Biosynthesis of Lipids

Suaib Luqman

  1. Objectives

 

  • v To understand the biosynthesis of lipids
  • v How do fatty acid, triacylglycerol, phospholipid and cholesterol synthesize

 

  1. Concept Map

3. Description

 

Lipids not only play an array of cellular roles including energy storage, membrane constituents but also serve as anchors for membrane proteins (prenyl groups, phosphatidylinositol), cofactors (vitamin K), detergents (bile salts), extracellular and intracellular messengers (eicosanoids, phosphatidylinositol derivatives), hormones (vitamin D derivatives, sex hormones), pigments (Retinal, carotene) and transporters (dolichols). Essentially, all organisms has the ability to synthesize a variety of lipids and in this module we study the biosynthetic pathways for some of the most common cellular lipids. The reaction sequences in the biosynthetic pathways of lipids are endergonic and reductive. They usually utilize NADPH (reduced electron carrier) as a reductant and ATP as a source of metabolic energy.

 

In animals, an excess of carbohydrate is transformed to triacylglycerol. The process involves synthesis of fatty acids from acetyl-CoA followed by esterification of fatty acids in triacylglycerol production by a mechanism called lipogenesis. FAs are prepared by FAS that polymerizes and subsequently reduce acetyl-CoA units. The FAs acyl chains extends by a series of reactions which add the acetyl group, ebb it to an alcohol, desiccate it to an alkene group and after that condense it again to an alkane group. The enzymes involved in the biosynthesis of fatty acid are divided into two groups: (1) In animals and fungi, FAS reactions are carried out by a solitary multifunctional protein and (2) In plant plastids and bacteria, separate enzymes execute each step in the pathway. The synthesis of fatty acids might be later converted to TAGs through packaging in lipoproteins and finally secreted from the liver. TAGs synthesis takes place in the endoplasmic reticulum where acyl groups in fatty acyl-CoAs are shifted to the hydroxyl groups of glycerol-3-phosphate and diacylglycerol.

 

However, the synthesis of unsaturated FAs engrosses a desaturation reaction, where a double bond is introduced into the fatty acyl chain. In humans, for example, stearoyl-CoA desaturase-1 produces oleic acid by the desaturation of stearic acid. The linoleic acid (doubly unsaturated) as well as α-linolenic acid (triply unsaturated) cannot be amalgamated in mammalian tissues, and are therefore termed as essential fatty acids and must be taken through the diet.

 

Isoprenoids, Carotenoids and Terpenoids are primed by the assemblage and amendment of isoprene units bestowed from the reactive precursor’s isopentenyl pyrophosphate (IPP) and dimethylallyl pyrophosphate (DMPP). In animals and archaea, the mevalonate (MVA) pathway produces these compounds from acetyl-CoA units, while in bacteria and plants, the non-mevalonate pathway uses glyceraldehyde 3-phosphate and pyruvate as substrates. These activated isoprene donors is also important in steroid biosynthesis where the isoprene units are tied together to make squalene followed by folding into a set of rings to make lanosterol which further be converted into cholesterol and ergosterol.

 

In this module, we first study the biosynthesis of fatty acids (primary components of both TAGs and phospholipids) followed by the assemblage into TAGs and the simpler membrane phospholipids. At last, we deliberate on the synthesis of cholesterol (component of membranes and precursor of steroids like bile acids, sex hormones, and adrenocortical hormones).

 

Biosynthesis of Fatty Acids (FAs)

 

The biosynthesis of FAs is the conception from acetyl CoA and malonyl CoA precursors via the deed of enzymes complex called FAS (fatty acid synthases). It is an imperative part of the lipogenesis process, which collectively with glycolysis, function to make fats from blood sugar.

 

   Synthesis of Straight Chain FAs

 

 

Straight chain FAs synthesis crop up in two categories: Saturated and Unsaturated.

In E. coli, synthesis of saturated FAs by FAS II occurs by means of the six recurring reactions until the 16-carbon palmitic acid is produced (as shown in the Table below). FAS II complex restrain multiple enzymes that act as one unit and exists in prokaryotes, fungi, parasites and plants.

 

In animals and some yeast/fungi system, the synthesis of saturated FAs occurs via similar reactions using FAS I, a hefty dimeric protein demeanouring all enzymatic activities necessary to create a fatty acid. FAS I is however, less efficient than FAS II; nevertheless, it permit formation of medium chain FAs via premature chain termination. As soon as 16:0 carbon FAs has been created, it endures a number of modifications, resulting in elongation and/or desaturation. Elongation starts with stearate (18:0) primarily in the ER by some membrane bound enzymes. The enzymatic steps implicated in the elongation procedure are predominantly the same as those demeanoured by FAS, but the four major consecutive steps of the elongation are executed by physically associated individual proteins.

 

Abbreviations: Acyl Carrier Protein (ACP), Coenzyme A (CoA), Nicotinamide Adenine Dinucleotide Phosphate (NADP).

 

Source: Adapted from https://en.wikipedia.org/wiki/Fatty_acid_synthesis

 

FAs Biosynthesis Regulation

 

 

The Acetyl-CoA gets transformed to malonyl CoA by acetyl CoA carboxylase and ordained to nosh into the FA synthesis pathway. The enzyme Acetyl-CoA carboxylase is the point of regulation in saturated straight chain FA synthesis focusing both allosteric regulation and phosphorylation. Allosteric regulation takes place in most organisms while regulation by phosphorylation occurs typically in mammals. Allosteric control occurs via activation by citrate and feedback inhibition by palmitoyl-CoA. During the high levels of palmitoyl-CoA, the final product of saturated FA synthesis allosterically inactivates acetyl-CoA carboxylase to avert a build-up of FAs in cells. Activation of acetyl-CoA carboxylase by citrate occurs under high levels as it indicates sufficient acetyl-CoA to equip into the Krebs cycle and generate energy.

