FA and TAG Metabolism (PDF)

Metabolism of FAs and TAGs  Making Fatty Acids Many FAs are obtained preformed in the diet. XS dietary CHO and protein can be converted to FAs, which are stored as TAGs In adult humans, FA synthesis occurs primarily in the liver, lactating mammary glands and to a lesser exten...

Metabolism of FAs and TAGs  Making Fatty Acids Many FAs are obtained preformed in the diet. XS dietary CHO and protein can be converted to FAs, which are stored as TAGs In adult humans, FA synthesis occurs primarily in the liver, lactating mammary glands and to a lesser extent the adipose tissue FA synthesis takes place in the cytosol of the cell It incorporates Cs from acetyl CoA into the growing FA chain It is an anabolic process – needs ATP and NADPH  1. Production of cytosolic acetyl coenzyme A 2. Carboxylation of acetyl coenzyme A to malonyl coenzyme A 3. Fatty acid synthase The CoA portion cannot cross the mit membrane Condensation reaction between Acetyl CoA and OAA to generate citrate by citrate synthase Q:When would this happen? Ans: When energy charge is high When ATP conc is high it inhibits isocitrate dehydrogenase Citrate and isocitrate build up High citrate indicative of high cellular energy charge 1. Production of cytosolic acetyl coenzyme A 2. Carboxylation of acetyl coenzyme A to malonyl coenzyme A 3. Fatty acid synthase The carboxylation of acetyl CoA (2C) to form malonyl CoA (3C) is catalyzed by acetyl CoA carboxylase (ACC). This is the rate-limiting and regulated step in FA synthesis  Short term regulation of ACC Allosteric Covalent regulation of regulation of malonyl CoA acetyl CoA synthesis by carboxylase by acetyl CoA AMPK which carboxylase itself is regulated both covalently and allosterically  Long-term regulation of ACC Prolonged consumption of diet with excess calories (particularly high-CHO) causes an increase in ACC synthesis – so increased FA synthesis Synthesis of ACC is upregulated by Insulin via a sterol regulatory element-binding protein, SREBP-1  ACC contains a Biotin prosthetic group Remember: Biotin as a prosthetic group for Pyruvate carboxylase in gluconeogenesis – both carboxylation reactions 1. Production of cytosolic acetyl coenzyme A 2. Carboxylation of acetyl coenzyme A to malonyl coenzyme A 3. Fatty acid synthase Fatty acid synthase sequentially adds 2-C units from malonyl- CoA to the growing fatty acyl chain to form palmitate After addition of each 2-C unit – the growing chain undergoes 2 reduction reactions that require NADPH (Note: remember PPP) Fatty acid synthase is a large enzyme – 2 identical subunits each with 7 catalytic activities – an acyl carrier protein (ACP segment) which contains a phosphopantetheine residue (derived from the vit panthothenic acid) This figure is For clarification of the process only You do not need to learn the details here No! you do not need to know the names of these enzymes or reactions Major source of reductant for FA synthesis 1. PPP major source of supplier of NADPH 2 NADPH produced for every molecule of glucose in PPP 2. Cytosolic conversion of malate to pyruvate – Malate is oxidized and decarboxylated by cytosolic malic enzyme (NADP+ – dependent) – Note: Malate can also arise from the reduction of OAA by cytosolic NADH-dependent malate dehydrogenase  Further elongation of FA chains Palmitate (16 carbon) fully saturated LCFA is the primary end product of fatty acid synthase activity. Palmitate elongated by the addition of 2-C units to the carboxylate end in the smooth ER. Malonyl CoA is the 2-C donor and NADPH supplies the electrons Brain has additional elongation capabilities to make VLCFAs (>22) which it requires for specific lipids  Desaturation of Fatty acid chains Desaturases in the SER carry out desaturation of FAs Require O2, NADH, cytochrome SER (Smooth Endoplasmic Reticulum)  Routes to Metabolism Depends on the chain length of the FA >C20 = very long chain C12-C20 = long chain C6-C12 = medium chain C4 = short chain Long chain unsaturated – require additional steps-isomerisation and redox rxns Water soluble medium chain –do not require carnitine –occurs only in the liver Excess FAs – may undergo microsomal ω-oxidation Very long chain –straight chain and branched chain –metabolised in peroxisomes FA degradation  Release of FAs from TAG  Activation of Hormone- sensitive lipase (HSL)  Fate of glycerol  Enters glycolysis in the fed state  Fasted state it is converted to glucose  Very important in prolonged fasting  Fate of FAs Note:Glycerol cannot be metabolised in adipose tissue – no glycerol kinase -Transported to liver where it can be phosphorylated. The resulting glycerol phosphate can be used to i) form TAG in the liver,or ii) can be converted to DHAP by reversal of the glycerol phosphate dehydrogenase reaction – DHAP can participate in glycolysis or gluconeogenesis. FA oxidation takes place in the mitochondrial matrix Mechanisms must be put in place in order to transport the LCFA across the membranes  1st Reaction of the Carnitine Shuttle  Catalyzed by a family of isoenzymes present in the outer mitochondrial membrane Acyl-CoA synthetases General reaction Fatty Acid + CoA + ATP fatty acyl-CoA +AMP+ PPi LCFA transported from cytosol to mitochondrial matrix – site of oxidative degradation Note Malonyl CoA potent inhibitor of CPT-1- why does that make good economic sense?  Sources of Carnitine Can be obtained from i) the diet, found mainly in meat ii) Synthesized from aa lysine and methionine by an enzymatic pathway found in the liver and kidney but not in skeletal or heart muscle  Carnitine Deficiencies Result in 1. Decreased ability of tissues to use LCFA as a metabolic fuel 2. Can cause accumulation of toxic amounts of FFAs and branched-chain acyl groups in cells.  CPT-1(Carnitine palmitoyl transferase 1) Deficiency -Genetic CPT-1 deficiency affects the liver, where an inability to use LCFA for fuel impairs that tissues ability to synthesize glucose during a fast --This can lead to severe hypoglycemia , coma, and death  CPT-2 Deficiency Occurs mainly in skeletal and cardiac muscle Symptoms range from cardiomyopathy, to muscle weakness with myoglobinemia following prolonged exercise - An example of how the impaired flow of a metabolite from one cell compartment to another results in pathological presentation  -Oxidation of Fatty Acids The major pathway for catabolism of FA 2-C fragments are successfully removed from the carboxyl end of the fatty acyl CoA End products – acetyl CoA, NADH and FADH2 -oxidation of Fatty Acids 4 reactions remove each acetyl-CoA unit from the carboxyl end of a saturated fatty acyl-CoA Each step is catalyzed by enzymes with chain-length specificity The 4 steps are repeated NOTE: Acetyl CoA is a +ive allosteric modulator of pyruvate carboxylase  Oxidation of Unsaturated Fatty Acids Requires 2 additional enzymes 1. An isomerase 2. A reductase This diagram is for clarification only – you do not need to remember the no of ATPs made at any stage of this process  Regulation of FA Oxidation Fatty acids are a precious fuel Liver – FAs formed have 2 fates 1……….  oxidation in mitochondria 2………. Conversion to TAGs and PLs Pathway taken depends on rate of transfer of LCFatty acyl-CoA into mitochondria  Regulation o Controlled by regulating the entry of FAs into the mitochondria - Malonyl-CoA (inhibits CPT-1) – prevents futile cycling 2 of the enzymes of  oxidation are also regulated by metabolites that signal energy sufficiency a) When [NADH]/[NAD ] ratio is high, -hydroxyacyl-CoA + dehydrogenase is inhibited b) Thiolase is inhibited by high concentrations of acetyl-CoA More than 25 enzymes and specific transport proteins participate in mitochondrial fatty acid metabolism. At least 15 of these have been implicated in inherited diseases in the human.