Lipid Biosynthesis These are endergonic and reductive reactions

*Lipid Biosynthesis - These are endergonic and reductive reactions, use ATP as source of energy and reduced electron carrier usually NADPH as reductant. * Fatty acid synthesis: - F. A synthesis is not the reversal of the degradative pathway, different sets of enzymes. using 1 -synthesis in cytosol, degradation in mitochondria (mitochondrial matrix) 2 -intermediates of F. A synthesis are covalently linked to -SH group of ACP 3 -the growing F. A is elongated by sequential addition of 2 -carbon units. 4 -the reductant in fatty acid synthesis is NADPH, 5 -elongation of F. A is stopped at C 16 and further elongation or insertion of double bonds are carried by other enzyme systems. * Large proportion of F. A used in the body is supplied by diet excess CHO and protein are converted into F. A. - F. A are synthesized mainly in liver and lacting mammary gland to lesser extent in adipose tissue and kidney.

*Formation of Malonyl Co. A from acetyl Co. A and HCO 3*This step is mediated by Acetyl Co. A carboxylase that consist of three domains. Biotin Prosthetic group 2 nd unit of Acetyl Co. A carboxylase 3 rd domain first part of acetyl Co. A carboxylase NET : Acetyl Co. A + ATP + HCO 3 - Malonyl Co. A + ADP + Pi + H+

Acetyl Co. A carboxylase enzyme

• The F. A grows by 2 carbon units donated by Malonyl Co. A • for palmitate synthesis 16 C 7 steps

Single polypeptide 77 a. a Linked to Ser & ACP *intermediates are attached to acyl carrier protein (ACP)

• The F. A synthase complex consist of 7 separated polypeptides. • These portions act together to catalyze the formation of F. A from acetyl Co. A and Malonyl Co. A Acetyl Co. A is attached to KS by -SH group thioester linkage. Malonyl Co. A is attached to ACP through -SH group forming thioester linkage.

*The -SH of KS is charged to acetyl Co. A by Acetyl - Co. A - ACP transacetylase (AT) KS : - ketoacyl-ACP synthase *The -SH group of ACP is bonded to malonyl Co. A by enzyme Malonyl - Co. A - ACP transferase (MT) Of phosphopantetheine group

*CO 2 is released is the same HCO 3 - that condensed into acetyl Co. A to form malonyl Co. A by acetyl Co. A carboxylase. Why ? Good nucleophile • Coupling the decarboxylation and condensation *HCO 3 - that reacts with Acetyl Co. A to form malonyl Co. A is released in form CO 2. The cleavage of acetyl group from F. A is highly exergonic. & condensation of acetyl Co. A endergonic. So introduction of CO 2 make the condensation more thermodynamically favored. exergonic Is the first step in F. A synthesis -ketoacyl - ACP synthase

*electron donor for this reduction NADPH Enzyme : - ketoacyl ACP reductase KR DD isomer

-D-hydroxyacyl dehydratase enzyme

NADPH is the electron donor Enoyl ACP reductase enzyme

This process is catalyzed by acetyl Co. A ACP transacetylase (AT) *Then the butyryl group is transferred from phosphopantetheine -SH of ACP to the Cys-SH of - ketoacyl ACP-synthase (KS)

By MT By KS

*The biosynthesis of F. A requires energy Needs ATP and reducing power of NADPH *For synthesis of 16 C F. A (palmitic acid) 7 cycles of condensation, 7 reduction, 7 dehydration and 7 reduction. 7 acetyl Co. A + 7 CO 2 + 7 ATP 7 malonyl Co. A + 7 ADP + 7 Pi Acetyl Co. A + 7 malonyl Co. A + 14 NADPH + 14 H+ 8 Co. A + 14 NADP+ + 6 H 2 O 8 acetyl Co. A + 7 ATP + 14 NADPH + 14 H+ H 2 O + 7 ADP + 7 Pi + 14 NADP+ palmitate + 7 CO 2 + palmitate + 8 Co. A + 6

