Lipid Metabolism

Palmitoyl CoA, or other long-chained fatty acids...

inhibits Acetyl CoA carboxylase by causing depolymerization of the enzyme - the end-product of fatty acid synthase

Dependence of the catalytic activity of acetyl CoA carboxylase on the concentration of citrate

(A) Citrate can partly activate the phosphorylated carboxylase (B) The dephosphorylated form of the carboxylase is highly active even when citrate is absent - Citrate partly overcomes the inhibition produced by phosphorylation

ACC inhibits fatty acid degradation because

its product, malonyl CoA, prevents the entry of fatty acyl CoA into the mitochondria by inhibiting carnitine acyl transferase 1

Fatty acid degradation and synthesis...

mirror each other in their chemical reactions

Activated fatty acids are transported into the ______________ before they are oxidized

mitochondria - after being activated by linkage to CoA, the fatty acid is transferred to carnitine, a reaction catalyzed by carnitine acyltransferase I, for transport into the mitochondria - carnitine translocase transports the acyl carnitine into the mitochondria - in the mitochondria, carnitine acyltransferase II transfers the fatty acid to CoA; fatty acyl CoA now ready to be degraded

Acetyl CoA carboxylase is inhibited when

phosphorylated by AMP-dependent kinase (AMPK) - Inhibition due to phosphorylation is reversed by protein phosphatase 2A Active ACC: favors fatty acid synthesis Inactive ACC (phosphorylated): favors fatty acid degradation - glucagon & epinephrine stimulate AMPK, while insulin stimulates protein phosphatase 2A

Odd-chain fatty acids yield __________ in the final thiolysis step

propionyl CoA - β-Oxidation of fatty acids with odd numbers of carbons generates propionyl CoA in the last thiolysis reaction

The second stage of fatty acid synthesis is

activation of acetyl CoA to form malonyl CoA - Formation of malonyl CoA is the committed step of fatty acid synthesis - Form malonyl CoA --> fatty acid synthesis - Inhibit malonyl CoA formation --> fatty acid oxidation & formation of other intermediates via citric acid cycle

The formation of ketone bodies

bodies—acetoacetate, d-3-hydroxybutyrate, and acetone—are formed from acetyl CoA primarily in the liver Enzymes catalyzing these reactions are: (1) 3-ketothiolase (2) hydroxymethylglutaryl CoA synthase (3) hydroxymethylglutaryl CoA cleavage enzyme (4) d-3-hydroxybutyrate dehydrogenase Acetoacetate spontaneously decarboxylates to form acetone

Pathological conditions result if fatty acids...

cannot enter the mitochondria - Muscle, kidney, and heart use fatty acids as a fuel - Pathological conditions results if the acyltransferase or the translocase are deficient - Carnitine deficiencies can be treated by carnitine supplementation

The degradation of a monounsaturated fatty acid

cis-delta^3-Enoyl CoA isomerase allows the beta-oxidation of fatty acids with a single double bond to continue

Triacylglycerols in adipose tissue are converted into free fatty acids in response to...

hormone signals - phosphorylation of perilipin restructures the lipid droplet and releases the coactivator of ATGL - activation of ATGL by binding with its coactivator initiates the mobilization - hormone-sensitive lipase releases a fatty acid from diacylglycerol - monoacylglycerol lipase completes the mobilization process

Insulin and glucagon also control rates of

synthesis and degradation of the enzymes involved in fatty acid synthesis, especially acetyl CoA carboxylase and fatty acid synthase - The enzymes of fatty acid synthesis are regulated by adaptive control - If adequate fats are not present in the diet, the synthesis of enzymes required for fatty acid synthesis is enhanced

Citrate

synthesized in the mitochondria (by CAC) - transported to the cytoplasm and cleaved by ATP-citrate lyase to generate acetyl CoA for fatty acid synthesis

The formation of malonyl CoA is...

the committed step in fatty acid synthesis - Malonyl CoA is synthesized by acetyl CoA carboxylase (ACC), a biotin-requiring enzyme

The third stage of fatty acid synthesis is

the repetitive addition and reduction of two carbon units to synthesize even, saturated fatty acid - Synthesis occurs on an acyl carrier protein, a molecular scaffold. - Fatty acid synthesis consists of a series of condensation, reduction, dehydration, and reduction reactions

