Functional Annotation Report: ilvC / Ketol-Acid Reductoisomerase (Q88DZ0) in Pseudomonas putida KT2440

Summary

The gene ilvC (ordered locus PP_4678; UniProt Q88DZ0) of Pseudomonas putida strain KT2440 (ATCC 47054 / DSM 6125 / NCIMB 11950) encodes ketol-acid reductoisomerase (KARI), also called acetohydroxy-acid isomeroreductase (AHIR/AHAIR), EC 1.1.1.86. This is the second enzyme of the branched-chain amino acid (BCAA) biosynthesis pathway, which produces L-valine, L-isoleucine, and L-leucine. The enzyme is a soluble, cytoplasmic, 338-residue class I ("type 1") KARI that uses two Mg²⁺ ions and NADPH to carry out, within a single active site, two chemically distinct reactions: an Mg²⁺-dependent alkyl (methyl or ethyl) migration/isomerization followed by an NADPH-dependent reduction. Gene identity was rigorously verified: the gene symbol, organism, EC number, protein family (ketol-acid reductoisomerase family), and the diagnostic InterPro domains (KARI_N IPR013116, KARI_C IPR000506, KARI_prok IPR014359) are all internally consistent, and the recovered literature describes exactly this enzyme class. There is no ambiguity — ilvC unambiguously denotes KARI.

Mechanistically, KARI converts (S)-2-acetolactate → (2R)-2,3-dihydroxy-3-isovalerate (the valine/leucine branch) and (S)-2-aceto-2-hydroxybutyrate → (2R,3R)-2,3-dihydroxy-3-methylvalerate (the isoleucine branch). It sits immediately downstream of acetohydroxyacid synthase (AHAS, encoded by ilvBN, which supplies its substrates) and immediately upstream of dihydroxyacid dehydratase (DHAD, ilvD). In P. putida KT2440, PP_4678 (ilvC) is genomically clustered with AHAS genes in a canonical ilvBN-ilvC-type arrangement, placing the enzyme physically and transcriptionally adjacent to its substrate-producing partner. The product ultimately feeds not only the three BCAAs but also 2-ketoisovalerate-derived pantothenate and coenzyme A biosynthesis.

The enzyme carries out its function in the cytoplasm, where the entire soluble BCAA biosynthetic pathway resides. Because this pathway is present in plants, fungi, and bacteria but absent in animals, KARI is a validated target for antimicrobials and herbicides, and — because of its NADPH dependence — a heavily engineered node in microbial isobutanol biofuel production. The findings below are supported by four converging lines of evidence: (1) authoritative enzymological and structural literature on KARI mechanism, (2) the curated UniProt/HAMAP annotation specific to Q88DZ0, (3) genomic-context analysis of the PP_4678 locus in KT2440, and (4) direct bioinformatic analysis of the Q88DZ0 sequence identifying its cofactor-binding and metal-coordinating residues.

Key Findings

F001 — ilvC encodes ketol-acid reductoisomerase, the second step of BCAA biosynthesis

The core functional assignment is definitive. Q88DZ0 is annotated as EC 1.1.1.86 under HAMAP-Rule MF_00435. KARI is a bifunctional enzyme that catalyzes two chemically distinct steps within a single active site: (1) an Mg²⁺-dependent alkyl migration (isomerization), in which a methyl or ethyl group migrates to convert (S)-2-acetolactate to 3-hydroxy-3-methyl-2-oxobutanoate, and (2) an NADPH-dependent reduction of the resulting keto group. The net conversions are:

• Valine/leucine branch: (S)-2-acetolactate → (2R)-2,3-dihydroxy-3-isovalerate
• Isoleucine branch: 2-aceto-2-hydroxybutyrate → (2R,3R)-2,3-dihydroxy-3-methylvalerate

The substrate specificity is strict for the acetohydroxy-acid intermediates of the ilv pathway. This is directly established by primary enzymology: KARI "catalyzes the conversion of 2-acetolactate into (2R)-2,3-dihydroxy-3-isovalerate or the conversion of 2-aceto-2-hydroxybutyrate into (2R,3R)-2,3-dihydroxy-3-methylvalerate. KARI catalyzes two reactions—alkyl migration and reduction—and requires Mg(2+) and NADPH for activity" (PMID: 19362563). Its ordinal position is equally clear: "Ketol-acid reductoisomerase (KARI) is the second enzyme in the branched-chain amino acid biosynthesis pathway" (PMID: 23036858).

