VLDL formation Apolipoprotien B-100 has a repeating  -helix/  -sheet structure: Lipids are packaged as apolipoprotein B-100 is being synthesized: From Shelness & Sellers (2001) Curr Opin Lipidology 12:

VLDL formation VLDL stands for Very Low Density Lipoprotein As it is synthesized, VLDL contains: One molecule of apoliprotein B-100 Triacylglycerol Phospholipid Cholesterol ester Microsomal Triacylglycerol Transfer Protein(MTP) assists in the formation of the VLDL Other components are added to the VLDL in the blood.

VLDL formation Apolipoprotein B-100 synthesis is required for the transport of lipid out of the liver –If protein synthesis is reduced (e.g. by malnutrition) fat droplets accumulate in the liver. –If the rate of lipid synthesis is greatly elevated with respect to protein synthesis (e.g. in type I diabetes or glucose 6-phosphatase deficiency) fat droplets accumulate in the liver.

Triacylglycerol Oxidation During starvation adipose tissue does not release triacylglycerol. It releases fatty acids and glycerol (produced by adipose lipases). In the fed state triacylglycerol is transported in the blood as a lipoprotein complex. In the blood the triacylglycerol is hydrolyzed to produce fatty acids and glycerol (lipoprotein lipase or hepatic lipase). Triacylglycerol 3 fatty acids + glycerol Lipases

Triacylglycerol Oxidation Glycerol can be converted to glucose –Glycerol kinase is present in liver but not normally present in adipose Glycerol Glycerol-3-Phosphate DHAP ATP ADP + Pi NAD + NADH + H + H 2 C-OH | HOCH | H 2 C-OH | HOCH O | | H 2 C-O-P-O - || O - H 2 C-OH | O=C O | | H 2 C-O-P-O - || O - Glycerol kinase Glycerol-3- phosphate dehydrogenase

Triacylglycerol Oxidation Fatty acids must be activated to Acyl-CoA Fatty acid + CoA + ATP Acyl-CoA + AMP + PPi PPi + H 2 O 2 Pi Acyl-CoA synthetase Pyrophosphatase

Triacylglycerol Oxidation Regulation Fatty acid oxidation takes place in the mitochondria. Transport into the mitochondria is the primary rate limiting step of fatty acid oxidation. The maximum rate of fatty acid oxidation is transcriptionally regulated by PPARα. –Unsaturated fatty acids increase PPARα activity –Fibrates, a class of triacylglycerol lowering drugs, increase PPARα activity. –Note PPAR will be persented in Thursday’s lecture

Triacylglycerol Oxidation Carnitine Shuttle Acyl-CoA + Carnitine Acyl-Carnitine + CoA Mitochondrion Acyl-CoA + Carnitine Acyl-Carnitine + CoA CAT-II CAT - Carnitine Acyl-CoA Transferase Inhibited by Malonyl-CoA CAT-II CAT-I Inner membrane Outer membrane

Triacylglycerol Oxidation β-oxidation of acyl-CoA –Two carbons at a time are oxidized and removed as acetyl-CoA –For each two carbons removed, 1 FADH 2 and 1 NADH + H + are produced –For palmitoyl-CoA, the reaction is: Palmitoyl-CoA + 7FAD + 7NAD + 7CoA + 7H 2 O 8Acetyl-CoA + 7FADH 2 + 7NADH + 7 H +

Triacylglycerol Oxidation The first step of the oxidation is catalyzed by Acyl-CoA dehydrogenase. –There are three types, differing in chain length specificity LCAD - Long chain MCAD - Medium chain SCAD - Short chain –In New York State, all newborns are screened for MCAD deficiency –This disorder is covered in the “Baby Ian” case study on the MGB web site.

Triacylglycerol Oxidation Oxidation of unsaturated fatty acids occurs by the beta oxidation pathway, but with two additional enzymes that isomerize (from cis to trans) and reduce the double bond(s). Very long chain fatty acids chain fatty acids gre oxidized in peroxisomes to long chain amd medium chain acyl-CoA which enter the mitochondria via the carnitine shuttle. Adrenoleukodystrophy (ALD) is an X- lined disorder in which the entry of very long chain fatty acids into the peroxisome is blocked.

Ketone Bodies

Ketone body synthesis - LIVER (mitochondria) O || || CH 3 -C-CH 2 -CO - OH O | || CH 3 -C-CH 2 -CO - O || CH 3 -C-S-CoA O O || || CH 3 -C-CH 2 -C-S-CoA OH O | || CH 3 -C-CH 2 -C-S-CoA | CH 2 | COO - 2 Acetyl-CoA Acetoacetyl-CoA HMG-CoA  -hydroxybutyrate Acetoacetate Acetyl-CoA Acetyl-CoA CoA CoA NAD + NADH+H + O || CH 3 -C-S-CoA mitochondrial!

Ketone Bodies Acetyl-CoA can be converted into ketone bodies: –Acetoacetate: –  -hydroxybutyrate: These are exported by the liver and used as fuel by other tissues In a non-enzymatic side reaction, small amounts of acetone are produced from acetoacetate O O || || CH 3 -C-CH 2 -CO - OH O | || CH 3 -C-CH 2 -CO - O || CH 3 -C-CH 3

Ketone Body Use O O || || CH 3 -C-CH 2 -CO - OH O | || CH 3 -C-CH 2 -CO -  -hydroxybutyrate Acetoacetate 2 Acetyl-CoA NAD + NADH+H + Succinyl-CoA Succinate Acetoacetyl-CoA CoA thiolase CoA transferase (thiphorase) NOT present in liver O || CH 3 -C-S-CoA