gcd (PP_1444) - Pseudomonas putida KT2440 - Research Notes

Gene Identity

• UniProt: Q88MX4 (TrEMBL, unreviewed)
• Gene: gcd (PP_1444)
• Product: Quinoprotein glucose dehydrogenase (mGDH)
• EC: 1.1.5.2
• Organism: Pseudomonas putida KT2440

Protein Structure and Localization

• 803 amino acids, 86.6 kDa
• Membrane-bound with 5 predicted transmembrane helices (aa 12-142)
• Large periplasmic PQQ beta-propeller domain (aa 171-778) PMID:12534463
• Belongs to bacterial PQQ dehydrogenase family (PTHR32303:SF4 = Quinoprotein Glucose Dehydrogenase)
• Domain architecture: N-terminal membrane anchor + C-terminal catalytic PQQ domain facing periplasm
• CDD: cd10280 (PQQ_mGDH) confirms membrane-bound glucose dehydrogenase classification
• InterPro: IPR017511 (PQQ_mDH), IPR018391 (PQQ_b-propeller repeat), IPR002372 (PQQ repeat domain)

Core Enzymatic Function

• Catalyzes oxidation of D-glucose to D-glucono-1,5-lactone (spontaneously converts to gluconate) in the periplasm
• Uses PQQ (pyrroloquinoline quinone) as a tightly-bound redox cofactor (Km for PQQ <0.1 uM) PMID:27287323
• Electrons are transferred to ubiquinone in the membrane (connects to respiratory chain)
• Kinetics: Km for glucose = 4.91 mM, Vmax = 67.64 uM/min PMID:27287323

Role in Glucose Catabolism

P. putida uses three convergent peripheral pathways for glucose catabolism, all converging at 6-phosphogluconate PMID:17483213:
1. Glucokinase pathway: glucose -> G6P -> 6PG (cytoplasmic, via Glk + Zwf)
2. Direct gluconate pathway: glucose -> gluconate (via Gcd, periplasmic) -> transport -> 6PG (via GnuK)
3. 2-ketogluconate loop: glucose -> gluconate -> 2KG (via Gad, periplasmic) -> transport -> 6PG (via KguK + KguD)

Gcd initiates pathways 2 and 3. About 50% of glucose flux goes through the 2-ketogluconate loop PMID:20581202.

All pathways converge at 6-phosphogluconate which feeds into the Entner-Doudoroff pathway via Edd/Eda.

Regulation

• gcd expression is highest when glucose is the sole carbon source PMID:27287323
• Low phosphate conditions increase both GDH activity and PQQ levels PMID:27287323
• PQQ availability (not enzyme abundance) limits activity under optimal conditions PMID:27287323
• PtxS repressor controls compartmentalized glucose metabolism PMID:20581202
• GnuR represses gluconate/glucose catabolism genes

Biological Roles

• Phosphate solubilization: Gluconic acid produced by Gcd chelates mineral-bound phosphates (Ca3(PO4)2, hydroxyapatite), making P available for plant uptake PMID:27287323
• Rhizosphere competence: Key for P. putida's role as rhizosphere bacterium
• gcd mutant phenotype: Loss of periplasmic glucose oxidation, no gluconate/2-KG accumulation, significant growth defect on glucose
• Pyoverdine regulation: gcd mutants show increased pyoverdine production (siderophore) due to loss of gluconic acid-mediated pH drop PMID:23392768

Biotechnology Applications

• gcd deletion used to redirect glucose flux (avoid gluconate accumulation) for metabolic engineering
• Overexpression of Gcd improves bioelectrochemical system performance PMID:28921555
• gcd deletion used for muconic acid production engineering

Additional Findings from Deep Research (Falcon)

• Substrate breadth: Gcd can oxidize mannose (from fructose isomerization) in addition to glucose, indicating broad aldose C-1 oxidation activity [Nguyen et al. 2021, DOI:10.1111/1751-7915.13862]
• Flux dominance: Under aerobic growth, ~90% of glucose entering the periplasm is oxidized to gluconate by Gcd [Chen et al. 2024, DOI:10.1111/1751-7915.70059]
• GnuR regulation: GnuR (PP_2321) is a newly characterized repressor of peripheral glucose/gluconate catabolism genes; Δgcd mutant used to distinguish glucose vs gluconate transcriptional responses [Chen et al. 2024]
• BES applications: Overexpression of gcd increased electron transfer rate by >4x and 2-KG production by 644% in bioelectrochemical systems [Yu et al. 2018, PMID:28921555]
• Systems engineering: Weimer et al. 2024 achieved 96% glucose-to-2-KG conversion (mol/mol) using metabolic engineering approaches [DOI:10.1186/s12934-024-02509-8]
• gcd deletion phenotype: Complete loss of gluconate/2-KG formation confirmed by Chen et al. 2024; significant growth defect on glucose

Key References

• PMID:27287323 - An & Moe 2016 - Regulation of PQQ-dependent GDH in P. putida KT2440
• PMID:17483213 - del Castillo et al. 2007 - Convergent peripheral pathways for glucose catabolism
• PMID:20581202 - Compartmentalized glucose metabolism controlled by PtxS
• PMID:12534463 - Nelson et al. 2002 - P. putida KT2440 genome sequence
• PMID:23392768 - Ponraj et al. 2013 - Periplasmic glucose oxidation and pyoverdine synthesis
• PMID:28921555 - Yu et al. 2018 - Bioelectrochemical system with gcd overexpression