1.13.12.15: 3,4-dihydroxyphenylalanine oxidative deaminase

aromatic amine + 1/2 O2

aromatic aldehyde + NH3 + H2O

oxidative deamination, unusual oxygen-consuming reaction catalyzed by the enzyme toward aromatic amines (serotonin, dopamine, and alpha-methyldopamine) and D-tryptophan methyl ester

production in equivalent amounts depending on the nature of the substrate, and ammonia with concomitant O2 consumption in a 1:2 molar ratio with respect to the products. A ketimine accumulates during the linear phase of product formation. This species is reactive since it is converted back to pyridoxal 5'-phosphate when the substrate is consumed. Superoxide anion and hydrogen peroxide are both generated during the catalytic cycles.

D-tryptophan methyl ester + 1/2 O2

methyl 3-(1H-indol-3yl)-2-oxopropanoate + NH3

production depending on the nature of the substrate, and ammonia with concomitant O2 consumption in a 1:2 molar ratio with respect to the products

glycine + O2

oxoacetic acid + NH3

L-alanine + O2

2-oxo-propanoic acid + NH3

L-DOPA + O2

L-glutamate + O2

2-oxopentanedioic acid + NH3

L-phenylalanine + H2O + O2

2-oxo-3-phenylpropanoic acid + NH3

L-tryptophan + O2

3-(1H-indol-3-yl)-2-oxopropanoic acid + NH3

Cereibacter sphaeroides

685904

50% of the activity with 3,4-dihydroxy-L-phenylalanine

L-tyrosine + O2

3-(4-hydroxyphenyl)-2-oxopropanoic acid + NH3

Cereibacter sphaeroides

685904

80% of the activity with 3,4-dihydroxy-L-phenylalanine

additional information

The novelty in DDC is the possibility of catalyzing a reaction involving dioxygen although the enzyme lacks of any cofactor or metal related to O2 chemistry. The external aldimine intermediate undergoes a decarboxylation or a deprotonation leading to a quinonoid species, that is protonated at C4 producing the ketimine intermediate. Although it cannot be ruled out that this intermediate could be attacked by dioxygen, it seems much more likely, regarding enzymes proceeding through a carbanion chemistry on DDC, that the more electron dense quinonoid intermediate, in equilibrium with the ketimine, is reactive toward O2. Aerobiosis shifts the quinonoid-ketimine equilibrium toward quinonoid, while anaerobiosis shifts the equilibrium toward ketimine. The reaction between dioxygen and the quinonoid give rise directly to a superoxide anion and semiquinone. Superoxide is deprotonated and its anionic form is thus able to couple with the semiquinone giving rise to a peroxide species that is further protonated, and thus forming a hydroperoxy-pyridoxal 5'-phosphate intermediate. This rearranges to produce aldehyde, ammonia and hydrogen peroxide.