1. Lecture 11 Lipogenesis

2. Overview glucose G6P pyruvate acetyl-CoA pyruvate LIPOGENESIS Fat PDH GLYCOLYSIS GLUT-4 No GS KREBS CYCLE CO 2 fatty acids ESTERIFICATION X Produces reductant PPP Consumes reductant and ATP NADH release ultimately produces ATP Key steps (eg, GLUT-4, PDH, lipogenesis) are stimulated when insulin binds to its receptor on the cell surface

3. Pyruvate Dehydrogenase Oxidative decarboxylation of pyruvate Pyruvate + CoA + NAD  acetyl-CoA + NADH + CO 2 Loss of carbon dioxide renders the reaction totally irreversible in vivo –No pathways in humans to make acetate into ‘gluconeogenic’ precursors Can’t make glucose from acetyl-CoA No way of going back once the PDH reaction has happened Key watershed between carbohydrate and fat metabolism Regulated by reversible phosphorylation –Active when dephosphorylated Inactivated by PDH kinase Activated by PDH phosphatase –Insulin stimulates PDH phosphatase Insulin thus stimulates dephosphorylation and activation of PDH Note about Coenzyme A –Often written as CoA-SH to emphasise that the functional part of the molecule is the sulphydryl group –Forms thioesters which are, themselves, quite ‘high energy’ bonds –Most common carrier of fatty acids and acetates

4. Fate of Acetyl-CoA Burnt in the Krebs Cycle –Stays in the mitochondria –Carbon atoms fully oxidised to CO 2 –Lots of NADH produced to generate ATP Lipogenesis –Moved out into the cytoplasm –Activated for fat synthesis In both cases the first step is citrate formation –Condensation of acetyl-CoA with oxaloacetate Catalysed by citrate synthase Relies on fact that methyl hydrogens in acetyl-CoA come off, leaving –ve charged carbon that attacks the carbonyl-carbon in oxaloacetate –Regenerates Coenzyme A –Citrate is a tricarboxylic acid It can be transported out of the mitochondria via carriers in the inner mitochondrial membrane or can be sent into the next reaction of the Krebs Cycle The ‘fate’ will depend on the need for energy (ATP/energy charge) and the stimulus driving lipogenesis

5. ATP-Citrate Lyase Once in the cytoplasm, the citrate is cleaved –By ATP-Citrate Lyase (ACL) –Using CoA to generate acetyl-CoA and oxaloacetate Reaction requires ATP  ADP + phosphate ACL is inhibited by hydroxy-citrate (OHCit) –OHCit is found in the Brindleberry –Sold as a fat synthesis inhibitor –Would we expect it to prevent the formation of fatty acids And, if so, would that actually help us lose weight? Oxaloacetate produced by ACL needs to return ot the matrix –Otherwise the mitochondrial oxaloacetate pool becomes depleted –Remember, oxaloacetate is really just a ‘carrier’ of acetates Both in the Krebs's cycle and in the transport of acetyl-CoAs into the cytoplasm –Oxaloacetate cannot cross the inner mitochondrial membrane Some interesting interconversions occur to get it back in!

6. Acetyl-CoA Carboxylase Activates acetyl-CoA and ‘primes’ it for lipogenesis Unusual in that it ‘fixes’ carbon dioxide –In the form of bicarbonate –A carboxylation reaction Acetyl-CoA + CO 2  malonyl-CoA –Reaction requires ATP  ADP + phosphate –Participation of the cofactor, biotin Biotin is involved in other carboxylation reactions ACC is stimulated by insulin –Malonyl-CoA is committed to lipogenesis Pattern of phosphorylation is important in ACC stimulation –Also stimulated allosterically by citrate and inhibited allosterically by long-chain fatty acyl-CoAs