Transesterification and Carnitine

Transesterification and Carnitine's Role

• Fatty acylCoA and carnitine undergo transesterification.
• The fatty acylcarnitine ester is formed by carnitine acyltransferase I, then transported from the cytosol into the mitochondrial matrix by acyl-carnitine/carnitine transporter, located on the inner mitochondrial membrane.
• In the intramitochondrial space, the fatty acyl group is transferred from carnitine to coenzyme A by carnitine acyltransferase II, forming fatty acyl-CoA again.
• Carnitine reenters the intermembrane space via the acyl-carnitine/carnitine transporter.
• These steps transfer fatty acids into the mitochondrion through:
  • Esterification to CoA
  • Transesterification to carnitine followed by transport
  • Transesterification back to CoA, linking separate pools of coenzyme A and fatty acylCoA in the cytosol and mitochondria.
• These coenzyme A pools have different functions.
• The mitochondrial matrix coenzyme A is largely used in oxidative degradation of pyruvate, fatty acids, and some amino acids.
• The cytosolic coenzyme A is used in fatty acid biosynthesis.
• Fatty acylCoA in the cytosolic pool is used for membrane lipid synthesis or moved into the mitochondrial matrix for oxidation and ATP production.

Fatty Acid Oxidation Stages

• Fatty acylCoA are transported into mitochondria, then oxidized in three stages to yield energy in the form of ATP.
• The first stage is β-oxidation, which dehydrogenates the hydrocarbon chain and gradually removes α and β carbons in the form of acetyl CoA.
• In the second stage, acetyl groups of acetylCoA are oxidized to CO₂ in the citric acid cycle within the mitochondrial matrix.
• AcetylCoA from fatty acids enters a final common oxidation pathway with acetylCoA from glucose, glycolysis, and pyruvate oxidation.
• The first two stages of fatty acid oxidation produce the reduced electron carriers NADH and FADH2.
• In the third stage, NADH and FADH2 donate electrons to the mitochondrial respiratory chain, through which the electrons pass to oxygen with concomitant phosphorylation of ADP to ATP.
• Energy released by fatty acid oxidation is conserved as ATP.

Fatty Acids Synthesis

• Synthesis occurs in the cytoplasm.
• Acetyl-CoA (the starter molecule) must be exported from the mitochondrion.
• Acetyl-CoA carboxylase catalyzes the committed step in formation of malonyl-CoA.
• Elongation occurs through the sequential addition of 2-carbon units (malonyl-CoA).
• Malonyl-CoA is needed to produce fatty acids.

Fatty Acid Synthase (FASN)

• The essential components of FASN are:
  • Condensing enzyme (CE)
  • Acyl carrier protein (ACP)
• FASN elongates C16:0 (palmitate); other enzymes can elongate further.

Substrates for Synthesis

• Sequential assembly of an acetyl "starter" molecule:
  • CoA (in Condensing Enzyme) with malonyl-CoA (in ACP)
• Acetyl-CoA is produced.
• Starts with citrate (from Krebs).

Fatty Acids Synthesis Regulation

• Increased glucose leads to citrate production, overwhelming the Krebs cycle.
• High carbohydrate diets result in fat production.
• High insulin levels activate citrate production, carboxylate dehydrogenase, and lipid macromolecule production.

Formation of Malonyl-CoA

• Acetyl-CoA and bicarbonate form Malonyl-CoA.
• Acetyl-CoA carboxylase catalyzes formation from Acetyl-CoA + HCO₃⁻ + ATP → malonyl-CoA + ADP.

Fatty Acid Synthesis Sequence

• Long carbon chains of fatty acids are assembled in a repeating four-step sequence.
• A saturated acyl group produced becomes the substrate for condensation with an activated malonyl group.
• With each passage, the fatty acyl chain extends by two carbons.
• The product’s chain length reaches 16 carbons.
• Palmitate (16:0) is the precursor of other fatty acids, including stearate and longer-chain saturated fatty acids.
• Palmitate is the precursor of monounsaturated acids, including palmitoleate and oleate.
• Mammals cannot convert oleate to linoleate or α-linolenate; these are essential dietary fatty acids.
• The number and position of double bonds symbolize unsaturated fatty acids.

