Amino Acid Oxidation: Urea Cycle Overview

Amino Acid Oxidation (Urea Cycle)

• Amino acids have an amino group and a carboxylic acid group attached to the same carbon atom, called the α-carbon.
• Amino acids (AAs) differ based on their side chains (R groups). These R groups vary in structure, size, and electric charge, influencing their solubility in water.
• The many paths for AA catabolism have two broad parts: one involving the amino groups, and the other involving the carbon skeletons.
• Four AAs—alanine, glutamate, glutamine, and aspartate—play key roles in transporting and distributing amino groups.
• Free ammonia is toxic to the body.
• Each amino acid has a unique catabolic fate.
• AAs undergo oxidative degradation when released during protein turnover is not needed for new protein synthesis or when ingested AAs exceed the body's need for protein synthesis.
• Daily protein turnover for humans is 300g.
• AAs contain nitrogen atoms which need to be eliminated without developing harmful ammonia.
• The urea cycle occurs in the liver and was first described in 1932.
• Steps of the Urea Cycle occur cyclically, with the help of Hans Krebs and Kurt Henseleit.

Metabolic Fates of Amino Groups

• Amino groups, if not reused, are channeled into a single excretory product: urea.
• Glutamate, glutamine, alanine, and aspartate are readily converted into citric acid cycle intermediaries.
• Glutamate and glutamine convert to alpha-ketoglutarate.
• Alanine is converted to pyruvate.
• Aspartate is converted to oxaloacetate.

TCA Cycle

• The TCA cycle accepts 3-, 4-, and 5-carbon skeletons.
• The breakdown of amino acids forms carbon skeletons.
• Deaminated aspartate yields oxaloacetate.
• Deaminated glutamate yields α-ketoglutarate.

Protein Metabolism and Urea Cycle

• During digestion, proteins are hydrolyzed into amino acids.
• Amino acids are oxidized via the Krebs cycle after various processes, including deamination, decarboxylation, and hydrogenation.
• The body eliminates ammonia by converting it into urea.

Urea Cycle

• Ammonia is transformed into urea within the liver's mitochondria (hepatocytes).
• Excreted from the body in urine.
• The carbon and oxygen of urea originate from CO2.
• Urea is generated in the liver, then transported via blood to the kidneys for excretion.

Nitrogen Removal from Amino Acids

• Step 1: Remove amino group
• Step 2: Transport amino group to liver for nitrogen excretion
• Step 3: Entry into mitochondria
• Step 4: Prepare nitrogen to enter the urea cycle
• Step 5: Urea cycle

Nitrogen Removal from Amino Acids

• Transamination: a transfer of the amino group from an amino acid to an α-keto acid, converting the original amino acid into an α-keto acid and vice-versa.
• Oxidative deamination: the removal of an amino group as ammonia. The aminotransferase enzyme is often involved.

Nitrogen Carriers

• Glutamate: transfers one amino group within cells.
• Glutamine: transfers two amino groups between cells, releasing its amino group in the liver.
• Alanine: transfers amino groups from tissues (like muscle tissue) into the liver. It uses transamination with glutamate to interconvert pyruvate and alanine.

Glucose-Alanine Cycle

• Alanine plays a key role in transporting amino groups to the liver.
• Alanine carries ammonia and pyruvate skeletons from muscle to liver.
• The ammonia is excreted, and the pyruvate is used to produce glucose returned to the muscle.

Step 3: Entry of Nitrogen to Mitochondria

• Glutamine from extrahepatic tissues releases ammonia.
• Alanine from muscle tissue releases ammonia.

Step 4: Preparing Nitrogen to Enter Urea Cycle

• The nitrogen is prepared to enter the urea cycle. (Details about this step are provided in Step 4.)

Reaction Steps of the Urea Cycle

• The first two steps (formation of carbamoyl phosphate and citrulline) occur in the mitochondria.
• The remaining steps (formation of argininosuccinate, cleavage to arginine and fumarate, and arginine hydrolysis) occur in the cytosol.

Step 5: Urea Cycle

• The urea cycle is a complex process involving multiple enzymes and steps. In detail, this step describes the actual catalytic transformations within the urea cycle.

