Fuel Metabolism Flashcards
Biotin Cycle
•Biotin is a prosthetic group cofactor for enzymes catalyzing carboxylation reactions.
- pyruvate carboxylase (PC)
- 3-methylcrotonyl-CoA carboxylase (MCC)
- propionyl-CoA carboxylase (PCC)
- acetyl-CoA carboxylase (ACC)
•Biotin is covalently attached to a lysine residue on the apoenzymes in a reaction catalyzed by holocarboxylase synthetase. When the holoenzymes (apoenzyme + biotin) are degraded, biocytin (biotin + lysine conjugate) is released. Biotinidase then cleaves biocytin to recycle biotin. If all this biocytin were to be excreted, the individual would become biotin deficient.
How are human requirements for biotin met?
- Human requirements for biotin are primarily met through synthesis by intestinal bacteria.
- Consuming raw eggs can lead to biotin deficiency because egg whites contain avidin that combines tightly with biotin to prevent its intestinal absorption.
- Some antibiotics can also cause a biotin deficiency.
- Symptoms include dermatitis, alopecia and enteritis.
- In some individuals, an apparent biotin deficiency can be caused by genetic defects of the carboxylases, of holocarboxylase synthetase or most commonly of biotinidase.
- Biotinidase defects in children usually present at 4 to 6 months of age with seizures, alopecia, dermatitis and psychomotor retardation.
- Complications are prevented by daily oral supplementation of biotin.
Pyruvate Carboxylase (PC)
- Pyruvate carboxylase - initial reaction of gluconeogenesis from lactate and alanine.
- The pyruvate carboxylase reaction initiates the gluconeogenesis pathway from the primary precursors lactate and alanine, as well as some other amino acids.
- Deficiencies of biotin or biotinidase limit the availability of biotin for being covalently attached to the enzyme.
- A defect of holocarboxylase synthetase prevents the attachment even though biotin is available.
- Regardless of the cause, inactivity of pyruvate carboxylase leads to an inability to synthesize glucose under conditions of food deprivation longer than an overnight fast or during stress. Hence hypoglycemia can be a consequence.
- The inability to convert lactate to glucose leads to lacticacidemia and hence metabolic acidosis with attendant responses of tachycardia.
Propionyl-CoA Carboxylase (PCC)
- Propionyl CoA is produced in pathways involving certain amino acids (i.e., isoleucine, methionine) and beta-oxidation of odd-chain fatty acids.
- Normally propionyl CoA is metabolized via carboxylation (biotin-dependent propionyl CoA carboxylase) to methylmalonyl CoA, which in turn is converted to succinyl CoA in the citric acid cycle.
- Defects associated with biotin availability or covalent attachment to the carboxylase leads to formation of 3- hydroxypropionate and more importantly of methylcitrate.
- Methylcitrate is formed via citrate synthase, with propionyl CoA competing with acetyl CoA for condensation with oxaloacetate.
- Not surprisingly, the methylcitrate is toxic to the cell as it interferes with normal function of the citric acid cycle and with the mitochondrial citrate transporter.
Acetyl-CoA Carboxylase
- Fatty Acid metabolism
- Glucose is the primary precursor for synthesis of fatty acids via its metabolism to acetyl CoA.
- Although acetyl CoA is required for fatty acid synthesis, it is produced in the mitochondria from the metabolism of pyruvate via pyruvate dehydrogenase (PDH).
- Because the pool of coenzyme A in the mitochondria remains separate from the pool in the cytoplasm, acetyl CoA is not transported directly across the membrane.
-Instead citrate carries these carbons from the mitochondria to the cytoplasm. This is a similar event as seen for cholesterol biosynthesis.
•Pyruvate, transported into the mitochondrial matrix, is converted either to acetyl CoA or to oxaloacetate (via pyruvate carboxylase, PC) in equal amounts.
-The opposite regulation of these reactions permits this to occur.
- These intermediates combine to form citrate in the citric acid cycle via citrate synthase (CS).
- The high concentration of citrate favors its transport out of the mitochondrial matrix via the tricarboxylate translocase, driven by the concentration gradient of citrate.
- In the cytoplasm citrate is cleaved by citrate lyase (CL) producing acetyl CoA, for lipogenesis, and oxaloacetate as products.
-The reaction requires energy obtained by hydrolysis of ATP.
- To recycle carrier carbons, oxaloacetate is reduced to malate via a cytoplasmic malate dehydrogenase (MDH).
- NADP subsequently oxidizes malate to pyruvate and CO2 in a reaction catalyzed by malic enzyme (ME).
- The pyruvate product re-enters the mitochondria to be available for synthesis of oxaloacetate.
- Thus oxaloacetate from the citric acid cycle is never consumed in this overall process of carbon transport.
