The rate-limiting step for norepinephrine synthesis: (A) Conversion of phenylalanine to Tyrosine, (B) Conversion of Tyrosine to DOPA, (C) Conversion of DOPA to dopamine, (D) Conversion of dopamine to norepinephrine
Answer is B. In the synthesis of Norepinephrine, Tyrosine is hydroxylated to 3,4dihydroxyphenylalanine by tyrosine hydroxylase. This enzyme catalyses the rate limiting reaction and require tetrahydrobiopterin as coenzyme.Ref: Biochemistry, U. Satyanarayana, Fouh edition, page no: 349