*Fatty acid synthase of different organisms F. A synthase from : - bacteria, plants : 7 activities in 7 separated polypeptides - Yeast : 7 activities in two separated polypeptides. - Vertebrates : 7 activities in one large polypeptide One large polypeptide with two domains with 7 activities

Fatty Acid Biosynthesis

Source of Acetyl Co for F. A synthesis

Source of Acetyl Co for F. A synthesis - Major source of acetyl Co. A is from mitochondrial oxidation of pyruvate and from catabolism of amino acids. - Acetyl Co. A produced from - oxidation of F. A is NOT a significant source of acetate because synthesis and oxidation of F. A are reciprocally regulated. - Acetyl Co. A is produced in the mitochondrial matrix - synthesis of F. A occurs in cytosol - Acetate is shuttled out of mitochondria as citrate because membrane is NOT permeable for acetate.

Source of Acetyl Co for F. A synthesis

Source of NADPH for F. A synthesis - Oxidation of malate to pyruvate will produce NADPH Reducing power of NADH is converted into NADPH Oxaloacetate Malate Pyruvate - The other NADPH are produced pentose phosphate pathway

Regulation of F. A synthesis and degradation - The reaction catalyzed by acetyl Co. A carboxylase is the rate limiting step. - Palmitoyl Co. A : -ve feed back in the biosynthesis, it inhibits the carboxylase enzyme. - When mitochondrial acetyl Co. A + ATP increase production of citrate which transfers out of mitochondria increase cystolic citrate. 1 - citrate is an allosteric activator for this carboxylase. 2 - citrate is the precursor of cystolic acetyl Co. A high level of citrate increase F. A synthesis - citrate inhibit the PFK 1 reducing the flow of carbon through glycolysis. - insulin stimulate F. A synthesis by activating the carboxylase. - Glucagon + Epinephrine decrease F. A synthesis by increasing the phosphorylation

F. A synthesis and degradation are reciprocally regulated (not active at the same time) -Malonyl Co. A: the 1 st intermediate in the synthesis of F. A inhibits the carnitine acyl transferase I, so inhibits the - oxidation. -Regulation of the level of gene expression? -Intake of large amount of CHO for long periods increase expression of carboxylase and F. A synthase enzyme.

Long-chain saturated F. A are synthesized from palmitate by Fatty acid elongation system Found in smooth endoplasmic reticulum and mitochondria The donor for two carbon units = acetyl Co. A Act on the F. A in the phospholipid, phosphatidylcholine or linked glycerol Desaturation by fatty acyl Co. A desaturase (liver)

Desaturation of F. A occurs in the phospholipid, phosphatidylcholine or linked glycerol

*Biosynthesis of Triacylglycerols - Ingested or synthesized F. A have two fates : 1 - Incorporation in TG for the storage of metabolic energy. 2 - Incorporation in phospholipids components of membranes. And the activated pathway is dependent on the need of cells. Rapid growth for phospholipid synthesis of membranes - if the organism has a lot of food and fats increase demand storage of F. A in TG * Few hundreds mg of glycogen stored in the body, but about 15 kg of lipids (in the normal person) can supply the body by energy for 12 weeks. * Excess of CHO stored as fats, While fats can’t used for glucose synthesis. * TG and phospholipids have the same precursors “Glycerol 3 -phosphate” and “fatty acyl Co. A” * TG is biosynthesized primarily in liver and also in adipocytes.

Biosynthesis of TG production of L-glycerol 3 -phosphate adipocyte liver In liver and kidney mainly and adipocytes, (cytosol) Just in liver

*phosphatidic acid is formed in trace amount in the body, but it is the central intermediate in the biosynthesis of phospholipid and in TG Synthesis by acyl Co. A synthetase )diacylglycerol 3 -phosphate)

TG is synthesized in the liver, adipocytes, mammary glands and intestinal mucosal cells. Transesterification by acyl transferase Is the parent compound of TG and phospholipid

Glycerol 3 -phosphate is synthesized in the liver and adipocyte from glucose Glucose entrance is not insulin dependant Glucose entrance is insulin dependant Fates of TG in liver and adipose tissue - TG are stored in the cytosol of the cells in anhydrous form, and ready for mobilization - in liver, TG stored are very little. - TG with cholesterol and cholesterol esters are packed in lipoprotein form and exported to other organs via blood.

