# Heme-Dependent Chemistry in Tyrosine Oxidation

> **NIH NIH R01** · UNIVERSITY OF TEXAS SAN ANTONIO · 2021 · $61,120

## Abstract

PROJECT DESCRIPTION
A distinguishing trait of heme enzymes is that a high-valent iron-oxo species is a common oxidant for mediating a
remarkable array of oxidation reactions. However, one conundrum is that each enzyme in general promotes only a
specific type of reaction. How the reaction type is determined after the formation of the key oxidant remains an open
question whose answers have implications for our fundamental understanding of enzyme catalysis as well as de novo
enzyme design and protein engineering. Because tyrosine is an important building block of natural products, this
application focuses on a mechanistic characterization of three heme-dependent tyrosine-oxidizing enzymes. Each of
these enzymes employs a mononuclear heme cofactor to oxidize its tyrosine-based substrate. Intriguingly, a
cytochrome P450 protein, CYP121 from Mycobacterium tuberculosis, catalyzes an unusual oxidative carbon-carbon
cross-coupling reaction instead a more common hydroxylation reaction. We found that SfmD is a new member of the
tryptophan dioxygenase superfamily that promotes regioselective monooxygenation of a methylated tyrosine substrate.
The peroxidase LmbB2 performs a peroxygenase-type of reaction with an axial ligand of histidine instead of cysteine.
These enzymes not only catalyze tyrosine-based oxidation reactions but they are also related to antimicrobial drug
development. Given the similarities of the heme-based oxidant and the structure of the substrates, the inevitable
question arises regarding the governing factors that determine the catalytic activity of these enzymes. In Aim #1, we
will determine the mechanistic and structural characteristics of CYP121. Using a battery of spectroscopic and structural
approaches coupled with synthetic probes, we will unveil a novel carbon-carbon coupling mechanism mediated by the
P450 enzyme. In Aim #2, we will characterize the structure and mechanism of SfmD with emphasis on how the
substrate is positioned to the iron-bound oxidant and the capture of catalytic intermediates. We have already identified
that this protein is a novel heme-based oxygenase. Aim #3 is focused on studying the peroxidase reaction catalyzed
by LmbB2 that is responsible for L-3,4-dihydroxyphenylalanine (L-DOPA) formation through L-tyrosine
hydroxylation. We will utilize small-molecule probes to interrogate mechanistic hypotheses. The in-depth analysis of
these three related catalytic systems will test our hypothesis regarding how the heme-bound oxidant is generated and
directed to the aromatic substrates, unravel the structure-function relationships of the heme enzymes of seemingly
unrelated superfamilies at a higher level, and develop underlying mechanisms further aiding rational drug design and
discovery processes.

## Key facts

- **NIH application ID:** 10475429
- **Project number:** 3R01GM108988-09S1
- **Recipient organization:** UNIVERSITY OF TEXAS SAN ANTONIO
- **Principal Investigator:** Aimin Liu
- **Activity code:** R01 (R01, R21, SBIR, etc.)
- **Funding institute:** NIH
- **Fiscal year:** 2021
- **Award amount:** $61,120
- **Award type:** 3
- **Project period:** 2014-08-01 → 2022-08-31

## Primary source

NIH RePORTER: https://reporter.nih.gov/project-details/10475429

## Citation

> US National Institutes of Health, RePORTER application 10475429, Heme-Dependent Chemistry in Tyrosine Oxidation (3R01GM108988-09S1). Retrieved via AI Analytics 2026-05-23 from https://api.ai-analytics.org/grant/nih/10475429. Licensed CC0.

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