LIPIDBIOSYNT
EnergyStorage
Fatty acid synthesis is regulated, both in plants andanimals. Excess carbohydrate and protein in the diet are converted into fat.Only a relatively small amount of energy is stored in animals as glycogen orother carbohydrates, and the level of glycogen is closely regulated.
Protein storage doesn’t take place in animals. Except for thesmall amount that circulates in the cells, amino acids exist in the body onlyin muscle or other protein-containing tissues. If the animal or human needsspecific amino acids, they must either be synthesized or obtained from thebreakdown of muscle protein. Adipose tissue serves as the major storage areafor fats in animals. A normal human weighing 70 kg contains about 160 kcal ofusable energy. Less than 1 kcal exists as glycogen, about 24 kcal exist as amino acids in muscle, and thebalance-more than 80 percent of the total-exists as fat. Plants make oils forenergy storage in seeds. Because plants must synthesize all their cellularcomponents from simple inorganic compounds, plants-but usually not animals-canuse fatty acids from these oils to make carbohydrates and amino acids for latergrowth after germination.
Fatty Acid Biosynthesis
The biosynthetic reaction pathway to a compound is usuallynot a simple opposite of its breakdown. Chapter 12 of Volume 1 discusses thisconcept in regard to carbohydrate metabolism and gluconeogenesis. In fatty acidsynthesis, acetyl-CoA is the direct precursor only of the methyl end of the growing fatty acidchain. All the other carbons come from the acetyl group of acetyl-CoA but onlyafter it is modified to provide the actual substrate for fatty acid synthase,malonyl-CoA.
Malonyl-CoA contains a 3-carbon dicarboxylic acid, malonate, bound to Coenzyme A. Malonate isformed from acetyl-CoA by the addition of CO2 using the biotin cofactor of theenzyme acetyl-CoA carboxylase.
HCO3
– Acetyl-CoA + HCO3
– + ATP Malonyl-CoA + ADP + Pi
Formation of malonyl-CoA is the commitment step for fattyacid synthesis, because malonyl-CoA has no metabolic role other than serving asa precursor to fatty acids.
Fatty acid synthase (FAS) carries out the chain elongationsteps of fatty acid biosynthesis. FAS is a large multienzyme complex. Inmammals, FAS contains two subunits, each containing multiple enzyme activities. In bacteria andplants, individual proteins, which associate into a large complex, catalyze theindividual steps of the synthesis scheme.
Initiation
Fatty acid synthesis starts with acetyl-CoA, and the chaingrows from the “tail end” so that carbon 1 and the alpha-carbon of the completefatty acid are added last. The first reaction is the transfer of the acetylgroup to a pantothenate group of acyl carrier protein (ACP), a region of thelarge mammalian FAS protein. (The acyl carrier protein is a small, independentpeptide in bacterial FAS, hence the name).
The pantothenate group of ACP is the same as is found onCoenzyme A, so the transfer requires no energy input: Acetyl~S-CoA + HS-ACP®HS-CoA + Acetyl~S-ACP.
In the preceding reaction, the S and SH refer to the thiogroup on the end of Coenzyme A or the pantothenate groups. The ~ is a reminderthat the bond between the carbonyl carbon of the acetyl group and the thio group is a “highenergy” bond (that is, the activated acetyl group is easily donated to anacceptor). The second reaction is another transfer, this time, from thepantothenate of the ACP to cysteine sulfhydral (–SH) group on FAS.
Acetyl~ACP + HS-FAS ® HS-ACP + Acetyl~S-FAS
Note that at this point, the FAS has two activated substrates,the acetyl group bound on the cysteine –SH and the malonyl group bound on thepantothenate –SH. Transfer of the 2-carbon acetyl unit from
Acetyl~S-cysteine to malonyl-CoA has two features:
Release of the CO2 group of malonyic acid that was originally
put on by acetyl-CoA carboxylase
Generation of a 4-carbon b-keto acid derivative, bound to thepantothenate of the ACP protein
The ketoacid is now reduced to the methylene (CH2) state in a
three-step reaction sequence.
The elongated 4-carbon chain is now ready to accept a new2-carbon unit from malonyl-CoA. The 2-carbon unit, which is added to thegrowing fatty acid chain, becomes carbons 1 and 2 of hexanoic acid (6-carbons).
Release
The cycle of transfer, elongation, reduction, dehydration,and reduction continues until palmitoyl-ACP is made. Then the thioesteraseactivity of the FAS complex releases the 16-carbon fatty acid palmitate fromthe FAS.
Note that fatty acid synthesis provides an extreme example ofthe phenomenon of metabolic channeling: neither free fatty acids with more thanfour carbons nor their CoA derivatives can directly participate in thesynthesis of palmitate. Instead they must be broken down to acetyl-CoA and reincorporated intothe fatty acid.
Fatty acids are generated cytoplasmically while acetyl-CoA ismade in the mitochondrion by pyruvate dehydrogenase.This implies that a shuttlesystem must exist to get the acetyl-CoA or its equivalent out of themitochondrion. The shuttle system operates in the following:
way: Acetyl-CoA is first converted to citrate by citratesynthase in the TCA-cycle reaction. Then citrate is transferred out of themitochondrion by either of two carriers, driven by the electroosmotic
gradient: either a citrate/phosphate antiport or a citrate/malateantiport as shown in Figure 2-2.