F002 — KARI is an Mg²⁺/NADPH-dependent, two-domain enzyme using an induced-fit mechanism

Structurally, KARI is built from two domains: an N-terminal Rossmann-like NAD(P)H-binding domain (KARI_N, IPR013116) and a C-terminal knotted α-helical domain (KARI_C, IPR000506) that houses the two catalytic Mg²⁺ ions. The active site lies at the interface between the two domains. Crystallographic studies demonstrate that the binding of Mg²⁺ and NADPH triggers a domain-closure/reorganization (induced fit) that assembles the catalytically competent active site. As stated for the plant enzyme, "the binding of Mg(2+) and NADPH opens the interface between the N- and C-domains, thereby allowing access for the substrates to bind" (PMID: 23036858). For the class I enzymes to which the prokaryotic ilvC belongs, "the class I KARI structures indicate that the active sites close upon binding NAD(P)H" (PMID: 25849365), directly confirming the induced-fit closure mechanism. Isothermal titration calorimetry on the E. coli enzyme further shows Mg²⁺ binding increases conformational disorder while NADPH binding increases rigidity, consistent with the ordered assembly of the active site.

F003 — Q88DZ0 is a class I / "type 1" prokaryotic KARI

UniProt annotates Q88DZ0 explicitly as "Ketol-acid reductoisomerase type 1 / type I" and assigns the prokaryotic KARI signature (KARI_prok, IPR014359). KARIs partition into two classes: class I (short, single ~340-residue subunit, predominant in bacteria and fungi) and class II (arising from an internal duplication of the C-domain, ~490+ residues, found in plants and some bacteria such as E. coli). Q88DZ0's 338-residue length and prokaryotic signature place it firmly in class I. The literature notes that "most sequenced members of the industrially important ketol-acid reductoisomerase (KARI) family are class I enzymes, [while] structural studies to date have focused primarily on the class II KARIs, which arose through domain duplication" (PMID: 25849365). The class distinction is functionally relevant because it correlates with the length of the β2αB cofactor-specificity loop that governs NADPH-versus-NADH preference.

F004 — KARI operates in the cytoplasm; it is a validated antimicrobial target absent from animals and a key biofuel-engineering node

The BCAA biosynthesis pathway — and KARI with it — is "found in plants, fungi and bacteria but not in animals. This difference in metabolism between animals and microorganisms makes KARI an attractive target for the development of antimicrobial agents" (PMID: 23036858). The functional importance of KARI in vivo is demonstrated genetically across taxa: in Mycobacterium tuberculosis, "Mtb shows survival deficit in macrophages and in mice after ketol-acid reductoisomerase down-regulation" (PMID: 36354071); and in the fungal pathogen Fusarium graminearum, deletion of the KARI homologue FgIlv5 abolishes growth without exogenous isoleucine and valine and reduces virulence (PMID: 24493249). As a soluble cytosolic enzyme, KARI is also a central node in engineered isobutanol production. Because glycolysis yields NADH while wild-type KARI uses NADPH, cofactor-swapped variants were engineered: "an NADH-dependent pathway enables anaerobic isobutanol production at 100% theoretical yield" (PMID: 21515217), underscoring KARI's NADPH specificity as the natural default and its role as an engineerable bottleneck.

