Download MendeleySubstrate Analogs Implicate a Free Radical Pathway in Tyrosine Hydroxylase Catalysis.
Traore, E.S., Wang, Y., Griffith, W.P., Liu, A.(2025) ACS Catal 15: 18270-18281
- PubMed: 41737883 Search on PubMedSearch on PubMed Central
- DOI: https://doi.org/10.1021/acscatal.5c05776
- Primary Citation Related Structures:
8VMK - PubMed Abstract:
Heme-dependent tyrosine hydroxylases (TyrH) are critical enzymes in catecholamine biosynthesis, yet the role of the substrate's α -amino group in their monooxygenation mechanism has been unclear. Using 3-(4-hydroxyphenyl)propionic acid (HPPA), an l-tyrosine analog lacking the α -amino group, we observed a distinct dimerization pathway that competes with the expected hydroxylation reaction. Several lines of evidence confirm that this process originates from a radical intermediate. First, the formation of this (HPPA) 2 dimer is selectively inhibited by a free radical scavenger. Second, 18 O-labeling experiments show phenolic oxygen scrambling, indicating a disruption of substrate aromaticity during catalysis. Finally, EPR spectroscopy using nitrosobenzene as a substrate analog revealed a substrate-based free radical. This mechanistic divergence clarifies the role of the α -amino group. Its absence in HPPA creates a kinetic bottleneck for the final O atom transfer step, allowing a fraction of the substrate radical to form the off-pathway dimer. Thus, the native substrate's α -amino group acts as a crucial kinetic modulator, ensuring the rapid and efficient commitment of the substrate radical to productive hydroxylation. These results collectively establish a peroxidase-like free radical pathway for TyrH and reveal the nonessential yet significant role the amino group plays in controlling reaction outcomes.
- Department of Chemistry, University of Texas at San Antonio, San Antonio, Texas 78249, United States.
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