Protein Synthesis and the Urea Cycle PDF

Protein Synthesis and the Urea Cycle Learning Objectives Albumin: Synthesis Function Clinical significance Clotting factors: Synthesis...

Protein Synthesis and the Urea Cycle Learning Objectives Albumin: Synthesis Function Clinical significance Clotting factors: Synthesis Function Clinical significance Urea cycle: Role in metabolism Ammonia neurotoxicity Urea cycle disorders Protein Synthesis What is produced when DNA is transcribed? Pre-mRNA Where does transcription occur? Nucleus How is mature mRNA produced? Post-transcriptional modification to pre-mRNA e.g. 5’ capping, 3’ Poly-A tail Protein Synthesis and the Urea Cycle 1  How does mature mRNA enter the cytoplasm? Through nuclear pores Where does translation of mature mRNA occur? Ribosome What happens to the protein after translation? Protein folding and post-translational modifications What is the primary structure of a protein? Polypeptide chain Protein Synthesis and the Urea Cycle 2  What type of bonds stabilise the primary structure of proteins? Peptide What type of protein structure are alpha helices and beta sheets? Secondary structure How does bonding differ between alpha helices and beta sheets? Alpha helices: hydrogen bonds are within the same polypeptide chain Beta sheets: Hydrogen bonds between different strands or regions of a chain (parallel/antiparallel) What is the tertiary structure of the protein? 3D overall fold of the protein containing secondary structures What is a multi-subunit complex where each subunit is a distinct polypeptide chain? Quaternary structure Protein Synthesis and the Urea Cycle 3  Major proteins synthesised in the Liver Which plasma proteins are synthesised by the liver? Albumin CRP Hormone binding globulins Apolipoproteins Other transport proteins Caeruloplasmin Ferritin What is the primary plasma protein synthesised by the liver that maintains oncotic pressure? Albumin What pathways is the liver part of? Protein Synthesis and the Urea Cycle 4  Inhibitors of clotting Fibrinolysis Inhibitors of fibrinolysis Complement What role does the liver play in the complement cascade? It synthesises all factors of the complement cascade. Albumin What are the properties of albumin? 66 kDalton protein Negatively charged 10-15g produced by the liver per day What are the functions of albumin? Plasma oncotic pressure Carrier protein What compounds does albumin transport? Hormones Vitamins Electrolytes (Ca2+. Mg2+, etc) Drugs Hypoalbuminaemia What are the causes of low albumin levels? Inflammation Protein Synthesis and the Urea Cycle 5  Liver disease Renal disease Burns/trauma Sepsis Malnutrition What are the consequences of low albumin? Oedema Effusions Carrier protein - may need to adjust for this Albumin calculations What are the two categories of fluid analysed in albumin calculations? Exudates vs Transudates What is exudate? Exudate is fluid that leaks around the cells of the capillaries caused by inflammation. What is transudate? Transudate is fluid pushed through the capillary due to high pressure within the capillary. Is exudate or transudate higher in protein? Exudate What do we need to adjust for in albumin calculations? Electrolytes (especially Ca2+) and hormone levels (e.g. free testosterone) Protein Synthesis and the Urea Cycle 6  Which medical condition is associated with the need for albumin calculations? Renal disease Clotting Cascade Where are the clotting factors synthesised? Liver Which clotting factors are synthesised in the liver? All blood clotting factors, except: factor III (tissue thromboplastin) - Produced by damaged endothelial cells factor IV (calcium) - obtained through diet and bone resorption. factor VIII (von Willebrand factor), are synthesised in the liver - Synthesised by endothelial cells and megakaryocytes What coagulation factors are produced in the liver? Protein Synthesis and the Urea Cycle 7  Fibrinogen, Prothrombin, Factors V, VII, IX, X, XI, XII, XIII, Protein C, and Protein S. What is the name of the serine protease that inhibits the proteolytic activity of factor Xa (FXa) and factor VIIa (FVIIa)? Tissue Factor Pathway Inhibitor (TFPI) Vitamin K is needed for the synthesis of which clotting factors? X, IX, VII and II Vitamin K is essential to what enzyme in the liver? Gamma-glutamyl carboxylase enzyme in the liver What does the gamma-glutamyl carboxylase enzyme do? It adds a carboxyl group to glutamic acid residues on clotting factors factors II, VII, IX, and X Protein Synthesis and the Urea Cycle 8  Chronic Liver Disease and Bleeding