F005 — Protein-specific annotation: 338-residue two-domain enzyme, 2 Mg²⁺/subunit, step 2/4 of both Val and Ile routes

The curated UniProt Q88DZ0 record (HAMAP-Rule MF_00435) provides protein-specific detail. The sequence is 338 amino acids long, consistent with a compact single-subunit class I KARI. Its domain architecture comprises an N-terminal KARI Rossmann domain (~residues 1–181) containing the Rossmann NAD(P)-binding fingerprint (…GYGSQGHA… around residues 22–29) and a C-terminal KARI knotted domain (~182–327). An active-site residue is annotated at position 107, with multiple metal/substrate binding sites (residues 24–27, 47, 50, 52, 82–85, 133, 190, 194, 226, 230, 251). The enzyme binds two Mg²⁺ per subunit and uses NADPH. The curated Rhea reactions are:

• RHEA:22068 — (2R)-2,3-dihydroxy-3-methylbutanoate + NADP⁺ = (2S)-2-acetolactate + NADPH + H⁺
• RHEA:13493 — (2R,3R)-2,3-dihydroxy-3-methylpentanoate + NADP⁺ = (S)-2-ethyl-2-hydroxy-3-oxobutanoate + NADPH + H⁺

Pathway assignments are L-valine biosynthesis from pyruvate, step 2/4 and L-isoleucine biosynthesis from 2-oxobutanoate, step 2/4.

F006 — PP_4678 is genomically clustered with acetohydroxyacid synthase genes and feeds Val/Ile and pantothenate/CoA pathways

Genomic-context analysis of P. putida KT2440 (KEGG/GenBank) places PP_4678 (ilvC) at complement(5316083..5317099), immediately preceded on the same minus strand by PP_4679 (~163 aa, acetohydroxyacid synthase small/regulatory subunit, ilvN/ilvH-like; 57-bp gap to ilvC) and PP_4680 (~574 aa, acetohydroxyacid synthase large/catalytic subunit, ilvB/ilvI-like; ~2-bp gap to PP_4679). The transcription order on the minus strand is therefore AHAS-large → AHAS-small → ilvC, a canonical ilvB(I)-ilvN(H)-ilvC cluster. This physically co-locates KARI with the AHAS enzyme that produces its substrates. KEGG annotations assign KO K00053 (KARI, EC 1.1.1.86); modules M00019 (Val/Ile biosynthesis) and M00570 (Ile biosynthesis from threonine); and pathways ppu00290 (Val/Leu/Ile biosynthesis), ppu00770 (pantothenate & CoA biosynthesis), ppu01210 (2-oxocarboxylic acid metabolism), and ppu01230 (amino-acid biosynthesis). The pantothenate/CoA link reflects that the KARI-DHAD product 2-ketoisovalerate is a branch-point precursor for pantothenate.

F007 — Sequence analysis predicts NADPH specificity and the two-Mg²⁺ acidic ligand cluster

Direct bioinformatic analysis of the Q88DZ0 sequence corroborates the annotation. (1) A canonical Rossmann dinucleotide-binding fingerprint GXGXXG is present at residues 23–28 (G23-Y-G25-S-Q-G28) in the N-domain. (2) The cofactor-specificity region immediately downstream (β2αB loop) contains basic residues Arg47 and Lys48 (…GLRKGSAT…, residues 45–52) plus Ser50/Thr52; positively charged residues at these positions are the structural hallmark that coordinates the 2′-phosphate of NADPH, indicating NADPH (not NADH) preference — consistent with the enzyme's formal name "Ketol-acid reductoisomerase (NADP(+))." (3) The C-terminal domain carries a conserved cluster of acidic residues — Asp190, Glu194, Glu226, Glu230 (all UniProt-annotated binding sites) — forming two Asp/Glu pairs that coordinate the two catalytic Mg²⁺ ions. (4) An annotated active-site residue His107 lies at the N/C-domain interface. This is grounded in the finding that specificity-loop residues determine cofactor preference in class I KARIs: "insertions in the specificity loops that confounded previous attempts to classify them according to loop length" (PMID: 25849365).