Urea Cycle - Review

• Carbamoyl phosphate formation in the mitochondria is a prerequisite for the urea cycle.
• Citrulline is formed from carbamoyl phosphate and ornithine.
• Aspartate provides additional nitrogen to form argininosuccinate in the cytosol.
• Arginine and fumarate are formed from argininosuccinate.
• Arginine is hydrolyzed into urea and ornithine.

Urea Cycle Recap (In Pictures)

• Shows the release of ammonia by glutamate dehydrogenase for entry into the urea cycle.
• Illustrates the involvement of alanine, pyruvate, and aspartate in the process.
• Provides a visual representation of the steps in the urea cycle.

Regulation of Urea Cycle

• Dietary protein intake regulates urea cycle activity.
• Prolonged starvation increases urea production.
• The rate of synthesis of urea cycle enzymes and carbamoyl-phosphate synthetase I are regulated by the demand for urea cycle activity.
• Enzymes are synthesized at high rates during starvation and high protein diets, and at low rates during well-fed conditions with carbohydrate and fat diets.
• N-acetylglutamic acid is an allosteric activator of CPS-I. High Arg concentration stimulates the process.

Some Human Genetic Disorders

• Table illustrating various disorders affecting amino acid catabolism.
• Includes medical conditions, approximate incidence, defective processes, associated defective enzymes, and their symptoms/effects.

Essential and Nonessential Amino Acids

• Lists essential and nonessential amino acids, including those conditionally essential.
• Includes Phenylketonuria, indicating that a mutation in phenylalanine hydroxylase causes this.

Lipids - Review

• Lipids comprise a diverse group of compounds (fats, oils, steroids, waxes) generally recognized by their insolubility in water and solubility in nonpolar solvents.
• Lipids are critical in biological systems, forming cell membranes, generating energy, and including essential vitamins.
• The three major categories are simple lipids, compound lipids, and steroids, further grouped into saponifiable and nonsaponifiable categories.

Steroid Classification

• Sterols: Have aliphatic side chains, often with hydroxyl groups.
• Sex hormones: Have ketone or hydroxyl groups and a two-carbon side chain.
• Cardiac glycosides: Contain a lactone ring (with a sugar).
• Bile acids: Contain a five-carbon side chain ending with a carboxylic acid.
• Sapongenins: Have oxacyclic ring systems.

Stages of Fatty Acid Oxidation

• Stage 1: β-oxidation—Fatty acids are broken down into acetyl-CoA molecules.
• Stage 2: Oxidation of acetyl-CoA—Acetyl-CoA is oxidized in the citric acid cycle.
• Stage 3: Electron transport chain and oxidative phosphorylation—ATP is generated from NADH and FADH2.

Beta Oxidation

• Beta-oxidation is the catabolic breakdown of fatty acids in the mitochondria of eukaryotes to generate acetyl-CoA.
• Acetyl-CoA enters the citric acid cycle, and NADH and FADH2 are used in the electron transport chain.
• Free fatty acids and water are the substrates.
• Products are acetyl CoA, NADH, FADH2.
• The process is called 'beta-oxidation' because beta-carbon of the fatty acid undergoes oxidation to a carbonyl group.

Oxidation of Odd-Chain and Unsaturated Fatty Acids

• Odd-chain fatty acids produce acetyl-CoA and propionyl-CoA.
• Propionyl CoA can be converted into succinyl CoA via three enzymatic steps.
• Unsaturated fatty acids require additional enzymes beyond the four that characterize beta-oxidation's repetitive steps.

Fatty Acid Synthesis

• A process by which saturated fatty acids (like palmitate, 16 carbons), are synthesized.
• Excess carbohydrates are converted to fatty acids to create energy storage.

Regulation of Fatty Acid Oxidation

• Enzyme carnitine palmitoyl transferase I (CPTI) is the rate-limiting enzyme in fatty acid oxidation; inhibited by malonyl CoA, a product of fatty acid synthesis.
• Hormonal regulation with hormones like glucagon and epinephrine promoting FA oxidation, and insulin inhibiting it.

Ketone Bodies

• Ketone bodies (acetoacetate, acetone, and hydroxybutyrate) are formed from acetyl-CoA in the liver.
• These are transported to other tissues, converted to acetyl-CoA, and used as fuel.
• The human liver produces ketone bodies and these are not generated in other tissues.
• They are often overproduced in cases of diabetes and starvation.