* 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, using different sets of enzymes. • 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 C16 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 and to lesser extent in adipose tissue and kidney.
* Formation of Malonyl CoA from acetyl CoA and HCO3- *This step is mediated by Acetyl CoA carboxylase that consist of three domains. Biotin Prosthetic group 3rd domain 2nd unit of Acetyl CoA carboxylase first part of acetyl CoA carboxylase NET : Acetyl CoA + ATP + HCO3- Malonyl CoA + ADP + Pi + H+
The F.A grows by 2 carbon units donated by Malonyl CoA • 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 CoA and Malonyl CoA Acetyl CoA is attached to KS by -SH group thioester linkage. Malonyl CoA is attached to ACP through -SH group forming thioester linkage.
* The -SH of KS is charged to acetyl CoA by Acetyl - CoA - ACP transacetylase (AT) Of phosphopantetheine group KS : - ketoacyl-ACP synthase * The -SH group of ACP is bonded to malonyl CoA by enzyme Malonyl - CoA - ACP transferase (MT)
Coupling the decarboxylation and condensation • exergonic * CO2 is released is the same HCO3- that condensed into acetyl CoA to form malonyl CoA by acetyl CoA carboxylase.Why ? Good nucleophile * HCO3- that reacts with Acetyl CoA to form malonyl CoA is released in form CO2. Is the first step in F.A synthesis The cleavage of acetyl group from F.A is highly exergonic. & condensation of acetyl CoA endergonic. So introduction of CO2 make the condensation more thermodynamically favored. - ketoacyl - ACP synthase
* electron donor for this reduction NADPH Enzyme : - ketoacyl ACP reductase KR D- D isomer
Enoyl ACP reductase enzyme NADPH is the electron donor
This process is catalyzed by acetyl CoA 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
7 acetyl CoA + 7 CO2 + 7 ATP 7 malonyl CoA + 7 ADP + 7 Pi Acetyl CoA + 7 malonyl CoA + 14 NADPH + 14 H+ palmitate + 7 CO2 + 8 CoA + 14 NADP+ + 6 H2O 8 acetyl CoA + 7 ATP + 14 NADPH + 14 H+ palmitate + 8 CoA + 6 H2O + 7 ADP + 7 Pi + 14 NADP+ * The biosynthesis of F.A requires energy Needs ATP and reducing power of NADPH * For synthesis of 16C F.A(palmitic acid) 7 cycles of condensation, 7 reduction, 7 dehydration and 7 reduction.
* 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
Source of Acetyl Co for F.A synthesis • - Major source of acetyl CoA is from mitochondrial oxidation of pyruvate and from catabolism of amino acids. • - Acetyl CoA produced from - oxidation of F.A is NOT a significant source of acetate because synthesis and oxidation of F.A are reciprocally regulated. • Acetyl CoA 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 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 CoA carboxylase is the rate limiting step. • - Palmitoyl CoA : -ve feed back in the biosynthesis, it inhibits the carboxylase enzyme. • - When mitochondrial acetyl CoA + ATP increase 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 CoA • high level of citrate increase F.A synthesis • - citrate inhibit the PFK1 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 CoA: the 1st 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 Desaturation by fatty acyl CoA desaturase (liver) Found in smooth endoplasmic reticulum and mitochondria The donor for two carbon units = acetyl CoA Act on the F.A in the phospholipid, phosphatidylcholine or linked glycerol
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 synthesis of membranes increase demand for phospholipid • - if the organism has a lot of food and fats 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 CoA” • * TG is biosynthesized primarily in liver and also in adipocytes.
Biosynthesis of TG production of L-glycerol 3-phosphate adipocyte liver Just in liver In liver and kidney mainly and adipocytes, (cytosol)
Synthesis by acyl CoA synthetase * phosphatidic acid is formed in trace amount in the body, but it is the central intermediate in the biosynthesis of phospholipid and in TG (diacylglycerol 3-phosphate)
TG is synthesized in the liver, adipocytes, mammary glands and intestinal mucosal cells. Is the parent compound of TG and phospholipid Transesterification by acyl transferase
Glycerol 3-phosphate is synthesized in the liver and adipocyte from glucose Glucose entrance is insulin dependant Glucose entrance is not 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 CoA) 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 CDP-HEAD Activation of head group Activation of diacyl glycerol
Strategy 1 * Synthesis of cardiolipin and phosphatidyl inositol (PI)
Biosynthesis of Sphingolipids 1st step Palmitoyl CoA + 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.
* Thromboxane synthase found in platelet Thromboxane lead to contraction of blood vesicles and platelet aggregation PGH2 Thromboxane A2 other Thromboxanes 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 PGH2. * COX also called prostaglandin H2 synthase. NSAIDS inhibits COX By enzyme : Thromboxane synthase PGE2, PGF2, ...
* Different leukotrienes : (different sites of -OOH peroxide) * leukotrienes found in leukocyte, heart, brain.
Release of Arachidonic acid from membrane lipid (20 : 5,8,11,14) Steroidal anti inflammatory Steroidal SAD