InhA — Enoyl-ACP Reductase
The primary target of isoniazid — the most widely used first-line anti-tuberculosis drug. InhA is a validated, extensively studied druggable target with one of the highest druggability scores in the ResistAI database.
Protein overview
Clinical significance
Tuberculosis kills ~1.6 million people annually. Isoniazid resistance — predominantly driven by mutations in inhA and katG — is the most common mechanism of first-line drug resistance in M. tuberculosis. InhA mutations (e.g. C15T promoter variant, I21V, I47T) confer low-level isoniazid resistance by reducing enzyme-drug binding affinity. Direct InhA inhibitors — compounds that bypass the need for KatG-mediated activation — represent a major strategy in the current TB drug discovery pipeline.
Druggability analysis — fpocket
Key finding
InhA scores 0.983 — placing it in the top 1% of all 2,433 analysed resistance proteins. This is consistent with decades of experimental drug discovery: InhA harbours a well-defined, hydrophobic NADH/NAD⁺ binding pocket that has been successfully targeted by both prodrugs (isoniazid) and direct inhibitors (GSK693, triclosan analogues, thiadiazoles).
Interpretation
fpocket identified 14 surface cavities on the AlphaFold-predicted InhA structure. The top-ranked pocket (score 0.983) corresponds to the substrate-binding channel adjacent to the NAD⁺ cofactor — the site occupied by isoniazid-NAD adduct in crystal structures. The high volume, hydrophobicity, and geometric regularity of this cavity explain the exceptional druggability score.
Note: Druggability scores are structural proxies computed by fpocket on AlphaFold-predicted structures (Le Guilloux et al. 2009). Thresholds: high ≥ 0.7, medium ≥ 0.4. Experimental validation is required to confirm binding site tractability.
ESM-2 embedding similarity
480-dim cosine similarityESM-2 similarity search against all 2,433 proteins reveals InhA's evolutionary context — and, critically, identifies fabI (E. coli enoyl-ACP reductase) as a close homolog (similarity 0.98). This is biologically validated: InhA and FabI are both members of the short-chain dehydrogenase/reductase (SDR) superfamily and share the same catalytic mechanism. Cross-species druggability comparison enables prioritisation of inhibitor scaffolds with broad-spectrum potential.
Mycobacterium bovis (BCG)
Mycobacterium tuberculosis (CDC 1551)
Mycolicibacterium smegmatis
Escherichia coli O157:H7
Shigella flexneri
Key finding: ESM-2 correctly identifies FabI (E. coli) as a homolog of InhA despite no explicit structural alignment — demonstrating that sequence-level embeddings capture functionally relevant evolutionary relationships. FabI inhibitors such as triclosan have been explored as InhA inhibitor scaffolds, validating this similarity.
Key resistance literature
Inhibition of the InhA enzyme of Mycobacterium tuberculosis by isoniazid
Science · 1994
Crystal structure of Mycobacterium tuberculosis InhA in complex with the active metabolite of isoniazid
J Biol Chem · 2004
Direct InhA inhibitors — a new approach to tuberculosis drug discovery
Drug Discov Today · 2017
GSK693 — a novel direct InhA inhibitor with potent activity against M. tuberculosis
Antimicrob Agents Chemother · 2015
Pipeline summary
UniProt ID P9WGR1 resolved — InhA sequence from M. tuberculosis H37Rv fetched via UniProt REST API
3D structure retrieved from AlphaFold DB v4 — high-confidence predicted model
fpocket detected 14 binding cavities — top pocket scored 0.983 (high druggability, top 1% of all 2,433 proteins)
ESM-2 generated 480-dim embedding — ChromaDB cosine search identified FabI homologs (similarity 0.98) and InhA orthologues across mycobacteria
RAG retrieved literature on isoniazid resistance mechanisms and direct InhA inhibitor development
Llama 3.3 70B generated PMID-cited summary confirming InhA as a high-priority validated drug target