- The acetyl CoA derived from citrate cleavage is processed to malonyl CoA via acetyl CoA carboxylase for lipogenesis.
- Malonyl CoA is the source of adding carbons for the synthesis of fatty acid chains via fatty acid synthase.
- Obviously biotin-related problems will diminish the activity of acetyl CoA carboxylase and hence impair lipogenesis.
3-methylcrotonyl-CoA Carboxylase (MCC)
•Leucine metabolism pathway
Conditions Favoring Fatty Acid Synthesis
•Following carbohydrate intake, the elevated blood glucose increases circulating insulin and decreases glucagon to produce a high ratio of insulin to glucagon.
- In adipose tissue, insulin inhibits the hormone-sensitive lipase thereby limiting the delivery of fatty acids to liver.
- In liver, the high ratio of insulin to glucagon favors glucose utilization and thus the availability of acetyl CoA for fat synthesis increases.
- This hormone balance supports activation of acetyl CoA carboxylase.
- Following carbohydrate intake, the blood levels of fatty acids are low.
-Low circulating fatty acids leads to small amounts of palmitoyl CoA. When the concentration of palmitoyl CoA is small, then acetyl CoA carboxylase remains active; that is the inhibitory effect of palmitoyl CoA is lost.
- Additionally, a large influx of glucose favors formation of citrate to promote the polymerization of acetyl CoA carboxylase to reduce the amount of the inactive monomeric form.
- This effect of citrate occurs in conjunction with elevated insulin that favors lipogenesis.
- During the synthesis of fatty acids, it is imperative that they not be taken up into the mitochondria for oxidation.
- Thus malonyl CoA, which is produced from the acetyl CoA carboxylase reaction, blocks the production of the form of palmitate that is transported into mitochondria; that is palmitoyl carnitine that is formed via carnitine palmitoyl transferase-I (CPT-I).
- In this way, oxidation of newly synthesized palmitate is prevented due to decreased formation of palmitoyl carnitine for transport into the mitochondrial matrix.
- Instead, the newly synthesized fatty acid is used to form triacylglycerol for storage.
Conditions Favoring Fatty Acid Oxidation
- In contrast to the synthetic conditions, the starvation state lowers the ratio of insulin to glucagon.
- Glucagon promotes lipolysis in adipose tissue to deliver fatty acids to the liver.
- In fight or flight responses, epinephrine is elevated and the same event occurs except that the release of fatty acids provides for increased energy demands by muscle.
- The hormone balance in starvation favors glucose production limiting the supply of carbons for lipogenesis.
- Low concentrations of citrate and elevated palmitoyl CoA, from lipolysis, favor deactivation of acetyl CoA carboxylase with phosphorylation via glucagon completing the inactivation.
- The decrease in acetyl CoA carboxylase activity, due to the combined effects of fatty acyl CoA and glucagon, lowers the concentration of cytoplasmic malonyl CoA thereby relieving the inhibition of CPT-I.
- Once CPT-I is active, fatty acids can be transported into the mitochondria for beta-oxidation.
- When production of acetyl CoA from beta- oxidation exceeds the capacity of the citric acid cycle, an increase in ketogenesis occurs to supply fuel during prolonged starvation.
- Recall that ketogenesis occurs in the liver when the supply of acetyl CoA from beta-oxidation of fatty acids exceeds the capacity of the citric acid cycle to oxidize the acetyl CoA.
-The acetyl CoA “spills over” into the ketogenesis pathway and ketone bodies become available as a fuel for brain, muscle, kidney, etc…
Carnitine Palmtoyl Transferase Deficiency
- The carnitine shuttle uses carnitine to facilitate hydrophobic long chain fatty transport from the mitochondrial intermembrane space, which equilibrates with the cytosol, into the mitochondrial matrix for the production of energy via β-oxidation.
- Individuals with deficiency in this system lack the ability to effectively utilize fats for energy production or for ketone body production.
- Disease severity varies and is related to the type and degree of deficiency in the shuttle mechanism.
- Disease manifestation is intuitive in that patients lack the ability to use lipids and to produce ketone bodies for energy and thus lack the ability to preserve glucose levels.
- Patients are therefore very susceptible to periods of fasting.
-Laboratory studies of patients during this period demonstrate a hypoketotic hypoglycemia.
•Severe forms of the disease are fatal in infancy.
- The infantile form involves multiple organ systems and often results in altered mental status and seizure activity during hypoglycemic events.
- Episodes are triggered by febrile illness, infection, or fasting.
•The myopathic form, a milder version, is more commonly seen in adults and includes rhabdomyolysis, myoglobinuria, recurrent muscle pain, and weakness.
-Symptoms are most often exercise-induced, but fasting, a high-fat diet, sleep deprivation, or febrile illness can also trigger myopathic symptoms.