Triacylglycerol Biosynthesis

• Begins with glucose.
• Mammals break down and resynthesize triacylglycerol molecules in a triacylglycerol cycle during starvation.
• Lipolysis of triacylglycerol in adipose tissue releases some fatty acids into the bloodstream, while the remainder are used for resynthesis.
• Fatty acids released into the blood provide energy in muscle.
• The liver synthesizes triacylglycerol, transporting it in the blood back to adipose tissue for storage, involving extracellular lipoprotein lipase and adipocytes.

Clinical Aspects of Fatty Acid Disorders

• Abnormal essential fatty acid metabolism correlates to clinical disorders.
• Linked to Cystic Fibrosis, Hepatorenal Syndrome, Sjögren Syndrome, Multisystem Neuronal Degeneration, Alcoholism and Cirrhosis Liver, Acrodermatitis Enteropathica, Crohn's Disease, and Reye's Syndrome.
• Zellweger's syndrome patients have high levels of very long chain polyenoic acids in the brain, due to the inherited absence of peroxisomes and peroxisomal oxidation.
• Diets with high P:S (polyunsaturated:saturated FA) ratios are beneficial, lowering serum cholesterol and LDL levels.

Ketone Bodies

• Acetyl-CoA from oxidation of fatty acids can enter the citric acid cycle or be converted to ketone bodies (acetone, acetoacetate, β-hydroxybutyrate) for export.
• Produced acetone is exhaled.
• Total ketone bodies in the blood of well-fed individuals typically do not exceed 1 mg/100 ml.
• Loss via urine is usually less than 1 mg/24 hrs.
• Acetoacetate and β-hydroxybutyrate are transported to tissues other than the liver where they are converted to acetyl-CoA and oxidized for energy.
• Ketone bodies are water-soluble and energy yielding.
• Acetone, produced in smaller quantities, cannot be metabolized.
• The brain can adapt to using acetoacetate or β-hydroxybutyrate.

Ketogenesis Details

• Acetoacetate is the primary ketone body.
• Beta-hydroxybutyrate and acetone are secondary.
• Synthesis occurs in the liver, with enzymes located in the mitochondrial matrix.
• Acetyl CoA is the precursor for ketone bodies, formed by oxidation of fatty acids, pyruvate, or amino acids.
• The first step in acetoacetate formation is the enzymatic condensation of two acetyl-CoA molecules, catalyzed by thiolase, which is reversible.
• Acetoacetyl-CoA then condenses to form HMG CoA.

HMG CoA Consumption

• HMG CoA is for ketogenesis. If not,
• HMG CoA is used for cholesterol synthesis.
• HMG CoA Cleaves:
  • HMG CoA is cleaved to free acetoacetate and acetyl-CoA in the liver
• acetoacetate is reversibly reduced by D-β-hydroxybutyrate dehydrogenase.
  • This enzyme is specific for this step
  • Does not act on L-β-hydroxyacyl-CoA
  • Shouldn't be confused with L-β-hydroxyacyl-CoA dehydrogenase of the β-oxidation pathway.

Continued Ketogenesis Steps

• In healthy people, acetone forms in small amounts from acetoacetate through spontaneous decarboxylation.
• Because individuals with diabetes produce large quantities of acetoacetate, blood contains significant amounts of acetone, which is volatile and smells.
• Diagnosing diabetes is sometimes useful, but it can also happen without the condition so is not a 100% reliable confirmation.

Ketolysis

• Ketolysis is the process of using ketone bodies for energy.
• In extrahepatic tissues, D-β-hydroxybutyrate is oxidized to acetoacetate by D-β-hydroxybutyrate dehydrogenase.
• Acetoacetate is activated by transfer of CoA from succinyl-CoA, catalyzed by ketoacyl-CoA transferase.
• Acetoacetyl-CoA is cleaved by thiolase to yield two acetyl-CoAs, then enters the citric acid cycle.
• Ketone bodies formed in the liver are utilized by extrahepatic tissues.
• Because they are water-soluble, it is easily transported.
• The two bodies serve as important energy sources for peripheral tissues, cardiac muscle/renal cortex, and skeletal muscle.
• Ketone bodies: More significant in the long-term.
• Ketone Bodies Production: More significant when glucose decreases
• Starvation: They become useful long-term.
• Observed in starvation and diabetes mellitus.

