Lecture 11

1. Lecture 11 Lipogenesis

2. Overview glucose Fat ESTERIFICATION GLUT-4 No GS X fatty acids glucose G6P Consumes reductant and ATP GLYCOLYSIS PPP LIPOGENESIS Produces reductant pyruvate acetyl-CoA pyruvate acetyl-CoA PDH Key steps (eg, GLUT-4, PDH, lipogenesis) are stimulated when insulin binds to its receptor on the cell surface KREBS CYCLE NADH release ultimately produces ATP CO2

3. Pyruvate Dehydrogenase • Oxidative decarboxylation of pyruvate Pyruvate + CoA + NAD  acetyl-CoA + NADH + CO2 • 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 CO2 • 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 + CO2 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