Fatty acid biosynthesis (and most biosynthetic reactions)requires NADPH to supply the reducing equivalents. Oxaloacetate is used togenerate NADPH for biosynthesis in a two-step sequence.
The first step is the malate dehydrogenase reaction found inthe TCA cycle. This reaction results in the formation of NAD from NADH (theNADH primarily comes from glycolysis). The malate formed is a substrate for themalic enzyme reaction, which makes pyruvate, CO2, and NADPH. Pyruvate istransported into the mitochondria where pyruvate carboxylase uses ATP energy toregenerate oxaloacetate.
Palmitate is the starting point for other fatty acids thatuse a set of related reactions to generate the modified chains and head groupsof the lipid classes. Microsomal enzymes primarily catalyze these chain modifications. Desaturationuses O2 as the ultimate electron acceptor to introduce double bonds at thenine, six, and five positions of an acyl-CoA.
Elongation is similar to synthesis of palmitate because ituses malonyl-CoA as an intermediate. See Figure 2-3.
Synthesisof Triacylglycerols
Glycerol accepts fatty acids from acyl-CoAs to synthesizeglycerol lipids. Glycerol phosphate comes from glycolysis-specifically from thereduction of dihydroxyacetone phosphate using NADH as a cofactor. Then theglycerol phosphate accepts two fatty acids from fatty acyl-CoA. The fatty acyl-CoA isformed by the expenditure of two high-energy phosphate bonds from ATP.
Cholesterol Biosynthesis and its Control
Despite a lot of bad press, cholesterol remains an essentialand important biomolecule in animals. As much as half of the membrane lipid ina cellular membrane is cholesterol, where it helps maintain constant fluidityand electrical properties. Cholesterol is especially prominent in membranes of the nervoussystem.
Cholesterol also serves as a precursor to other importantmolecules. Bile acids aid in lipid absorption during digestion. Steroidhormones all derive from cholesterol, including the adrenal hormones thatmaintain fluid balance; Vitamin D, which is an important regulator of calcium status; and the male andfemale sex hormones.
Although humans wouldn’t survive in one sense or anotherwithout cholesterol metabolites, cholesterol brings with it some well-knownside effects. Doctors find cholesterol derivatives, being essentially insolublein water, in the deposits (plaque) that characterize diseased arteries.
HMG CoA Reductase
HMG-CoA reductase is the committed and therefore theregulatory step in cholesterol biosynthesis. If HMG-CoA is reduced tomevalonate, cholesterol is the only product that can result. The reduction is atwo-step reaction, which releases the Coenzyme A cofactor and converts thethiol-bound carboxylic group of HMG-CoA to a free alcohol. Two NADPH moleculessupply the reducing equivalents because the thioester must first be reduced tothe level of an aldehyde and then to an alcohol.
Mevalonate Squalene
Mevalonate molecules are condensed to a 30-carbon compound,squalene. The alcohol groups of mevalonate are first phosphorylated. Then theymultiply phosphorylated mevalonate decarboxylates to make the two compoundsisopentenyl pyrophosphate (IPP) and dimethylallyl pyrophosphate (DMAPP).
mevalonate ® phosphomevalonate ® pyrophosphomevalonate
First, the other hydroxyl group of mevalonate accepts aphosphate from ATP. The resulting compound rearranges in an enzyme-catalyzedreaction, eliminating both CO2 and phosphate. The 5-carbon compound that results, IPP,is rapidly isomerized with DMAPP.
In plants and fungi, IPP and DMAPP are the precursors to manyso-called isoprenoid compounds, including natural rubber. In animals, they aremainly precursors to sterols, such as cholesterol. The first step iscondensation of one of each to geranyl pyrophosphate, which then condenses with anothermolecule of IPP to make farnesyl pyrophosphate. Some important membrane-boundproteins have a farnesyl group added on to them; however, the primary fate offarnesyl pyrophosphate is to accept a pair of electrons from NADPH and condensewith another molecule of itself to release both pyrophosphate groups.
The resulting 30-carbon compound is squalene; it folds into astructure that closely resembles the structure of the steroid rings, althoughthe rings are not closed yet.
Squalene ® Lanosterol
The first recognizable steroid ring system is lanosterol; itis formed first by the epoxidation of the double bond of squalene that wasoriginally derived from a DMAPP through farnesyl pyrophosphate, and then by thecyclization of squalene epoxide. The enzyme that forms the epoxide uses NADPHto reduce molecular oxygen to make the epoxide.
Lanosterol ® Cholesterol
This sequence of reactions is incompletely understood butinvolves numerous oxidations of carbon groups, for example, the conversion ofmethyl groups to carboxylic acids, followed bydecarboxylation. The end product,cholesterol, is the precursor to cholesterol esters in the liver and istransported to the peripheral tissues where it is a precursor to membranes (allcells), bile salts (liver), steroid hormones (adrenals and reproductivetissues), and vitamin D (skin, then liver, and finally kidney).
Cholesterol Transport, Uptake, and ControlCholesterol isexpor ted to the peripheral tissues in LDL and VLDL (see Chapter 1). About 70percent of the cholesterol molecules in LDL are esterified with a fatty acid(for example, palmitate) on the OH group (at Carbon 3; see Figure 2-5). Cellstake up cholesterol from the LDL by means of LDL receptors in the outer cellmembrane.