Overproduction of Ketone Bodies:

• Overproduction from starvation and untreated diabetes mellitus yields acidosis and related problems.
• Starvation involves increased degradation of fatty acids, which leads to acetyl-CoA overproduction.
• Gluconeogenesis depletes citric acid cycle intermediates, so acetyl-CoA diverts towards ketone body production.
• In untreated diabetes with insufficient insulin, extrahepatic tissues don't use glucose.
• Malonyl-CoA levels fall
  • Which relieves inhibition of carnitine acyltransferase
  • Leading to fatty acids entering to be degraded to acetyl-CoA in the mitochondria
• Accumulation of acetyl-CoA accelerates ketone body formation beyond the capacity of extrahepatic tissues to oxidize them.
• High blood levels of acetoacetate and D-β-hydroxybutyrate lower blood pH, resulting in acidosis, which is a condition.
• Extreme acidosis can lead to coma or death.
• In untreated diabetics can reach extraordinary levels-a blood concentration and urinary excretion
• Ketosis: This is what the condition is termed. Individuals on very low-calorie diets also have increased ketone bodies due to using fats for energy.

Ketonemia and Ketonuria

• The rise of ketones in blood (ketonemia) should be avoided.
• Rise of ketone bodies in blood above normal is known as condition.
• Elevated levels should be monitored in the blood and urine to avoid acidosis and ketosis (ketoacidosis).

Basics of Amino Acids and Protein Metabolism

• Metabolizes wide array of synthetic and degradative reactions, by which amino acids are assembled and broken down.
• This metabolism involves polypeptides and metabolic energy.
• Transforms and differs from those of carbohydrates or lipids; it concerns amino acids with nitrogen.
• The cell's amino acids become proteins, continuously being synthesized and degraded.
• There is no "storage form" of acid like for glycogen or triacylglycerols; mammals synthesize and obtain the rest from diets.

Dietary Proteins

• There are two kinds of of dietary proteins for body intake
  • Animal
  • Vegetable
• Some materials contain enzyme inhibitors that destroy vitamins
• Cooking destroy the harmful enzymes and bacteria.

Digestion of Proteins To Acids

• In humans, ingested proteins degrade into the constituent amino acids and occurs the gastrointestinal tract.
• As the dietary protein enters then the gastric mucosa secretes gastrin, which in turns lead to hydrochloric acid secretion
• Parietal cells and pepsinogen the gastric glands stimulates, the acidic gastric juice has antiseptic with a pH of 1-2.5.
• Pepsinogen converts with autocatalysis to pepsin and then hydrolyzes protein.
• There is an aromatic amino acid, with residues Phe, Trp, Tyr and cleaving Polypeptide changes.
• Low pH triggers and the pancreas secretes secretion which is why it increases high.
• The arrival then releases from the blood Choleocystokinin then comes the Pancreatic Enzymes in optimal activity with Trypsin, Chymotrypsin, which is how the exocrine cells secrets.

Enzymes for Proteolysis

• Trypsinogen converts then Catalyzes conversion of then synthesis get inactive which leads to active digestion within the pancreatic cells.
• That's is why there is a specific protein called Pancreatic Trypsin then inhibits Enzyme Productions and it hydrolize in the peptides in the stomach as well.
• Short peptides then gets completed and hydrolizes, from free amino, through which the humans have from animal which are almost hydrolyzed.
• In some content, there is the plant foods which get protected because there is indigestion. -Enzymes must be involved with the GIT and to the liver.

Amino Acid Degradation

• Dietary amino acids are not simply excreted.
• Converted to metabolic wastes such as glucose.
• Serve as precursors to fatty acids and ketone bodies.
• Important in the metabolism of protein.
• Can enter the Amino Acid Pool.