Mechanistic Model / Interpretation

Position in the branched-chain amino acid pathway

KARI (ilvC) is the second of four common enzymatic steps that build all three branched-chain amino acids from pyruvate (and, for isoleucine, 2-oxobutanoate). The pathway architecture in P. putida KT2440 is:

                       pyruvate + pyruvate  (Val/Leu branch)
                       pyruvate + 2-oxobutanoate  (Ile branch)
                                 |
                                 |  [ Step 1 ]  AHAS  (ilvBN / PP_4680-PP_4679)
                                 v
        (S)-2-acetolactate        OR   (S)-2-aceto-2-hydroxybutyrate
                                 |
                                 |  [ Step 2 ]  KARI  (ilvC / PP_4678)  <-- THIS GENE
                                 |     Mg2+-dependent alkyl migration
                                 |     + NADPH-dependent reduction
                                 v
   (2R)-2,3-dihydroxy-isovalerate  OR  (2R,3R)-2,3-dihydroxy-3-methylvalerate
                                 |
                                 |  [ Step 3 ]  DHAD  (ilvD)  -- dehydration
                                 v
        2-ketoisovalerate       OR       2-keto-3-methylvalerate
                     |                              |
                     |  [ Step 4 ] transaminase     | [ Step 4 ] transaminase
                     v                              v
                  L-VALINE                     L-ISOLEUCINE
                     |
                     +--> (via ilvD product 2-ketoisovalerate)
                          --> LEUCINE biosynthesis (leuABCD)
                          --> PANTOTHENATE / CoA biosynthesis (panBCD)

KARI performs the pivotal chemistry that converts the acetohydroxy-acid intermediates into dihydroxy-acids. Its bifunctionality is remarkable: a single active site accommodates two mechanistically unrelated reactions. The isomerization step is an alkyl migration (methyl for the valine branch, ethyl for the isoleucine branch), positioning the substrate for the second reaction; this step requires the two active-site Mg²⁺ ions coordinated by the Asp190/Glu194 and Glu226/Glu230 pairs. The reduction step then uses hydride from NADPH bound in the N-terminal Rossmann domain.

Structure–function logic

Localization and pathway integration

KARI is a soluble cytoplasmic enzyme — the entire BCAA biosynthetic pathway operates in the cytosol, with no signal peptide or membrane-targeting features. Its genomic clustering with AHAS (ilvBN) in KT2440 supports co-regulation and metabolic channeling of substrates directly from AHAS to KARI. Downstream, the shared intermediate 2-ketoisovalerate is a branch point not only for valine/leucine but also for pantothenate and coenzyme A biosynthesis, explaining KARI's assignment to pathway ppu00770 in addition to the BCAA pathways.

Note on catabolism versus biosynthesis

Much of the P. putida-specific historical literature (PMIDs 5030618, 4150713, 4150714, 4352175, 10217783, 19910413) concerns branched-chain amino acid catabolism (the bkd operon, branched-chain keto acid dehydrogenase, transaminases). These pathways degrade BCAAs and are distinct from the biosynthetic role of ilvC. KARI is unambiguously a biosynthetic enzyme; the catabolic literature is contextually relevant to P. putida BCAA metabolism but does not describe ilvC function directly.

Evidence Base

Supporting P. putida BCAA-catabolism literature (contextual, not describing ilvC directly): 5030618, 4150713, 4150714, 4352175, 10217783, 19910413.

Convergence of evidence

The functional assignment rests on four independent, converging evidence lines: 1. Enzymology/structure — primary literature defines KARI's reaction, cofactors, and mechanism. 2. Curated annotation — UniProt/HAMAP-Rule MF_00435 assigns EC 1.1.1.86, class I identity, sequence length, and catalytic residues specifically to Q88DZ0. 3. Genomic context — PP_4678 sits in a canonical ilvBN-ilvC cluster in KT2440. 4. Sequence analysis — the Rossmann fingerprint, NADPH-specificity loop residues, and two-Mg²⁺ acidic ligand cluster are all identifiable in the Q88DZ0 primary sequence.

