Evidence for two copper atoms/subunit in dopamine beta-monooxygenase catalysis

Area of Science:

• Biochemistry
• Enzymology
• Metalloprotein chemistry

Background:

• Dopamine beta-monooxygenase (DBM) catalyzes a critical step in neurotransmitter synthesis.
• The precise copper requirement for DBM activity remains incompletely understood.
• Previous studies were confounded by trace copper contamination.

Purpose of the Study:

• To determine the exact stoichiometry of copper required for DBM catalysis.
• To elucidate the role of copper ions in the enzyme's active site.
• To investigate the catalytic mechanism of dopamine to norepinephrine conversion.

Main Methods:

• Utilized rapid chemical-quench techniques with concentrated enzyme samples.
• Investigated enzyme kinetics under varying copper concentrations.
• Measured tritium isotope effects during dopamine hydroxylation.

Main Results:

• DBM activity was maximal at a stoichiometry of 2 mol of copper per mol of enzyme subunit.
• Low turnover numbers were observed when enzyme concentration exceeded copper levels.
• Exogenous Cu(II) addition fully restored enzyme activity.
• Other metal ions (Ni(II), Co(II), Mn(II), Fe(III), Zn(II)) did not stimulate activity.
• Tritium isotope effect remained invariant, irrespective of copper-to-enzyme ratio.

Conclusions:

• Both copper atoms bound per DBM subunit are essential for catalysis.
• The results implicate both copper atoms in the active site's redox chemistry.
• An effector role for the second copper is ruled out, supporting direct catalytic involvement.