MCAT Biochemistry Review
Chapter 11: Lipid and Amino Acid Metabolism
11.5 Fatty Acids and Triacylglycerols
Fatty acids are long-chain carboxylic acids. The carboxyl carbon is carbon 1, and carbon 2 is referred to as the α-carbon. Fatty acids found within the body occur as salts that are capable of forming micelles or are esterified to other compounds, such as the membrane lipids discussed in Chapter 8 of MCAT Biochemistry Review.
When describing a fatty acid, the total number of carbons is given along with the number of double bonds, written as carbons:double bonds. Further description can be given by indicating the position and isomerism of the double bonds in an unsaturated fatty acid. Saturated fatty acids have no double bonds while unsaturated fatty acids have one or more double bonds. Humans can synthesize only a few of the unsaturated fatty acids; the rest come from essential fatty acids found in the diet that are transported in chylomicrons as triacylglycerols from the intestine. Two important essential fatty acids are α-linolenic acid and linoleic acid. These polyunsaturated fatty acids, as well as other acids formed from them, are important in maintaining cell membrane fluidity, which is critical for proper functioning of the cell. The omega (ω) numbering system is also used for unsaturated fatty acids. The ω designation describes the position of the last double bond relative to the end of the chain and identifies the major precursor fatty acid. For example, linoleic acid (18:2 cis,cis-9,12) is the precursor of the ω-6 family, which includes arachidonic acid.α-linolenic acid (18:3 all-cis-9,12,15) is the primary precursor of the ω-3 family. Double bonds in natural fatty acids are generally in the cis configuration.
Trans double bonds are uncommon in natural fatty acids; they predominate in fatty acids found in margarine and other foods that use partial hydrogenation of vegetable oils in their preparation. Compared with liquid oils, these partially hydrogenated fatty acids are solids at room temperature. These fatty acids contribute to arterial diseases and decreased membrane fluidity.
Fatty acids used by the body for fuel are supplied primarily by the diet. In addition, excess carbohydrate and protein acquired from the diet can be converted to fatty acids and stored as energy reserves in the form of triacylglycerol. Lipid and carbohydrate synthesis are often callednontemplate synthesis processes because they do not rely directly on the coding of a nucleic acid, unlike protein and nucleic acid synthesis.
Fatty Acid Biosynthesis
Fatty acid biosynthesis, shown in Figure 11.5, occurs in the liver and its products are subsequently transported to adipose tissue for storage. Adipose tissue can also synthesize smaller quantities of fatty acids. Both of the major enzymes of fatty acid synthesis, acetyl-CoA carboxylase andfatty acid synthase, are also stimulated by insulin. Palmitic acid (palmitate) is the primary end product of fatty acid synthesis.
Figure 11.5. Fatty Acid Synthesis from Glucose
Following a large meal, acetyl-CoA accumulates in the mitochondrial matrix and needs to be moved to the cytosol for fatty acid biosynthesis. Acetyl-CoA is the product of the pyruvate dehydrogenase complex, and it couples with oxaloacetate to form citrate at the beginning of the citric acid cycle. Remember that isocitrate dehydrogenase is the rate-limiting enzyme of citric acid cycle; as the cell becomes energetically satisfied, it slows the citric acid cycle, which causes citrate accumulation. Citrate can then diffuse across the mitochondrial membrane. In the cytosol, citrate lyase splits citrate back into acetyl-CoA and oxaloacetate. The oxaloacetate can then return to the mitochondrion to continue moving acetyl-CoA.
Acetyl-CoA is activated in the cytoplasm for incorporation into fatty acids by acetyl-CoA carboxylase, the rate-limiting enzyme of fatty acid biosynthesis. Acetyl-CoA carboxylase requires biotin and ATP to function, and adds CO2 to acetyl-CoA to form malonyl-CoA. The enzyme is activated by insulin and citrate. The CO2 added to form malonyl-CoA is never actually incorporated into the fatty acid because it is removed by fatty acid synthase during addition of the activated acetyl group to the fatty acid.
Fatty Acid Synthase
Fatty acid synthase is more appropriately called palmitate synthase because palmitate is the only fatty acid that humans can synthesize de novo. Fatty acid synthase is a large multienzyme complex found in the cytosol that is rapidly induced in the liver following a meal high in carbohydrates because of elevated insulin levels. The enzyme complex contains an acyl carrier protein (ACP) that requires pantothenic acid (vitamin B5). NADPH is also required to reduce the acetyl groups added to the fatty acid. Eight acetyl-CoA groups are required to produce palmitate (16:0). Fatty acyl-CoA may be elongated and desaturated, to a limited extent, using enzymes associated with the smooth endoplasmic reticulum (SER). The steps involved in fatty acid biosynthesis are shown in Figure 11.6 and include attachment to an acyl carrier protein, bond formation between activated malonyl-CoA (malonyl-ACP) and the growing chain, reduction of a carboxyl group, dehydration, and reduction of a double bond. These reactions occur over and over again until the sixteen-carbon palmitate molecule is created. Many of these reactions are reversed in β-oxidation.
Figure 11.6. Action of Fatty Acid Synthase Reactions include activation of the growing chain (a) and malonyl-CoA (b) with ACP, bond formation between these activated molecules (c), reduction of a carbonyl to a hydroxyl group (d), dehydration (e), and reduction to a saturated fatty acid (f).
Triacylglycerol (Triglyceride) Synthesis
Triacylglycerols, the storage form of fatty acids, are formed by attaching three fatty acids (as fatty acyl-CoA) to glycerol. Triacylglycerol formation from fatty acids and glycerol 3-phosphate occurs primarily in the liver and somewhat in adipose tissue, with a small contribution directly from the diet, as well. In the liver, triacylglycerols are packaged and sent to adipose tissue as very-low-density lipoproteins (VLDL), leaving only a small amount of stored triacylglycerols.
