appear to enter the mucosal cells, although how they do so is not clear. Once inside the mucosal cells the fatty acid-bile salt complex dissociates and the bile salts pass into the portal circulation and are returned to the liver. The role of the bile salts in absorption seems to be even more important than in digestion, since if bile is totally excluded from the gut large amounts of fatty acids are found in the faeces.
The fatty acids and monoglycerides released from micelles are resynthesized into triglycerides within the epithelial cells, the mechanism of resynthesis being similar to that for triglyceride synthesis in adipose tissue (page 258). It should be noted that triglycerides that accumulate in the intestinal mucosal cells during fat absorption are different from those originally present in the food, both with respect to the arrangement of their fatty acids and the origin of the glycerol part of the molecule. Glycerol in totally resynthesized fat is derived from intermediates of glycolysis and not from the original glyceride.
From the mucosal cells, the droplets of resynthesized triglyceride pass into the central lacteals of the intestinal villi. They then move into the lymphatics where they appear as small droplets known as chylomicrons which are coated with a stabilizing layer of protein, phospholipid and cholesterol. From the lymphatics they pass into the systemic blood via the thoracic duct giving the plasma a milky appearance. This lipaemia is, however, only temporary since the chylomicrons are readily attacked by lipoprotein lipase which is produced in a number of tissues and released into the capillaries. This enables the tissues to take up fatty acids from the blood and use them for immediate energy production or for energy storage. The lipoprotein lipase activity of the extrahepatic tissues varies according to the nutritional state. After a meal adipose tissue has a high activity while that of muscle is low; consequently a large proportion of the ingested fatty acids is stored. On the other hand during starvation, exercise or exposure to cold, the lipoprotein lipase activity of muscle is high and that of adipose tissue low so that fatty acids are used mainly for energy production. This adaptation to the changing needs of the organism is probably achieved by hormonal control.
Most of the long chain fatty acids are absorbed into the lymphatic system but the small proportion of fatty acids that contain less than ten carbon atoms do not become re-esterified in the mucosal cells and find their way directly into the portal blood where they circulate as non-esterified fatty acids bound to serum albumin. On reaching the tissues these fatty acids are usually oxidized straight away rather than stored.
Body fats may be broadly divided into two types, namely the constant and variable elements. The constant element, which, as the name suggests, is not subject to appreciable variations in amount, is composed mainly of phospholipids and other compound lipids. It represents those lipids present in all regions of the body that fulfil a mainly structural role. The variable element is composed almost entirely of triglycerides and constitutes the main energy reserve. This depot fat is found as a layer under the skin and also surrounding the viscera. It shows wide variations in amount according to the nutritional state. In addition, small amounts of lipid material are present in transport forms in the blood. The plasma of a normal post-absorptive subject contains about 500 mg of total lipid per 100 ml. Free fatty acids are present only in small amounts. Except in the immediate post-absorptive period, the plasma is clear and limpid, since the water-insoluble triglycerides and free fatty acids are associated with various plasma protein fractions to give soluble lipoprotein complexes. It is now clear that adipose tissue fat undergoes rapid and continuous turnover even when the body weight is constant or falling. In line with this is the finding that the small amounts of unesterified or free fatty acids (FFA) in the plasma have a high turnover rate. When ample amounts of energy-producing materials are available more are added to than released from the tissue stores and the plasma concentration tends to fall. When extra energy is required, more FFA are released from the tissues and the plasma concentration rises. These two opposing functions of adipose tissue, the accumulation and mobilization of fat, are delicately balanced. However, at certain times, and under certain conditions, one or other process will predominate. Survival of many animal species depends on their ability to store sufficient fat in times of plenty to tide them over lean periods. The amount laid down must not, however, be so great that their mobility is impaired.
Although under normal circumstances this may not be an important consideration, when water supplies are restricted it is clearly advantageous that a large proportion of the energy requirement should be derived from fat or better still from alcohol! On the other hand, protein consumption should be kept low since the urea produced from its metabolism requires water for its elimination via the kidneys. The camel’s hump is, of course, mainly composed of fat.
In fasting conditions Man and other mammals use fat as the main source of energy and it has been estimated that, even in normally fed animals, the oxidation of fatty acids provides at least half of the energy used by the liver, kidneys, heart and resting skeletal muscle. Brain, however, normally derives little or no energy from this source and is dependent upon the blood glucose.
