Metabolism of Amino Acids: Oxidation and Urea Production

Vocabulary flashcards covering key enzymes, pathways, clinical terms, and regulatory concepts from the lecture on amino-acid metabolism and urea production.

Nitrogen Balance

Relationship between nitrogen intake and loss; can be positive, negative, or neutral.

Positive Nitrogen Balance

Nitrogen intake exceeds loss (e.g., growth, pregnancy, recovery).

Negative Nitrogen Balance

Nitrogen loss exceeds intake (e.g., starvation, kwashiorkor).

Kwashiorkor

Protein-deficiency disease causing muscle wasting and negative nitrogen balance.

Ureotelic

Organisms (e.g., terrestrial mammals) that excrete nitrogen mainly as urea.

Ammonotelic

Organisms (e.g., most bony fish) that excrete nitrogen directly as ammonia.

Uricotelic

Organisms (e.g., birds, reptiles) that excrete nitrogen mainly as uric acid.

Zymogen

Inactive enzyme precursor secreted by stomach or pancreas (e.g., pepsinogen).

Cholecystokinin (CCK)

Intestinal hormone that stimulates pancreatic enzyme secretion and gall-bladder contraction.

Secretin

Intestinal hormone that stimulates pancreatic HCO₃⁻ secretion to neutralize gastric acid.

Enteropeptidase

Intestinal enzyme that activates trypsinogen to trypsin, initiating pancreatic zymogen activation.

Hartnup Disease

Inherited defect in intestinal/renal transport of neutral amino acids.

Transamination

Reaction that transfers an amino group to an α-keto acid, usually forming glutamate.

Transaminase (Aminotransferase)

Enzyme catalyzing transamination; requires pyridoxal phosphate.(B6)

Alanine Aminotransferase (ALT/GPT)

Major transaminase converting alanine ↔ pyruvate; serum marker for liver damage.

Aspartate Aminotransferase (AST/GOT)

Major transaminase converting aspartate ↔ oxaloacetate; serum marker for heart/liver damage.

Pyridoxal Phosphate (PLP)

Vitamin B6-derived cofactor required for transaminase activity.

α-Ketoglutarate (αKG)

Common amino-group acceptor in transamination; forms glutamate.

Glucose–Alanine Cycle

Pathway in which muscle exports alanine to liver for gluconeogenesis while disposing nitrogen.

Glutamine Synthase

Enzyme that converts glutamate + NH₄⁺ + ATP → glutamine; key regulator of nitrogen disposal.

Glutaminase

Enzyme that hydrolytically deaminates glutamine → glutamate + NH₄⁺ (liver, kidney).

Glutamate Dehydrogenase (GDH)

Mitochondrial enzyme that oxidatively deaminates glutamate → αKG + NH₄⁺ + NAD(P)H.

Oxidative Deamination

Removal of an amino group as NH₄⁺ with simultaneous oxidation (e.g., by GDH).

Hydrolytic Deamination

Removal of an amino group using water, as catalyzed by glutaminase.

Blood Urea Nitrogen (BUN)

Clinical measure of serum urea; elevated values suggest renal dysfunction.

Urea Cycle

Liver pathway converting NH₄⁺, CO₂, and aspartate to urea; spans mitochondria and cytosol.

Carbamoyl Phosphate Synthetase I (CPS I)

Mitochondrial, rate-limiting enzyme of urea cycle producing carbamoyl phosphate; requires 2 ATP.

Ornithine Transcarbamoylase (OTC)

Mitochondrial enzyme transferring carbamoyl group to ornithine → citrulline.

Argininosuccinate Synthase

Cytosolic urea-cycle enzyme combining citrulline + aspartate + ATP → argininosuccinate.

Argininosuccinate Lyase

Cytosolic enzyme splitting argininosuccinate → arginine + fumarate.

Arginase

Cytosolic enzyme hydrolyzing arginine → urea + ornithine.

Krebs’ Bicycle (Aspartate–Argininosuccinate Shunt)

Interconnection between urea cycle and TCA cycle via fumarate and aspartate.

Ammonia Encephalopathy

Brain dysfunction due to elevated NH₄⁺, causing ATP depletion, neurotransmitter imbalance, swelling.

Adenylation (Enzyme Regulation)

Covalent attachment of AMP to glutamine synthase, decreasing its activity.

Creatine Kinase (CK)

Serum enzyme elevated after muscle or heart damage; assessed alongside transaminases.

β-Amino Acid Transporter

Intestinal active transport system specific for β-alanine, taurine, etc.

Branched-Chain Aminotransferase

Enzyme initiating catabolism of leucine, isoleucine, valine in muscle.

Urease

Enzyme not present in humans but in bacteria, converts urea → CO₂ + NH₃ (basis of breath test).

Lactulose (Therapy)

Non-absorbable sugar metabolized by gut bacteria to acids, trapping NH₄⁺ for treating hyperammonemia.

Hyperammonemia

Pathologic increase in blood ammonia, often due to urea-cycle defects or liver failure.