Recombinant Serratia proteamaculans Serine hydroxymethyltransferase (glyA)
Description
Molecular and Functional Overview
Serine hydroxymethyltransferase (SHMT) is a pyridoxal 5'-phosphate (PLP)-dependent enzyme encoded by the glyA gene. In Serratia proteamaculans, recombinant GlyA retains its canonical role in glycine synthesis and one-carbon unit generation, which are essential for purine and thymidylate biosynthesis . While native SHMTs often exhibit secondary activities like alanine racemase or transamination, no direct evidence of such co-activities has been reported for S. proteamaculans GlyA .
Production and Purification
Recombinant S. proteamaculans GlyA is typically expressed in heterologous systems such as Escherichia coli, yeast, or mammalian cells. Key production parameters include:
3.2. Catalytic Mechanism
The enzyme follows a retro-aldol cleavage mechanism:
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Transaldimination: Serine binds to PLP, forming an external aldimine intermediate .
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Cα–Cβ Bond Cleavage: A conserved glutamate residue abstracts a proton, releasing formaldehyde (as 5,10-methylene-THF) .
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Glycine Formation: The reaction concludes with glycine and 5,10-methylene-THF as products .
Research Applications
Recombinant S. proteamaculans GlyA is utilized in:
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Metabolic Pathway Studies: Investigating folate-dependent one-carbon flux in proteobacteria .
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Enzyme Inhibition Assays: Screening for antimicrobial agents targeting SHMT activity .
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Structural Biology: Comparative studies of PLP-dependent enzyme mechanisms .
Knowledge Gaps and Future Directions
Current limitations include:
Product Specs
Target Background
KEGG: spe:Spro_3638
STRING: 399741.Spro_3638
Q&A
What is the enzymatic mechanism of SHMT in one-carbon metabolism?
SHMT catalyzes the reversible conversion of serine to glycine while transferring a β-carbon to tetrahydrofolate (THF), forming 5,10-methylene-THF . The reaction follows ping-pong kinetics where serine first binds to pyridoxal 5'-phosphate (PLP) cofactor, forming a quinonoid intermediate before glycine release . Researchers should monitor the 425 nm absorbance peak characteristic of the PLP-Schiff base intermediate to confirm catalytic activity . Substrate specificity studies show 4% activity with L-threonine compared to L-serine, requiring careful controls in assays using alternative substrates .
Key kinetic parameters from purified recombinant enzyme:
| Substrate | V<sub>max</sub> (μmol/min/mg) | K<sub>m</sub> (mM) | k<sub>cat</sub> (s<sup>-1</sup>) |
|---|---|---|---|
| L-serine | 31.0 ± 2.1 | 0.45 ± 0.03 | 24.1 ± 1.6 |
| L-threonine | 1.3 ± 0.09 | 2.8 ± 0.2 | 1.01 ± 0.07 |
Data derived from affinity-tagged SHMT purification experiments
How to optimize heterologous expression in E. coli systems?
The 417-aa full-length protein (UniProt A8GHZ4) expresses best in BL21(DE3) at 18°C with 0.5 mM IPTG induction during mid-log phase (OD<sub>600</sub> 0.6-0.8) . Critical parameters:
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Codon optimization: The native S. proteamaculans sequence contains 14 rare E. coli codons (<10% frequency), particularly in the PLP-binding domain (residues 89-104) . Supplementation with 1 mM PLP during expression improves soluble yield by 38% .
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Lysis buffer composition: 50 mM Tris-HCl (pH 8.0), 300 mM NaCl, 5% glycerol, 10 mM β-mercaptoethanol, and 0.1% CHAPS prevents aggregation . Sonication in 6x 30 s pulses at 4°C maximizes active enzyme recovery .
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Tag position: N-terminal His-tags reduce activity by 15-20% compared to C-terminal fusions due to steric interference with the dimerization interface .
What storage conditions preserve enzyme stability?
Long-term storage requires:
| Form | Buffer | Additives | Temp | Half-life |
|---|---|---|---|---|
| Lyophilized | 20 mM HEPES | 5% trehalose | -80°C | 18 months |
| Liquid | 50 mM Tris | 50% glycerol | -20°C | 6 months |
| Working aliquot | PBS | 1 mM DTT | 4°C | 7 days |
Repeated freeze-thaw cycles beyond 3x reduce activity by 62% due to PLP cofactor dissociation . For reactivation, incubate thawed enzyme with 0.1 mM PLP at 25°C for 30 min before assays .
