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

Fatty Acyl Sythase

FAS - simplified

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’ CO 2 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 glucose G6P pyruvate acetyl-CoA pyruvate LIPOGENESIS Fat PDH GLYCOLYSIS GLUT-4 No GS KREBS CYCLE CO 2 fatty acids ESTERIFICATION X G6PDH G6PDH stimulated by demand for NADP Insulin stimulates GLUT-4. PDH and ACC. Also switches on the genes for FAS and esterification enzyme. Krebs cycle will be stimulated by demand for ATP ACC FAS glycerol 3-P Acetyl-CoA transport stimulated by increased production of citrate citrate