Biosynthesis of Fatty Acids and Triglycerides (With Diagram)

In this article we will discuss about the biosynthesis of fatty acids and triglycerides.

The Neutral simple lipids are esters of glycerol and long chain fatty acids containing generally 16 or 18 carbon atoms. The fatty acids may be saturated, like palmitic acid and stearic acid, or may be unsaturated, like oleic acid. Fatty acids are synthesized by step-wise addition of two-carbon units in the form of acetyl-CoA. It may be remembered that acetyl-CoA is an important intermediate, produced by decarboxylation of pyruvic acid in course of glucose breakdown pathway.

Although long-chain fatty acids are synthesized by addition of two-carbon units at a time, this two-carbon unit is donated not by acetyl-CoA directly, but by a carboxylated product of acetyl-CoA viz. malonyl-CoA. Thus, for each addition of a two-carbon unit, acetyl-CoA has to be converted to malonyl- CoA and the latter is again decarboxylated as it transfers the acetyl group to the elongating fatty acid chain. As carboxylation reactions require energy supplied by ATP hydrolysis, fatty acid synthesis is an energy consuming process.

The acetyl-group (CH₃-CO-) transferred to the lengthening fatty acid chain is converted to an ethyl group (CH₃-CH₂-) by stepwise reduction, dehydration and again reduction (CH3-CO- — > CH₃– CHOH– –> CH₃-CH=CH– –> CH₃-CH₂-CH₂). These reactions take place on an enzyme complex, called the acyl-carrier protein (ACP). This complex serves as an anchor to which the acyl intermediates remain attached during fatty acid synthesis. ACP contains a single thiol group (-SH) contributed by 4-phosphopantetheine which is linked to ACP via a serine residue. The ACP of E. coli is a small heat-stable protein having a molecular weight of 10,000 Daltons.

Fatty acid synthesis starts with the transfer of the acyl-group of acetyl-CoA to ACP-SH forming acetyl-S-ACP + HS-CoA. Acetyl-S-ACP is then carboxylated to malonyl-S-ACP catalysed by acetyl-S-ACP carboxylase. Next, malonyl-S-ACP reacts with another molecule of acetyl-S-ACP to yield acetoacetyl-S-ACP, CO₂ and ACP-SH.

Now that a two-carbon unit has been added, the product which is acetoacetyl-S-ACP is dehydrogenated, dehydrated and again reduced through three reactions described below. All the time, the reactions are carried out on ACP.

First, acetoacetyl-S-ACP is dehydrogenated by the enzyme β-keto acyl-S-ACP reductase to β-hydroxy butyryl-S-ACP, NADPH₂ acting as H-donor. The product is next dehydrated by an enoyl-ACP dehydratase to an unsaturated fatty acid, crotonyl-S-ACP which is then again dehydrogenated by crotonyl-S-ACP reductase to form butyryl-S-ACP.

Through these reactions, the first cycle of chain-elongation of fatty acid synthesis is completed by adding of a two-carbon unit to the carboxyl end. The cyclic events are repeated by adding each time a malonyl-S-ACP at the carboxyl end displacing-S-ACP and decarboxylation of malonyl-S-ACP, so that an acetyl-rest is added at a time. Thus, for synthesis of palmitic acid, a C-16 fatty acid, seven cycles of the above events have to be repeated. After completion of the synthesis, the fatty acid is released as CoA derivative from ACP.

Fatty acids are converted to neutral fats, which are triglycerides, by esterification with glycerol-3- phosphate to produce first phosphatidic acid and then triglyceride.

Biosynthesis of Murein:

A characteristic polymer that forms the backbone of all eubacterial cell wall is peptidoglycan or murein. Murein essentially consists of parallelly running polysaccharide chains, the repeating unit of which is a disaccharide of N-acetyl-glucosamine (NAG) and N-acetyl muramic acid (NAM) having a tetra-peptide bonded to its lactyl group. The tetra-peptide side chain contains L-alanine, D-alanine, D-glutamic acid and either meso-diaminopimelic acid or its decarboxylated product, L-lysine.

Biosynthesis of murein starts in the cytoplasm and then the precursors are transported to the cell membrane where further synthesis continues. Finally, the precursors are transported across the cell membrane into the periplasmic space in case of Gram-negative bacteria, or outside the membrane in case of Gram-positive bacteria and incorporated into the growing murein chain.

The biosynthesis of murein begins with addition of a lactyl-group donated by phosphoenol pyruvic acid to N-acetyl glucosamine resulting in formation of N-acetyl muramic acid. Prior to this step, N-acetyl glucosamine is produced from glucose in several steps. The formation of NAG and NAM is shown below. It is to be noted that in these biosynthetic processes UDP (uridine diphosphate) acts as a carrier.