Regulation of TG Biosynthesis CHO or protein consumed in excess of energy needs is stored in the form of TG Biosynthesis and degradation of TG are regulated reciprocally. Depending on the metabolic recourses and requirements -Different hormones affect TG metabolism Insulin, Glucagon, adrenal cortical hormones. Unable to utilize glucose and to convert it to TG - oxidation is continued of F. A (excess acetyl Co. A) increase keton bodies loss of weight

*Biosynthesis of membrane phospholipids - synthesis of membrane lipids requires in general : 1 - Synthesis of backbone molecule (glycerol or sphingosine) 2 - Attachment of F. A to the backbone by ester or amide linkage. 3 - Addition of hydrophilic head group through phosphodiester linkage. 4 - Alteration or exchange of head group to yield final phospholipids. -synthesis occur in smooth endoplasmic reticulum then goes to Golgi apparatus. - The polar head is linked to the glycerol by phosphodiester linkage. *All cells except RBC can synthesize phospholipid, but TG is synthesized in the liver, adipocyte, mammary glands and intestinal mucosal cells.

*Two strategies for attaching head groups In the biosynthesis one of the OH group is activated by attachment of a nucleotide (CDP) CDP-diacyl group Activation of diacyl glycerol CDP-HEAD Activation of head group

Strategy 1 * Synthesis of cardiolipin and phosphatidyl inositol (PI)

Strategy 2

Two strategies Biosynthesis of membrane phospholipids

Biosynthesis of Sphingolipids 1 st step Palmitoyl Co. A + serine 18 Carbons The reducing agent

Ceramide is parent compound of Sphingolipids *Polar lipids are targeted to specific cell membrane. After the synthesis in the S. E. R. the polar lipids (phospho, sphingo, glyco lipids) they transferred to different cell membranes by Golgi apparatus in form of vesicles.

* Degradation of Phospholipids enzyme responsible for degradation are present in all tissues and pancreatic juice. - phospholipases hydrolyze the esters of phosphodiester bonds of phosphoglycerides. * Degradation of Sphingomyelin - Sphingolipids are degraded by sphingomyelinase which remove phosphorylcholine leaving Ceramide. - Ceramide is cleaved by ceramidase into sphingosine and free fatty acid.

*Degradation of phospholipids

The End

Eicosanoids : synthesized from (20 : 5, 8, 11, 14) * potent biological signaling molecules act as short range messengers. *prostaglandins regulate many physiological processes, platelet aggregation, uterine contraction, pain, inflammation and secretion of mucin in GIT *COX found in smooth endoplasmic reticulum, act on archidonate and convert it PGH 2. * COX also called prostaglandin H 2 synthase. NSAIDS inhibits COX *Thromboxane synthase found in platelet Thromboxane lead to contraction of blood vesicles and platelet aggregation PGH 2 Thromboxane A 2 Thromboxanes other PGE 2, PGF 2, . . . By enzyme : Thromboxane synthase

*Different leukotrienes : (different sites of -OOH peroxide) *leukotrienes found in leukocyte, heart, brain.

*NSAIDS : Aspirin , Ibuprofen

Release of Arachidonic acid from membrane lipid (20 : 5, 8, 11, 14) Steroidal anti inflammatory Steroidal SAD

*Synthesis of phosphatadyl ethanolamine (PE) and phosphatadyl choline (PC) PC + PE are abundant in eukarytic cells - choline + ethanolamine obtained from diet or from the turnover of the body’s phospholipids. - Also choline can be synthesized from phosphatidyl serine. Choline ethanolamine CDP-choline CDPethanolamine *The reutilization of choline is important because de novo synthesis of choline three methyl groups needed is derived from methionine which is essential amino acid. * Role of PC in lung surfactant Dipalmitoyl phosphatidyl choline : major component in lung surfactant (extracellular fluid layer lining the alveoli) (Respiratory distress syndrome hyaline membrane disease) * PC is synthesized from PS