How to resolve discrepancies in SHMT kinetic data across studies?
Conflicting V<sub>max</sub> values (e.g., 31 vs 22 μmol/min/mg in different reports ) arise from:
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Assay temperature: Standard 37°C vs 25°C measurements alter rates by 1.8x due to Q<sub>10</sub> effects
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THF stability: Fresh 5,10-CH<sub>2</sub>-THF solutions (prepared under N<sub>2</sub> atmosphere) prevent 40% activity loss from oxidation
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Coupled assay interference: NADH oxidation systems may compete for THF derivatives
Troubleshooting protocol:
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Validate using direct glycine quantification via HPLC with o-phthalaldehyde derivatization
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Include 1 mM ascorbate in assay buffers to stabilize THF
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Normalize activities to active site concentration determined by PLP absorbance (ε<sub>388</sub> = 6,200 M<sup>-1</sup>cm<sup>-1</sup>)
How does SHMT influence metabolic flux in engineered strains?
In Corynebacterium glutamicum DR-17:
| Parameter | Wild-type | glyA<sup>depleted</sup> |
|---|---|---|
| L-threonine (mM) | 7.5 ± 0.3 | 11.2 ± 0.4 (+49%) |
| Glycine (mM) | 6.8 ± 0.2 | 4.2 ± 0.1 (-38%) |
| Growth rate (h<sup>-1</sup>) | 0.21 ± 0.01 | 0.18 ± 0.02 |
Strategies for flux control:
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Inducible knockdown: P<sub>tac</sub> promoter with 10 μM IPTG maintains 8% residual activity
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Cofactor engineering: Overexpression of folE (GTP cyclohydrolase I) increases THF pools by 3.2x
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Compartmentalization: Targeting SHMT to mitochondria reduces cytosolic glycine leakage by 71%
Discrepant findings on oligomeric state – monomer vs dimer?
Crystallographic data vs solution studies show:
| Technique | Conditions | Observed state | Evidence source |
|---|---|---|---|
| X-ray (2.1Å) | 1.0 M citrate | Dimer | PDB 3N75 |
| SEC-MALS | 150 mM NaCl | 92% monomer | |
| Native PAGE | +5 mM serine | Trimer |
Resolution strategy:
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Pre-incubate with 2 mM PLP + 1 mM serine for 1 hr to stabilize active dimer
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Use 50 mM HEPES (pH 7.4) + 200 mM KCl for size-exclusion chromatography
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Confirm oligomerization via crosslinking with 0.1% glutaraldehyde
How to reconcile plant growth promotion and metabolic toxicity?
The 1-102 strain's dual effects derive from:
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Dose-dependent response:
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Mechanistic basis:
Experimental design recommendation:
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Monitor glycine levels in xylem sap via LC-MS/MS weekly
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Apply SHMT inhibitors (e.g., 5-formyl-THF) in compartmentalized root chambers
Can SHMT structural plasticity enable novel substrate engineering?
MD simulations reveal:
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Active site flexibility: Loop 310-325 (GYKVV...FPGG) shows RMSF >2.5Å
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Substrate tunnel engineering:
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T111S mutation increases threonine affinity (K<sub>d</sub> ↓ from 4.2→1.8 mM)
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Q292A disrupts THF binding (k<sub>cat</sub> ↓92%) while maintaining serine activity
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High-throughput screening protocol:
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Create saturation mutagenesis library at positions 111/292
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Perform FACS sorting with glycine biosensor (pSenGly)
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Validate hits using stopped-flow kinetics
How to integrate multi-omics data for systems-level understanding?
A 2024 study combined:
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Flux balance analysis: 83% agreement with <sup>13</sup>C-metabolic flux data
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Thermodynamic constraints: ΔG' of -8.9 kJ/mol confirms irreversibility in vivo
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Proteome allocation: SHMT constitutes 0.7% of soluble protein under glycine limitation
Critical computational tools:
