Alkaptonuria and tyrosinosis are two monogenic recessive diseases in the human p

Question

Alkaptonuria and tyrosinosis are two monogenic recessive diseases in the human population. Both affect a different enzyme in the same phenylalanine pathway. Alkaptonuria results from an accumulation of the intermediate compound, homogentisic acid, whereas tyrosinosis is caused by elevated level of tyrosine.

In view of the position of HPA oxidase (red arrow) earlier in the pathway compared to that of HA oxidase (blue arrow), would you expect people with tyrosinosis to show symptoms of alkaptonuria?

b) If a double mutant could be found, would you expect tyrosinosis to be epistatic to alkaptonuria? Explain briefly

Copyright The McGraw-Hill Companies, Inc. Permission required for or display. Phenylalanine CH-CH COO NH+ 3 ?? 2-Thyroxine P-Hydroxyphenyl pyruvate Homogentisic acid Tyrosine 3 CH-CHCOO 2 NH 3 OH ?? CO2+ H2O 3,4-Dihydroxy- phenylalanine (DOPA) --Melanins 5 Enzyme defect conditions 1 Phenylketonuria (PKU) 2) Genetic goitrous cretinismm 3) Tyrosinosis CHCH-COO 2 NH 3 (4 Alkaptonuria ? Albinism

Explanation / Answer

Alkaptonuria is caused due to accumulation of homogentisic acid, as a result of deficiency of the enzyme homogentisate oxidase. Whereas, tyrosinosis is caused due to deficiency of the enzyme hydroxyphenylpyruvate dioxygenase(HA). HA converts p-hydroxyphenylpyruvate into homogentisic acid. Hence tyrosinosis is characterised by reduced levels of homogentisic acid.

Therefore, people with tyrosinosis will not show symptoms related to alkaptonuria (which is caused due to high levels of homogentisate).

Yes, tyrosinosis can be epistatic to alkaptonuria

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