Acyl carnitine translocase

- The entry of acyl carnitine into the mitochondrial matrix is mediated by a translocase - Carnitine returns to the cytoplasmic side of the inner mitochondrial membrane in exchange for acyl carnitine

The utilization of d-3-hydroxybutyrate and acetoacetate as a fuel

- d-3-Hydroxybutyrate is oxidized to acetoacetate with the formation of NADH - Acetoacetate is then converted into two molecules of acetyl CoA, which then enter the citric acid cycle

Fatty acids are processed in three stages

1) Degradation of triacylglycerols to release fatty acids and glycerol into the blood for transport to energy-requiring tissues 2) Activation of the fatty acids and transport into the mitochondria for oxidation 3) Degradation of the fatty acids to acetyl CoA for processing by the citric acid cycle

Sumary of Fatty Acid Degredation (steps)

1) Ligase (uses ATP, equivalent to 2 ATP) 2) Transferase (carnitine is the group transferred) 3) Oxidoreductase (produces FADH2 that is shuttled to electron transport chain - complex II) 4) Lyase (stereospecific addition of water to double bond) 5) Oxidoreductase (produces NADH) 6) Hydrolase (produces acetyl CoA)

Fatty acid oxidation (degradation) consists of four steps that are repeated

1) Oxidation of the β carbon, catalyzed by acyl CoA dehydrogenase, generates trans-Δ2-enoyl CoA and FADH2. 2) Hydration of trans-Δ2-enoyl CoA by enoyl CoA hydratase yields l-3-hydroxyacyl CoA. 3) Oxidation of L-3-hydroxyacyl CoA by L-3-hydroxyacyl dehydrogenase generates 2-ketoacyl CoA and NADH. 4) Cleavage of the 3-ketoacyl CoA by thiolase forms acetyl CoA and a fatty acid chain two carbons shorter. Fatty acid degradation is also called β-oxidation

The transfer of acetyl CoA to the cytoplasm

Acetyl CoA is transferred from mitochondria to the cytoplasm, and the reducing potential of NADH is concomitantly converted into that of NADPH by this series of reactions

Diabetic Ketosis

An overproduction of ketone bodies can occur when diabetes, a condition resulting from a lack of insulin function, is untreated - If insulin is absent or not functioning, glucose cannot enter cells - All energy must be derived from fats, leading to the production of acetyl CoA - Acetyl CoA builds up because oxaloacetate, which can be generated from glucose, is not available to replenish the citric acid cycle - Moreover, fatty acid release from adipose tissue is enhanced in the absence of insulin function (insulin signals to store or use fuels such as fatty acids)

The formation of malonyl CoA occurs in two steps

Biotin-enzyme + ATP + HCO3- <--> CO2-biotin + ADP + Pi + H+ CO2-biotin enzyme + acetyl CoA ---> malonyl CoA + biotin enzyme

Fatty acids are linked to

CoA (activation process) - Upon entering the cell cytoplasm, fatty acids are activated by attachment to coenzyme A - This two-step reaction proceeds through an *acyl adenylate* intermediate - The reaction is rendered irreversible by the action of *pyrophosphatase*

Filaments of acetyl CoA carboxylase

Electron micrograph showing the enzymatically active filamentous form of acetyl CoA carboxylase from chicken liver - The inactive form is a dimer of 265-kd subunits

Acetyl CoA carboxylase (ACC)

Enzyme in the committed step of fatty acid synthesis - subject to regulation on several levels, local & hormonal - regulated by conditions in the cell

Acidosis

Excess production of ketone bodie - moderately strong acids

Acetyl CoA, NADH, and FADH2 are generated by

Fatty acid oxidation - Oxidation of the β carbon, catalyzed by acyl CoA dehydrogenase, generates trans-Δ2-enoyl CoA and FADH2 - Hydration of trans-Δ2-enoyl CoA by enoyl CoA hydratase yields l-3-hydroxyacyl CoA - Oxidation of L-3-hydroxyacyl CoA by L-3-hydroxyacyl dehydrogenase generates 2-ketoacyl CoA and NADH - Cleavage of the 3-ketoacyl CoA by thiolase forms acetyl CoA and a fatty acid chain two carbons shorter

Pathway Integration: Fatty acid synthesis

Fatty acid synthesis requires the cooperation of various metabolic pathways located in different cellular compartments