How does liver disease affect the coagulation cascade? Reduced synthesis of clotting factors Reduced synthesis of inhibitors How does chronic liver disease lead to reduced synthesis of clotting factors? Hepatic dysfunction Vitamin K deficiency/malabsorption Why does Vitamin K deficiency occur in chronic liver disease? Malabsorption due to decreased bile production How does chronic liver disease affect protein production? Production of abnormal/dysfunctional proteins What are two reasons for enhanced fibrinolytic activity in chronic liver disease? Reduced production of fibrinolysis inhibitors. Reduced clearance of fibrinolysis activators. How does chronic liver disease affect clotting factor clearance? Reduced hepatic clearance of clotting factors. What is disseminated intravascular coagulation? blood clots form throughout the body, blocking small blood vessels How are platelets affected in chronic liver disease? Platelet abnormalities include reduced number and impaired function. Protein Synthesis and the Urea Cycle 9  What vascular complication develops due to chronic liver disease? Development of varices The Urea Cycle What happens to surplus amino acids in the fed state? They are metabolised into glucose, glycogen, fatty acids, or oxidised to generate ATP What happens during fasting or starvation to muscle proteins? Muscle proteins undergo catabolic wasting, yielding amino acids for gluconeogenesis to maintain blood glucose levels. Why is ammonia (NH₄⁺) a concern during amino acid metabolism? Ammonia is extremely toxic and must be detoxified. How is ammonia detoxified in the body? It is converted into urea for urinary excretion. What happens to ammonia that evades detoxification as urea? It is incorporated into glutamine by the enzyme glutamine synthetase. Protein Synthesis and the Urea Cycle 10  Glucose-Alanine Cycle What is the principal ammonia scavenger and transporter in muscle? Alanine How does ammonia from muscle get attached to alanine? Protein Synthesis and the Urea Cycle 11  1. Glutamate collects ammonia in muscle. 2. The enzyme alanine aminotransferase (ALT) transfers the amino group from glutamate to pyruvate (from glycolysis). 3. This forms alanine. What happens to alanine after it is formed in muscle? Alanine is transported in the blood to the liver. What occurs in the liver during the glucose-alanine cycle? 1. Alanine undergoes the reverse reaction to release ammonia. 2. Ammonia is converted to urea. 3. Pyruvate is recycled into glucose via gluconeogenesis. What enzyme is crucial for the glucose-alanine cycle? Alanine aminotransferase (ALT) Why is the glucose-alanine cycle efficient? It solves two problems: 1. Transports carbon atoms (pyruvate) and ammonia from muscle to liver. 2. Shifts the energetic burden of gluconeogenesis to the liver, allowing muscle ATP to be used for contraction. The Krebs’ Cycle Protein Synthesis and the Urea Cycle 12  What intermediate acts as a connecting link between the Krebs’ and urea cycle? Fumarate How is this intermediate the connecting link? Fumarate, produced in the urea cycle by argininosuccinate lyase, enters the citric acid cycle and is converted to oxaloacetate. How is aspartate formed to feed into the urea cycle? 1. Oxaloacetate accepts an amino group from glutamate via transamination. 2. This forms aspartate, which donates its amino group to the urea cycle. The Urea Cycle Protein Synthesis and the Urea Cycle 13  What enzyme initiates the urea cycle, and what does it require? Carbamoyl phosphate synthase 1 (CPS1), requiring N-acetylglutamate (NAG) as an allosteric effector. What is the role of ornithine transcarbamylase (OTC) in the urea cycle? OTC catalyses the reaction between carbamoyl phosphate and ornithine, forming citrulline. Which enzyme catalyses the formation of argininosuccinate? Argininosuccinate synthase Protein Synthesis and the Urea Cycle 14  What reaction is catalysed by argininosuccinate lyase? Cleaves argininosuccinate into fumarate and arginine. What is the function of arginase? Hydrolyses arginine into urea and regenerates ornithine. What happens when the urea cycle is blocked after CPS1? Carbamoyl phosphate (CP) accumulates, while ammonia cannot be detoxified into urea, leading to hyperammonemia (high blood ammonia levels). What are the two routes for amino acids to enter the urea cycle? 1. Transdeamination route (cytosolic transamination followed by mitochondrial deamination). Protein Synthesis and the Urea Cycle 15  2. Transamination route (two transamination reactions forming glutamate and aspartate). How does the transdeamination route contribute to the urea cycle? 