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

1. breakdown of purine
2. 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).