Lecture 12 Fatty Acyl Synthase and Pentose Phosphate Pathway
Malonyl-CoA • Activated acetyl-CoA • Tagged and primed for lipogenesis • But also a key regulator of fatty acid oxidation • ACC is not only present in lipogenic tissues • Also present in tissues that need to produce malonyl-CoA in ‘regulatory’ amounts • Malonyl-CoA inhibits carnitine palmitoyl transferase I (CPT-1) • a.k.a carnitine acyl transferase I (CAT-1) • CPT-1 is responsible for forming fatty acyl carnitine • An essential step in fatty acid oxidation • Only way of getting long chaing fatty acyl-CoAs into the mitrochondria • So when ACC is active in, say, muscle • Malonyl-CoA concentration rises • CPT-1 is inhibited • Fatty acid oxidation stops • Cell must use carbohydrate instead • Therefore insulin, by stimulating acetyl-CoA carboxylase, encourages carbohydrate oxidation and inhibits fatty acid oxidation
Lipogenesis • Fatty acyl synthase (FAS) is a multi-functional enzyme • Lots of different enzyme activities in the complex • Can you count them all? • Bringing in acetyl and malonly groups, catalysing the reaction between the decarboxylated malonyl and the growing fatty acid chain, the reduction/dehydration/reduction steps, moving the fatty acid to the right site and finally releasing it as FA-CoA • FAS has two free sulfhydry groups on an ‘acyl-carring protein’ • Keeps the intermediates in exaclty the right position for interaction with the right active sites • Each new 2C unit is added onto the carboxy-end • Each round of 2C addition requires • 2 molecules of NADPH • No ATP (!!) • The release of the carbon dioxide that went on during the production of malonyl-CoA • Thus the carboxylation of acetyl-CoA does not result in ‘fixing’ CO2 • FAs start getting ‘released’ as FA-CoA when chain length is C14 • Desaturation is done AFTER FAS
Pentose Phosphate Pathway • Provides NADPH for lipogenesis • NADPH - A form of NADH involved in anabolic reactions • Rate of NADPH production by PPP is proportional to demand for NADPH • Key regulatory enzyme is G6PDH • Glucose 6-phosphate dehydrogenase G6P + NADP 6-phosphogluconolactone + NADPH • The gluconolactone is further oxidised to give more NADPH • But this step also causes decarboxylation to give a 5-carbon sugar phosphate (ribulose 5-phosphate) • Need to put the 5-C sugar back into glycolysis • Accomplished by rearranging and exchanging carbon atoms between 5C molecules • Catalysed by enzymes called transaldolases and transketolases • So, 5C + 5C C7 + C3 by a transketolase (2C unit transferred) • Then C7 + C3 C6 + C4 by a transaldolase (3C unit transferred) • Then C4 + C5 C6 + C3 by a transketolase (2C unit transferred) • The C6 and C3 sugars can go back into glycolysis • Alternatively, the PPP can be used to make ribose 5-phosphate • Important in nucleotide pathways • Or the PPP can be used to generate NADPH as an anti-oxidant • Particularly in red blood cells where a deficiency in G6PDH can cause anemia
Esterification • Formation of Fat • Glycerol plus three fatty acids • Glycerol needs to be glycerol 3-phosphate • Derived from the reduction of glyceraldehyde 3-phosphate derived from glycolysis • Glycolysis important not just for the production of acetyl-CoA but for the production of fat! • Esterification enzyme uses FA-CoA • Not just FAs • FAs added one at a time • Both esterification enzyme and FAS are upregulated by insulin • Gene expression and protein synthesis • FAS is downregulated when lots of fat around • As in a Western diet!!
Regulatory Overview Fat glucose ESTERIFICATION GLUT-4 No GS X fatty acids glucose G6P G6PDH glycerol 3-P FAS LIPOGENESIS GLYCOLYSIS ACC pyruvate acetyl-CoA Acetyl-CoA transport stimulated by increased production of citrate pyruvate acetyl-CoA PDH citrate G6PDH stimulated by demand for NADP KREBS CYCLE Insulin stimulates GLUT-4. PDH and ACC. Also switches on the genes for FAS and esterification enzyme. CO2 Krebs cycle will be stimulated by demand for ATP