1. Alpha-ketoglutarate accepts an amino group, forming glutamate. 2. Glutamate is transported into the mitochondrion and deaminated by glutamate dehydrogenase to release ammonium. 3. Ammonium is incorporated into carbamoyl phosphate. How does the transamination route contribute to the urea cycle? 1. Glutamate transfers an amino group to oxaloacetate, forming aspartate. 2. Aspartate enters the urea cycle and condenses with citrulline to form argininosuccinate. What are the effects of valproate on the urea cycle? Valproate inhibits: 1. Conversion of carnitine to glutamate. 2. NAGS (N-acetylglutamate synthetase). 3. CPS1 (Carbamoyl phosphate synthase 1). How do organic acidemias (e.g., PA, MMA, IA) cause hyperammonemia? They compete with acetyl-CoA, inhibiting NAGS and disrupting the TCA cycle. They are distinguished by acidosis, ketosis, and lactic acidaemia. How do fatty acid oxidation disorders (FAOD) lead to hyperammonemia? Decreased carnitine limits glutamate production. Reduced acetyl-CoA inhibits CPS1 activity. Protein Synthesis and the Urea Cycle 16  Ammonia Toxicity Clinical Signs of Severe Ammonia Toxicity What reflex can be tested to assess ammonia toxicity? Doll’s eye reflex (oculocephalic reflex), where the eyes stay fixed in position when the head is turned. What does decerebrate posturing indicate in severe ammonia toxicity? It indicates brainstem dysfunction, often due to toxic effects of ammonia on the central nervous system. What are the characteristics of decerebrate posturing? Rigid extension of arms and legs, plantar flexion of feet, and backward arching of the head. Why does ammonia cause neurological symptoms like doll’s eyes and posturing? Ammonia disrupts brain metabolism, leading to cerebral edema and impaired brainstem function. Protein Synthesis and the Urea Cycle 17  Ammonia leads to neurotoxicity Protein Synthesis and the Urea Cycle 18  Why does elevated ammonia occur in severe liver disease? The blood is not adequately exposed to liver cells for ammonia removal due to: 1. Loss of liver parenchymal cells (viral/toxic damage). 2. Blood bypassing the liver (e.g., in cirrhosis, due to high resistance from fibrosis). How does ammonia cross into the brain, and why is this toxic? Ammonia crosses the blood-brain barrier easily. Once inside, it is converted to glutamate via glutamate dehydrogenase, depleting α- ketoglutarate. What happens when α-ketoglutarate is depleted in the brain? 1. Oxaloacetate decreases. 2. The citric acid cycle stops. 3. This leads to irreparable cell damage and neural cell death. Protein Synthesis and the Urea Cycle 19  What is the glutamine hypothesis of ammonia neurotoxicity? Hyperammonaemia activates glutamine synthase in astrocytes, causing: 1. Accumulation of glutamine. 2. Astrocytic swelling (glutamine acts as an osmolyte). 3. Cerebral edema and impaired astrocyte function. What are two major consequences of ammonia toxicity in the brain? Cerebral edema due to astrocyte swelling. Neural cell death due to impaired citric acid cycle activity. Treatment What is a key strategy to prevent ammonia buildup in urea cycle disorders? Avoid catabolism by providing glucose polymers when unwell. How is anabolism induced in urea cycle treatment? Give dextrose 10% at 2 ml/kg/hr, which stimulates insulin release. What dietary modifications are made in urea cycle disorders? 1. Low protein diet. 2. Supplementation with arginine. 3. Use of benzoate and phenylbutyrate. What are two advanced treatment options for severe urea cycle disorders? Haemofiltration. Liver transplantation or umbilical vein hepatocyte transfusion. Protein Synthesis and the Urea Cycle 20  Oral Emergency Regimen When should an oral emergency regimen (ER) for urea cycle defects be started? At the first sign of illness, such as nausea, loss of appetite, vomiting, or fever. What is the first step in Stage 1 of the emergency regimen? Take 200 ml of glucose polymer drink. What happens in Stage 2 of the emergency regimen? If unwell but able to tolerate oral intake: 1. Commence full emergency regimen. 2. Stop normal diet. 3. Continue normal medications. What should be done in Stage 3 of the emergency regimen? If unable to take drinks or medications: Seek immediate medical attention. Hospital admission may be required for IV glucose (e.g., 10% glucose at 2 ml/kg/hr). How often should emergency drinks be taken during the oral emergency regimen? Every 2 hours, day and night. Protein Synthesis and the Urea Cycle 21 Protein Synthesis and the Urea Cycle 22