Pyrimidines as cofactors for phenylalanine hydroxylase

It has been generally assumed that a tetrahydropterin (2-amino-5,6,7,8-tetrahydro-4-pteridinone) is essential for activity of the three aromatic amino acid hydroxylases. In this report it is shown that appropriately substituted pyrimidines can assume the role of cofactor for phenylalanine hydroxylase. 2,5,6-Triamino-4-pyrimidinone(V) and 5-benzylamino-2,6-diamino-4-pyrimidinone(VI) possess the same K_m values (0.1 mM and 0.003 mM) and stoichiometry of tyrosine generated to cofactor consumed (0.4 and 1.0) as their corresponding pteridine analogs, tetrahydropterin(III) and 6-phenyltetrahydropterin(IV). However, the rates with pyrimidines are lower. The ratio of rates $\text{V}\text{III}\text{= 0.045}$ and $\text{VI}\text{IV}\text{= 0.015}$. These results indicate that pteridine carbons 6 and 7 are not fundamental to cofactor binding or function, though they markedly influence the maximum velocity of hydroxylation. Pyrimidine cofactors of phenylalanine hydroxylase are valuable probes for the elucidation of the binding forces, transition states, and mechanism of oxygen activation of these hydroxylases.