Fatty acid synthesis and β-oxidation are reverse processes. Both involve transport across the mitochondrial membrane, followed by a series of redox reactions, but always in the opposite direction of one another. Understanding one process will enable you to answer questions about both pathways.
Most fatty acid catabolism proceeds via β-oxidation that occurs in the mitochondria; however, peroxisomal β-oxidation also occurs. Branched-chain fatty acids may also undergo α-oxidation, depending on the branch points, while ω-oxidation in the endoplasmic reticulum produces dicarboxylic acids. You should be aware that these processes exist; however, the mechanisms are beyond the scope of the MCAT. We will take an in-depth look at β-oxidation, which will be much more heavily tested. Insulin indirectly inhibits β-oxidation while glucagon stimulates this process.
When fatty acids are metabolized, they first become activated by attachment to CoA, which is catalyzed by fatty-acyl-CoA synthetase. The product is generically referred to as a fatty acyl-CoA or acyl-CoA. Specific examples would be acetyl-CoA containing a 2-carbon acyl group, or palmitoyl-CoA with a 16-carbon acyl group.
Fatty Acid Entry Into Mitochondria
Short-chain fatty acids (two to four carbons) and medium-chain fatty acids (six to twelve carbons) diffuse freely into mitochondria, where they are oxidized. In contrast, while long-chain fatty acids (14 to 20 carbons) are also oxidized in the mitochondria, they require transport via a carnitine shuttle, as shown in Figure 11.7.
Figure 11.7. Fatty Acid Activation and Transport
Carnitine acyltransferase I is the rate-limiting enzyme of fatty acid oxidation. Very long-chain fatty acids (over 20 carbons) are oxidized elsewhere in the cell.
β-Oxidation in Mitochondria
β-Oxidation reverses the process of fatty acid synthesis by oxidizing and releasing (rather than reducing and linking) molecules of acetyl-CoA. The pathway is a repetition of four steps; each four-step cycle releases one acetyl-CoA and reduces NAD+ and FAD (producing NADH and FADH2). The FADH2 and NADH are oxidized in the electron transport chain, producing ATP. In muscle and adipose tissue, acetyl-CoA enters the citric acid cycle. In the liver, acetyl-CoA, which cannot be converted to glucose, stimulates gluconeogenesis by activating pyruvate carboxylase. In a fasting state, the liver produces more acetyl-CoA from β-oxidation than is used in the citric acid cycle. Much of the acetyl-CoA is used to synthesize ketone bodies (essentially two acetyl-CoA molecules linked together) that are released into the bloodstream and transported to other tissues.
Figure 11.8. β-Oxidation
The four steps of β-oxidation, illustrated in Figure 11.8, are:
1. Oxidation of the fatty acid to form a double bond
2. Hydration of the double bond to form a hydroxyl group
3. Oxidation of the hydroxyl group to form a carbonyl (β-ketoacid)
4. Splitting of the β-ketoacid into a shorter acyl-CoA and acetyl-CoA
This process then continues until the chain has been shortened to two carbons, creating a final acetyl-CoA.
Fatty acids with an odd number of carbon atoms undergo β-oxidation in the same manner as even-numbered carbon fatty acids for the most part. The only difference is observed during the final cycle, where even-numbered fatty acids for the most part. yield two acetyl-CoA molecules (from the four-carbon remaining fragment) and odd-numbered fatty acids yield one acetyl-CoA and one propionyl-CoA (from the five-carbon remaining fragment), as shown in Figure 11.9. Propionyl-CoA is converted to methylmalonyl-CoA by propionyl-CoA carboxylase, which requires biotin (vitamin B7). Methylmalonyl-CoA is then converted into succinyl-CoA by methylmalonyl-CoA mutase, which requires cobalamin (vitamin B12). Succinyl-CoA is a citric acid cycle intermediate and can also be converted to malate to enter the gluconeogenic pathway in the cytosol. Odd-carbon fatty acids thus represent an exception to the rule that fatty acids cannot be converted to glucose in humans.
Figure 11.9. The Propionic Acid Pathway
Until now we've been discussing the oxidation of saturated fatty acids. In unsaturated fatty acids, two additional enzymes are necessary because double bonds can disturb the stereochemistry needed for oxidative enzymes to act on the fatty acid. To function, these enzymes can have at most one double bond in their active site; this bond must be located between carbons 2 and 3. Enoyl-CoA isomerase, shown in Figure 11.10, rearranges cis double bonds at the 3,4 position to trans double bonds at the 2,3 position once enough acetyl-CoA has been liberated to isolate the double bond within the first three carbons. In monounsaturated fatty acids this single step permits β-oxidation to proceed.
Figure 11.10. Reaction of Enoyl-CoA Isomerase
In polyunsaturated fatty acids, a further reduction is required using 2,4-dienoyl-CoA reductase to convert two conjugated double bonds to just one double bond at the 3,4 position, where it will undergo the same rearrangement as monounsaturated fatty acids, as shown in Figure 11.11.
Figure 11.11. Reaction of 2,4-Dienoyl-CoA Reductase
MCAT Concept Check 11.5:
Before you move on, assess your understanding of the material with these questions.
1. Draw the following fatty acids: palmitic acid, 18:3 (all-cis-9,12,15), ω-6.
2. What are the five steps in the addition of acetyl-CoA to a growing fatty acid chain?
3. How does β-oxidation of unsaturated fatty acids differ from that of saturated fatty acids?
4. True or False: Fatty acids are synthesized in the cytoplasm and modified by enzymes in the smooth endoplasmic reticulum.