The oxidation of fatty acids takes place in the mitochondria and involves a series of reactions by which fragments containing two carbon atoms are released one at a time in the form of acetyl-CoA. The reactions are repeated until the entire fatty acid chain has been converted into acetyl-CoA which enters the citrate cycle. Significant amounts of ATP are generated in the oxidative reactions which lead to the formation of acetyl-CoA, but even more is derived from the oxidation of the acetyl-CoA itself.
Oxidation of triglycerides must be preceded by lipolysis. Breakdown into their constituent fatty acids and glycerol occurs in the tissues under the influence of lipases. A special adrenaline-sensitive lipase in adipose tissue releases FFA and glycerol into the blood when extra energy is required. Glycerol, which represents only a minor fraction of the triglyceride (about 10% in tristearin), is activated by conversion to glycerol 3-phosphate under the influence of the enzyme, glycerol kinase. This is present in liver and kidney and various other tissues but is virtually absent from adipose tissue, skeletal and heart muscle. The glycerol 3-phosphate is then dehydrogenated to dihydroxyacetone phosphate which is a normal intermediate of carbohydrate metabolism involved in both glycolytic and gluconeogenetic pathways.
Similarly the fatty acids must be activated by conversion to their CoA derivatives before they can be metabolized. Formation of the fatty acyl-CoA derivatives is catalysed by various fatty acid thiokinases (fatty acid: CoA ligases) whose activity is linked with the breakdown of ATP to AMP and pyrophosphate, the liberated energy being used in the formation of the thiol ester bond:
Since the reaction is essentially irreversible, fatty acids may be activated and metabolized even when present in low concentration. Furthermore, the reaction will proceed when the ATP concentration is low. The expenditure of the ATP is well worth while in view of the much greater amounts produced during the oxidation of the fatty acids.
Although conversion of FFA to their CoA esters may occur in either the cytoplasm or the mitochondria, their oxidation occurs only within the mitochondria and, since the inner mitochondrial membrane is impermeable to acyl-CoA compounds, a special carrier is needed to take the acyl groups across. The carrier is the compound carnitine, to which the acyl group is transferred from acyl-CoA by a carnitine-CoA acyl transferase which is associated with the mitochondrial membrane.
Oxidation of fatty acyl-CoA compounds occurs by a well-recognized pattern of reactions similar to those occurring at the four-carbon stages of the citrate cycle. Thus oxidation of the β-carbon atom as shown in Figure 18.2 occurs by: (1) removal of 2H to give an unsaturated derivative; (2) addition of H2O to give a β-hydroxy derivative; (3) removal of 2H to give a β-keto derivative.
The first reaction of the sequence is catalysed by a fatty acyl:CoA dehydrogenase. The reaction requires a stronger oxidizing agent than NAD+ and the enzymes are flavoproteins containing FAD as the prosthetic group. Before the two hydrogen atoms which have been removed are passed along the carriers in the later stages of the electron-transport chain, they are transferred to a second flavoprotein, known as the electron-transport flavoprotein. It is not clear why an extra intermediate is required here but the by-passing of NAD+ means that only two molecules of ADP are converted to ATP in the course of this particular oxidation.
In the next reaction catalysed by enoyl hydrase, water is added at the double bond giving the corresponding β-hydroxyacyl-CoA. This is followed by the second dehydrogenation under the influence of β-hydroxyacyl-CoA dehydrogenase, which uses NAD+ as the electron acceptor. In the presence of a further molecule of CoA the resulting β-ketoacyl-CoA is split by β-ketothiolase. This reaction, which is known as thiolysis, produces one molecule of acetyl-CoA and one molecule of acyl-CoA in which the acyl group is two carbons shorter than the original. The reaction is highly exergonic and virtually irreversible.
Points worth noting are: (1) All the reactants are acyl derivatives of CoA. (2) All the enzymes are localized within the mitochondria with those of the citrate cycle and electron-transport chain. This ensures efficient utilization of the acetyl-CoA released by the fatty acid oxidation. (3) Only one activation step is necessary, regardless of the length of the fatty acid chain. This uses two high-energy bonds, since ATP is broken down to AMP and pyrophosphate during the thiokinase reaction.
The pathway represents a remarkably efficient and economical method of processing a great variety of fatty acids. It can be used for all those containing an even number of C atoms regardless of the chain length, and also for the unsaturated fatty acids found abundantly in foodstuffs. As a result of the combined action of two addition enzymes, unsaturated fatty acids are converted into standard intermediates for the β-oxidative pathway.