The reaction sequence for the degradation of fatty acids

Fatty acids are degraded by the repetition of a four-reaction sequence consisting of oxidation, hydration, oxidation, and thiolysis

Animals cannot convert fatty acids into

Glucose - Fats are converted into acetyl CoA, which is then processed by the citric acid cycle - Oxaloacetate, a citric acid cycle intermediate, is a precursor to glucose - However, acetyl CoA derived from fats cannot lead to the net synthesis of oxaloacetate or glucose because although two carbons enter the cycle when acetyl CoA condenses with oxaloacetate, two carbons are lost as CO2 before oxaloacetate is regenerated

Triacylglycerols are hydrolyzed by

Lipases - Triacylglycerols are stored in adipocytes as a lipid droplet - When needed, the triacylglycerols are degraded by lipases to release fatty acids and glycerol into the blood for transport to energy-requiring tissues

Ketone bodies are used as a major fuel when glucose levels are

Low - The plasma levels of fatty acids and ketone bodies increase in starvation, whereas that of glucose decreases - Kidney takes over gluconeogenesis

The conversion of propionyl CoA into succinyl CoA

Propionyl CoA, generated from fatty acids having an odd number of carbon atoms as well as from some amino acids, is converted into the citric acid cycle intermediate succinyl CoA

The mechanism of acetyl CoA carboxylase is analogous to

Pyruvate Carboxylase Pyruvate + CO2 + ATP + H2O --> oxaolacetate + ADP + Pi + 2H+

Ketone body

Source of energy during starvation - acetoacetate, 3-hydroxybutyrate, and acetone - synthesized from acetyl CoA in liver mitochondria and secreted into the blood for use as a fuel by some tissues such as heart muscle - use curtails protein degradation and thus prevents tissue failure - synthesized from fats, the largest energy store in the body

Lipolysis

Starts by epinephrine and glucagon, acting through 7-transmembrane (7TM) receptors - Protein kinase A phosphorylates perilipin, which is associated with the lipid droplet, and hormone-sensitive lipase - Phosphorylation of perilipin results in the activation of adipocyte triacylglyceride lipase (ATGL). ATGL initiates the breakdown of lipids Stimulated by glucagon or epinephrine - generates free fatty acids & glycerol - glycerol released during lipolysis is absorbed by the liver for use in glycolysis or gluconeogenesis

The oxidation of linoleoyl CoA

The complete oxidation of the diunsaturated fatty acid linoleate is facilitated by the activity of enoyl CoA isomerase and 2,4-dienoyl CoA reductase

Summary of fatty acid degradation (catabolism)

The fatty acids incorporated into triacylglycerols in adipose tissue are made accessible in three stages - Once free fatty acids are formed, they need to be activated by ATP, transported into the mitochondria, and then processed to acetyl CoA by fatty acid oxidation

The first three rounds in the degradation of palmitate

Two carbon units are sequentially removed from the carboxyl end of the fatty acid - Palmitate has 16 carbons - Each round of oxidation shortens palmitate by 2 carbons - It takes 7 cycles of oxidation to degrade palmitate to 8 molecules of acetyl CoA (last round generates 2 acetyl CoA - Each round (of 7) also produces 1 FADH2 & 1 NADH - In the end, have 8 acetyl CoA, 7 FADH2 & 7 NADH Hint: # carbons in fatty acid / 2 = # acetyl CoA; subtract 1 for # FADH2 & # NADH

Propionyl carboxylase

a biotin enzyme - adds a carbon to propionyl CoA to form methylmalonyl CoA

Succinyl CoA

a citric acid cycle component - subsequently formed from methylmalonyl CoA by methylmalonyl CoA mutase, a vitamin B12 requiring enzyme

3-Hydroxybutyrate

formed upon the reduction of acetoacetate

Acetoacetate

generated by the condensation of 2 acetyl CoA molecules

Acetone

generated by the spontaneous decarboxylation of acetoacetate

The first stage of fatty acid synthesis is

transfer of acetyl CoA out of the mitochondria into the cytoplasm - *Citrate* is transported into the cytoplasm and cleaved into oxaloacetate and acetyl CoA. - Fatty acid oxidation happens in the mitochondria, while synthesis occurs in the cytoplasm (compartmentalization)