protein breakdown and urea formation

Question 1

Nitrogen balance

Answer 1

• Proteins are structural or functional
• There are no specific protein stores
• Amino acids are supplied in the diet Excess protein is broken down and excreted

Question 2

Metabolism of amino acids

Answer 2

• Two parts: 
○ The carbon skeleton 
○ Nitrogen 
• Carbon skeleton 
○ Energy metabolism 
○ Biosynthetic pathways 
• Amino acids not stored, either used or broken down

Question 3

Removal of Nitrogen

Answer 3

• Nitrogen is toxic so has to be removed safely

* In mammals, nitrogen converted to the non-toxic neutral compound urea and excreted in the urine

Question 4

Transfer of amino acid nitrogen to urea (Step 1-transamination)

Answer 4

○ Enzyme involved is transaminase
○ Nitrogen group of amino acid A transferred to keto acid B to give to second amino acid B and Keto acid A
○ Keto acids are important metabolic intermediates
○ Alanine and Aspartate are transaminase:
§ Transfer an amino group from an amino acid to a α-keto acid
§ Alanine catalyses α-ketoglutarate to give pyruvate and glutamate
§ Aspartate catalyses α-ketoglutarate to give oxaloacetate and glutamate § High levels of alanine and aspartate in the blood are indicative of liver damage Whole reaction requires pyridoxal phosphate derived from vitamin B6

Question 5

Transfer of amino acid nitrogen to urea (Step 2-Oxidative deamination)

Answer 5

○ Glutamate can be converted to α-ketoglutarate by glutamate dehydrogenase
○ Fully reversible and can either use NAD or NADP
○ Ammonia is formed and is a substrate in the urea cycle ○ Elimination of free ammonia:
§ Glutamate gains nitrogen
§ ATP and glutamine synthase involved
§ Glutamine main transporter of nitrogen as it is readily soluble

Question 6

Transfer of amino acid nitrogen to urea (Step 3-Urea formation)

Answer 6

○ Means of excreting nitrogen
○ Enzymes are present in liver but not muscle
○ Takes place in mitochondria and cytoplasm ○ Substrates are bicarbonate, aspartate and ammonium ions(released from either glutamine or glutamate)
○ Formation of urea uses two amino groups One from glutamate and one from aspartate

Question 7

Interaction of urea cycle with the tricarboxylic acid cycle

Answer 7

• Ammonia first reacts to form carbamoyl phosphate
• Combines with ornithine to give citrulline
• Citrulline breaks down to give arginine which under the influence of arginase gives urea
• Fumarate converted to malate and then oxaloacetate
• Can be used to react with further amino acid to give a keto acid and asparate

Question 8

Muscle

Answer 8

• In prolonged exercise or starvation, branched amino acids are used for energy
• Enzyme of the urea cycle not present
• Two routes of transport used to transport nitrogen to the liver
○ Alanine
§ Nitrogen transferred to alanine via glutamate and pyruvate
○ Glutamine
§ Glutamate is made into glutamine

Question 9

Glucose alanine cycle

Answer 9

• Break down of protein
• Transamination to form alanine
• Alanine reacts to form glutamate and pyruvate
• Glutamate synthesises urea • Carbon skeleton that is formed is converted to pyruvate

Question 10

Fate of the carbon skeleton

Answer 10

• Will form α-keto acids. Some of the backbone of other amino acids can also feed into different components of the TCA cycle All amino acids can be inter-converted to α-ketoacids except two: lysine and threonine

Question 11

Protein and amino acid metabolism after a meal

Answer 11

metabolism after a meal
• Normal metabolism
○ High insulin, low glucagon ○ Most amino acids from a protein meal used for protein synthesis in peripheral tissues such as skeletal muscle
○ Excess amino acids used as sources of energy and nitrogen derived from their oxidation is incorporated into urea in the liver and excreted

Question 12

Protein and amino acid metabolism during starvation

Answer 12

• Normal individual:
○ Insulin is low, glucagon is high
○ Short term starvation:
§ Net flow of amino acids from muscle to liver
§ Increased production of glucose and urea
○ Long term starvation:
§ Tissue protein is spared because ketone bodies replace glucose as major energy fuel for the brain.

Question 13

Protein and amino acid metabolism in untreated diabetes

Answer 13

• Negative nitrogen balance due to decreased protein synthesis or increased protein breakdown
○ Seen in conditions of chronic infections, late stage cancer or trauma
○ Some of these effects are mediated by cytokines