NAG in its phosphorylated form is transferred to UTP to form UDP-NAG which is next converted to NAM:

To the UDP-N-acetyl muramic acid (UDP-NAM) so formed are added the amino acids, one by one, to build first a tripeptide side chain and then a pentapeptide side chain. The peptide chain is linked to the lactyl group of UDP-NAM, so that the α-amino group of the first amino acid, L-alanine, is linked with the carboxyl group of lactyl moiety by a peptide bond. The successive amino acids are then linked by peptide bonds with the preceding amino acid. To the tripeptide chain so formed is added a dipeptide, D-alanyl-D-alanine to complete the pentapeptide side chain of UDP-NAM (Fig. 8.75).

Up to the stage of synthesis of UDP-NAM-pentapeptide, the reactions occur in the cytoplasm of the bacterial cells. For the following reactions of the next stage, the UDP-NAM-pentapeptide is transferred to a membrane-bound phospholipid with liberation of UDP. This phospholipid, known as bactoprenol, is undecaprenyl phosphate, a terpenoid having 55 C-atoms and a phosphate group.

Its structure is shown below (Fig. 8.76):

Next, a disaccharide is formed by joining UDP-NAG with the bactoprenoid NAM-pentapeptide by 1——–> 4 glycosidic linkage and setting in this reaction UDP free. The formation of disaccharide takes place in the membrane.

At the next step, the disaccharide with attached pentapeptide is transported outside the membrane leaving the bactoprenoid carrier in the membrane. The disaccharide unit acts as a precursor and is added to the murein chain in the cell wall. Thus, accretion of new cell wall material occurs by transfer of newly synthesized disaccharide-pentapeptide units from bactoprenoid carriers across the membrane to the existing peptidoglycan (murein) chain in the cell wall. By each transfer, the chain is extended by a disaccharide unit (NAG-NAM-pentapeptide) at its reducing end (i.e. C-l of NAG or NAM).

The next step of murein biosynthesis consists of cross-linking of the pentapeptide side chains by addition of short chains of amino acids which connect L-lysine of one pentapeptide chain with the penultimate D-alanine of another pentapeptide, thereby displacing the terminal D-alanine which is set free. In case of Micrococcus lysodeikticus the cross-linking peptide consists of 5 glycine molecules.

The cross-linking of pentapeptide side chains of NAM is called transpeptidation. Several antibiotics, like penicillins, vancomycin, novobiocin, bacitracin etc. are known to inhibit transpeptidation. Another antibiotic, D-cycloserine, inhibits the formation D-alanyl-D-alanine and, hence, the dipeptide cannot be incorporated into pentapeptide. Transpeptidation between parallelly running adjacent peptidoglycan chains is diagrammatically represented in Fig. 8.77.

In presence of low concentration of penicillin, the bacteria excrete uridine nucleotides, known as Park nucleotides (after the discoverer), consisting of UDP-N-acetyl muramic acid and varying lengths of amino acid chain. This indicates that polymerization of murein in the cell wall occurs through transpeptidation of newly synthesized precursors. Because penicillin inhibits transpeptidation, the newly synthesized precursors cannot be incorporated into the peptidoglycan net.

An overall picture of biosynthesis of murein is presented in Fig. 8.78:

Other Cell Wall Polymers:

Apart from murein, several other polymers are present in the cell wall of bacteria. In Gram- positive bacteria, the thick, rigid covering of murein is surrounded by polymers, like teichoic acids and teichuronic acid. Teichoic acids are hydrophilic linear negatively charged molecules and they are linked to murein chain possibly by phosphodiester bonds with the N-acetyl muramic acid residues. Teichoic acids are synthesized from CDP-ribitol or CDP-glycerol precursors with the help of specific polymerases. Sugar side chains present in some teichoic acids are transferred from the UDP-derivatives.

Teichoic acids often contain N-acetyl glucosamine or glucose. These are added from their UDP- derivatives. In Gram-negative bacteria, the thin murein layer is surrounded by a soft covering consisting of a polymer made of lipids and polysaccharides of complex structure.

The cell-wall of Gram-positive acid-fast bacteria is characterized by an exceptionally high lipid content, a feature which is unusual for Gram-positive bacteria. The acid-fastness of these bacteria has been attributed to their high lipid content.

These lipids are mainly derivatives of mycolic acids which have a general structure as shown below, though other lipids are also present:

R₁ and R₂ represent long chain hydrocarbons which are highly hydrophobic in nature.

These mycolic acids are conjugated to polymers of arabinose and galactose (arabinogalactans) and are linked to the peptidoglycan polymer of the cell wall through phosphodiester bonds as shown in Fig. 8.79:

The peptidoglycan layer of the cell wall of acid-fast bacteria differs from that of other eubacteria in at least two respects. Firstly, the disaccharide repeating unit of murein consists of N-acetyl glucosamine and N-glycolyl muramic acid. Secondly, among the amino acids of the tetra peptide side- chains attached to the muramic acid residues, D-glutamic acid and m-diaminopimelic acid often contain amido (-CONH₂) groups.

The structure of a repeating unit of murein is shown below (Fig. 8.80):