Limitations and Knowledge Gaps

1. No direct experimental characterization of the P. putida KT2440 enzyme. All mechanistic and structural detail is inferred from orthologues (plant, E. coli, M. tuberculosis, R. eutropha, F. graminearum) and from bioinformatic/curated annotation. There is no published crystal structure, kinetic study, or knockout phenotype specifically for Q88DZ0/PP_4678.

2. Cofactor specificity is predicted, not measured. NADPH preference for Q88DZ0 is inferred from the β2αB loop basic residues (Arg47/Lys48) and the enzyme name. Direct kinetic determination of NADPH vs NADH preference for this specific enzyme has not been done.

3. Substrate-branch kinetics unknown. The relative activity toward the valine-branch (2-acetolactate) versus isoleucine-branch (2-aceto-2-hydroxybutyrate) substrate has not been measured for this enzyme; branch partitioning is assumed to follow the general KARI paradigm.

4. Residue numbering caveats. Active-site (His107) and metal-ligand residue positions are drawn from UniProt annotation and sequence analysis; they have not been validated by mutagenesis in this organism.

5. Regulation not experimentally defined. The genomic clustering with ilvBN implies co-regulation, but promoter mapping, transcriptional regulation, and feedback inhibition of KARI in KT2440 specifically remain uncharacterized (though feedback inhibition of KARI/AHAIR by BCAAs is documented in related bacteria such as R. eutropha and C. glutamicum).

Proposed Follow-up Experiments / Actions

1. Recombinant expression and steady-state kinetics. Express Q88DZ0 (His-tagged) in E. coli, purify, and determine k_cat/K_M for both branch substrates with NADPH and NADH; confirm the predicted strong NADPH preference and quantify the NADH/NADPH selectivity ratio.

2. Metal and cofactor dependence. Measure Mg²⁺ (and Mn²⁺ substitution) dependence and stoichiometry by ITC; verify the two-metal requirement and the roles of Asp190/Glu194/Glu226/Glu230 via alanine mutagenesis.

3. Structural determination. Solve the crystal structure (or generate an AlphaFold model followed by validation) of Q88DZ0 with bound Mg²⁺/NADPH to confirm the class I two-domain fold, the interface active site, and induced-fit closure.

4. Genetic knockout in KT2440. Delete PP_4678 and test for BCAA auxotrophy (growth rescue by valine + isoleucine), confirming the essential biosynthetic role in situ.

5. Operon/transcription mapping. Define the PP_4680–PP_4679–PP_4678 transcript(s) by RT-PCR/RNA-seq and identify the promoter and any BCAA-responsive regulation, testing co-regulation with AHAS.

6. Metabolic-engineering assessment. Evaluate Q88DZ0 (wild-type and NADH-swapped variants) as a KARI module for isobutanol / 2-ketoisovalerate-derived product biosynthesis in P. putida, exploiting the organism's robust solvent tolerance.

Conclusion

ilvC (Q88DZ0, PP_4678) of Pseudomonas putida KT2440 encodes ketol-acid reductoisomerase (KARI/AHAIR, EC 1.1.1.86), a soluble cytoplasmic, 338-residue class I enzyme that catalyzes the second step of branched-chain amino acid biosynthesis. Using two Mg²⁺ ions and NADPH in a single active site, it performs an alkyl-migration isomerization followed by reduction, converting (S)-2-acetolactate to (2R)-2,3-dihydroxyisovalerate (valine/leucine branch) and (S)-2-aceto-2-hydroxybutyrate to (2R,3R)-2,3-dihydroxy-3-methylvalerate (isoleucine branch). It acts downstream of acetohydroxyacid synthase (ilvBN, encoded immediately upstream) and upstream of dihydroxyacid dehydratase (ilvD), also feeding pantothenate/CoA biosynthesis. The assignment is supported by convergent enzymological, curated-annotation, genomic-context, and sequence-analysis evidence, though no experiment has yet been performed on the KT2440 enzyme itself.