During the oxidation of palmitate by the pathway described, 8 molecules of acetyl-CoA are produced. Each of these, on oxidation via the citrate cycle, provides for the conversion of 12 molecules of ADP to ATP so that a total of 96 molecules of ATP will be formed. In addition, during each of the seven β-oxidations required to produce the 8 molecules of acetyl-CoA, 1 molecule of reduced flavoprotein and 1 of NADH are formed. In the course of the oxidation of the reduced flavoprotein, 2 molecules of ATP are produced while the oxidation of each molecule of NADH gives rise to 3. Thus every β-oxidation results in the synthesis of 5 molecules of ATP giving a total of 35 (7 × 5). The balance sheet for the production of ATP is as follows:
|7 β-oxidations each producing 5 molecules of ATP||+ 35|
|Oxidation of 8 molecules of acetyl-CoA each producing 12 molecules of ATP||+96|
|Less 2 ATP equivalents expended during the initial fatty acid activation and conversion of ATP to AMP||− 2|
|Net gain 129|
The amount of energy that higher vertebrates can store in the form of carbohydrate is strictly limited and most of the surplus energy taken in when food is plentiful is stored as fat. Although fatty acids cannot give rise to carbohydrate in the body, all types of energy-producing materials may be converted into depot fat and carbohydrate appears to be a major source. In spite of the fact that the carbon residues of the amino acids are readily transformed into pyruvate or citrate cycle intermediates, which may in turn be converted into fat, significant amounts are unlikely to be derived from dietary protein in normal circumstances.
The long chain fatty acids are built up from units containing two C atoms derived from acetyl-CoA. The process is essentially reductive since, after condensation of the units, a −CO− group must be converted to a −CH2 group. In some respects the synthesis of the acyl residue resembles the oxidative process in reverse, but there are a number of important differences. In the first instance, whereas fatty acids are oxidized within the mitochondria, their synthesis is essentially a cytoplasmic process. Moreover, while the muscles are the principal site of fatty acid oxidation, fatty acid synthesis occurs mainly in the liver and adipose tissue.
During the conversion of carbohydrate to fatty acids the carbohydrate is first oxidized to pyruvate which enters the mitochondria and is oxidatively decarboxylated by pyruvate dehydrogenase to acetyl-CoA and CO2. Some of the acetyl-CoA will be drawn directly into the citrate cycle to replenish the stocks of ATP. If these are already high, a large proportion of the acetyl-CoA will be used for fatty acid synthesis. However, this is a cytoplasmic process and the mitochondrial wall is not readily permeable to acyl-CoA compounds. The problem is overcome by uniting the acetyl groups with oxaloacetate to form citrate which is able to pass between the extra- and intra-mitochondrial compartments. Thus when citrate is present in high concentration, due to a surplus of substrates for the final stages of oxidative metabolism, it diffuses from the mitochondria into the cytosol. Once in the cytosol, the citrate is subject to the action of the citrate cleavage enzyme ATP citrate lyase, which, in the presence of ATP, breaks it down into oxaloacetate and acetyl-CoA once again. In this way, oxaloacetate acts as a carrier of acetyl groups from the mitochondria into the cytoplasm. The acetyl groups are then used for fatty acid synthesis and oxaloacetate is converted to pyruvate by a two-stage process:
As a result of these reactions when energy supplies are plentiful, reducing power (NADH), obtained during the conversion of glucose to pyruvate by the glycolytic pathway, instead of being used for further energy production can be transferred to NADP+ and used for fat synthesis and energy storage. One NADPH is generated for every acetyl unit that passes into the cytosol from the mitochondria but the synthesis of 1 mol of palmitate requires 14 mol of NADPH (see below) and the remaining 6 are obtained from the pentose phosphate pathway. The pyruvate passes back into the mitochondria and is used either for regeneration of oxaloacetate or for conversion to acetyl-CoA (Figure 18.5).
In the first reaction of fatty acid synthesis acetyl-CoA is converted to malonyl-CoA by the very important enzyme, acetyl-CoA carboxylase which contains biotin (page 166) as its prosthetic group. ATP is needed to provide energy for the carboxylation and magnesium ions are also required:
Acetyl-CoA carboxylase is an allosteric enzyme and the reaction is the primary regulating step in fatty acid synthesis (page 341). It is also regulated through phosphorylation/dephosphorylation reactions which modify specific serine residues on the protein (page 344).
The subsequent series of reactions that lead eventually to the formation of palmitate in mammals are carried out by a multienzyme complex known as fatty acid synthase. The initial two carbons, which eventually form C-15 and C-16 of palmitate, are supplied in the form of acetyl-CoA which acts as a primer for the subsequent condensation of malonyl-CoA units which lose CO2 in the process. The hydrogens required for the reductive reactions are all supplied by NADPH and the overall process may be represented as follows: