Protein metabolism
Structures
Primary structure - amino acid sequence
Secondary structure - protein twisting and folding
* e.g. alpha-helix, beta-sheet
Tertiary structure - arrangement of chains and sheets into layers, crystals, fibres
Quaternary structure - Arrangement of subunits
Tidbits
- Amino acid turnover: 80-100g/day
- Oxidation of cysteine is the main source of most of the sulfates in urine.
- Creatinuria (creatine in urine) occurs normally in children and pregnant women. Occasionally in non-pregnant women. Associated with extensive muscle breakdown.
- NH4+ (from deamination) is mostly converted to urea in liver.
- Pyrimidines are catabolized to CO2 and NH3.
- Purines (double ring) are converted to uric acid.
Amino acids
Essential amino acids
- Valine, Leucine, Isoleucine, threonine, Methionine, Phenylalanine, Arginine, Lysine, Histidine
- When essential amino acids are missing, some proteins are not synthesized. The other amino acids which would have gone into the protein are deaminated and metabolised.
-> negative nitrogen balance
Ketogenic amino acids
- carbon skeleton can be used to produced ketone bodies
- Includes: leucine, isoleucine, phenylalanine, tyrosine
Glucogenic amino acids
- carbohydrate skeleton can be used to produce glucose
- Includes: alanine and others
Uric acid
... is formed from
- breakdown of purine
- by direct synthesis from 5-phosphoribosyl pyrophophate (5-PRPP) and glutamine
98% of filtered uric acid is reabsorbed
Active secretion by tubules
About 1g/24h is excreted
* 80% of is by active secretion
Gout
... is due to increased blood uric acid level causing urate deposit in joints and other tissues.
Primary gout
- enzyme abnormality causing excess production or selective deficit in renal secretion.
Secondary gout
- abnormality arising from other conditions.
- e.g. renal disease, thiazide diuretic (impaired excretion)
- e.g. leukemia and pneumonia (due to increased breakdown of uric acid-rich WBCs)
Colchicine - doesn't affect uric acid metabolism but relieves gout attacks by inhibiting the phagocytosis of uric acid crystals by leukocytes
Allopurinol - inhibits xanthine oxidase (which otherwise converts xanthine into uric acid).