Cystine has how many molecules of cysteine?

Ans. is 'b' i.e., 2* Cysteine is a sulphur containing aminoacid* It is a polar but uncharged aminoacid* Cysteine with a sulphydryl group can get oxidised and forms a dimer (2) called as cystine.* Cysteine acquires its polar nature only by virtue of its sulfhydryl group.* When it gets oxidised to form cystine, the polar nature is reduced.* This is principally why cystinuria presents with recurrent renal stones.* Cystinuria is caused by a defect of a neutral aminoacid transporter, which reabsorbs four aminoacids from tubular fluid - Cysteine, Ornithine, Lysine and Arginine.* When these four aminoacids do not get reabsorbed in the renal tubules, they get excreted in the urine. On supersaturation, cysteine forms a dimer which is cystine. Cystine is insoluble in water and hence it precipitates to form renal stones. Hence Cystinuria presents as recurrent renal stones.