 

De Novo FA Synthesis in Humans

 

In humans, FAs are produced predominantly in liver, lactating mammary glands and to a slighter extent, the adipose tissue. The majority of acetyl-CoA is produced from pyruvate by pyruvate dehydrogenase in the mitochondria followed by condensation with oxaloacetate by citrate synthase to form citrate, which is subsequently elated into the cytosol and busted to yield acetyl-CoA and oxaloacetate by ATP citrate lyase.

 

Oxaloacetate in reduced in the cytosol to malate by cytoplasmic malate dehydrogenase and malate is again transported backside into the mitochondria to play a part in the Citric acid cycle.

 

FAs Desaturation

 

 

It engrosses a procedure that requires cytochrome b5, molecular oxygen (O2) and NADH. In ER, the reaction results in the oxidation of both the FA and NADH. The majority of desaturation reactions engage the placement of a double bond between carbons 9 and 10 (e.g. palmitic acid to palmitoleic acid and stearic acid to oleic acid conversion is facilitated by the action of 9-desaturase). Other positions such as carbon 4, 5, and 6, can also be desaturated in humans via 4, 5, and 6 desaturases respectively. The unsaturated fatty acids are indispensable components to both prokaryotic and eukaryotic cell membrane primarily maintains membrane fluidity. They have also been allied with helping as signaling molecules in further processes including cell differentiation and DNA replication. The organisms use two pathways for desaturation: Aerobic and Anaerobic.

 

Aerobic desaturation

 

It is the most prevalent pathway for the synthesis of unsaturated FAs, operated in some prokaryotes and all eukaryotes. The pathway exploits desaturases to make unsaturated FAs from full-length saturated FA substrates. All desaturases entail oxygen and eventually devour NADH despite the fact that desaturation is an oxidative

 

process. They are explicit for the double bond they tempt in the substrate. In B. subtilis, 5 desaturase specifically induces a cis double bond at the 5 position. S. cerevisiae include one desaturase, Ole1p that engender the cis double bond at 9 position.

 

In B. subtilis, the aerobic desaturation pathway is regulated by a two unit system i.e. DesK and DesR. The former is a membrane-associated kinase and the latter is a transcriptional controller of the des gene. The regulation retorts to temperature change and the gene is upregulated as it goes down. At low temperature, unsaturated FAs stabilizes the membrane by increasing the fluidity. During a decrease in temperature, DesK (sensor protein) autophosphorylates into DesK-P and relocate its phosphoryl group to DesR. Two DesR-P proteins dimerizes and truss to the DNA promoters of the des gene and conscript RNA polymerase to initiate transcription.

Anaerobic desaturation

 

 

The anaerobic pathway is used by many bacteria for synthesizing unsaturated FAs. It does not require oxygen but reliant on enzymes to insert the double bond prior to elongation utilizing the typical FA synthesis machinery. The pathway has been well studied in E. coli.

The salient features of anaerobic desaturation are as under:

 

  • FabA, a β-hydroxydecanoyl ACP dehydrase, is explicit for 10 carbon saturated FA synthesis intermediate (β-hydroxydecanoylACP).
  • FabA initiate β-hydroxydecanoyl ACP dehydration triggering the release of water and incorporation of the double bond between C7 and C8 enumerating from the methyl end thereby creating the trans 2 decenoyl intermediate.
  • The trans 2 decenoyl intermediate pushed to the usual saturated FA synthesis pathway either by FabA that catalyzes the isomerization into the cis 3 decenoyl intermediate or by FabB, where the double bond hydrolyzes.
  • FabB, a β-ketoacyl ACP synthase, elongates and conduit intermediates into the normal FA synthesis pathway. When FabB respond with the cis 3 decenoyl intermediate after elongation, the product is an unsaturated FA.
  • The two major unsaturated FAs prepared are Palmitoleoyl ACP (16:1 ω7) and cis-vaccenoyl ACP (18:1ω7).

 

 

A number of bacteria that go through anaerobic desaturation enclose homologues of FabA and FabB. The Clostridia species are exceptions as they have a novel enzyme (yet to be fully identified) that catalyzes the formation of the cis double bond. The anaerobic desaturation pathway undergoes transcriptional regulation by FabR and FadR. The FabR acts as a repressor for the transcription of fabA and fabB whereas FadR is the comprehensively studied protein with attributed bifunctional characteristics. FadR acts as an activator of fabA and fabB transcription and also as a repressor for the β-oxidation regulon.

 

In general, both aerobic and anaerobic unsaturated FA synthesis does not take place within the same system. Still, P. aeruginosa and Vibrio ABE-1 are exceptions. Principally, P. aeruginosa undergoes anaerobic desaturation; it also endures two aerobic pathways (i) a 9 desaturase (DesA) that initiates a double bond formation in membrane lipids and (ii) uses two proteins (DesB and DesC) to act as a 9 desaturase that incorporates a double bond into a saturated FA CoA molecule. DesT, a repressor of fabAB expression also regulates the second pathway for anaerobic desaturation in presence of exogenous unsaturated FAs coordinating the expression of the two pathways within the organism.

 

  1. Summary

 

In this lecture we learnt about:

 

  • The Biosynthesis of Lipids I