Recombinant Haemophilus influenzae L-serine dehydratase (sdaA)

Enzyme Overview

L-serine dehydratase (SdaA) catalyzes the deamination of L-serine to pyruvate and ammonia, a critical step in amino acid catabolism and gluconeogenesis. In Haemophilus influenzae, serine utilization is linked to biofilm formation and nutrient adaptation, though its SdaA-specific mechanisms remain less characterized compared to homologs in Campylobacter jejuni or Pseudomonas aeruginosa .

Key Features

• Catalytic Mechanism: Pyridoxal-5′-phosphate (PLP)-independent, oxygen-labile iron-sulfur cluster (observed in C. jejuni SdaA) .

• Substrate Specificity: Highly specific for L-serine, with minimal activity on threonine (contrasts with E. coli PLP-dependent dehydratases) .

Comparative Kinetics

Role in Metabolism and Pathogenesis

• Biofilm Adaptation: In H. influenzae, serine depletion in biofilms correlates with upregulated SdaC (serine transporter) and metabolic reprioritization toward aspartate and serine-derived amino acids (e.g., glycine, tryptophan) .

• Colonization: C. jejuni SdaA is essential for avian gut colonization, suggesting potential parallels in H. influenzae mucosal adherence .

Key Studies

1. Serine Utilization in Biofilms

   • H. influenzae biofilms show increased serine uptake via SdaC but lack direct evidence for SdaA activity. Serine is converted to pyruvate, feeding central metabolism under nutrient stress .

   • Table: Metabolite Changes in H. influenzae Biofilms

     Metabolite	Biofilm vs. Planktonic	Proposed Role
     L-serine	Depleted	Precursor for pyruvate
     Aspartate	Depleted	Oxaloacetate synthesis
     Glycine	Increased	Derived from serine catabolism

2. Homologous Systems

   • C. jejuni SdaA mutants fail to colonize chickens, underscoring serine catabolism’s role in virulence .

   • P. aeruginosa SdaA/SdaB contribute 34% and 66% of total L-serine deamination, respectively, with glycine enhancing activity .

Recombinant Expression and Applications

• Cloning and Purification: C. jejuni SdaA has been overexpressed in E. coli, yielding oxygen-sensitive, iron-sulfur-containing enzyme . Similar approaches could apply to H. influenzae SdaA.

• Biotechnological Potential:

  • Diagnostics: Targeting SdaA could aid in detecting H. influenzae metabolic adaptation in chronic infections.

  • Therapeutic Targets: Inhibitors of serine catabolism (e.g., L-leucine analogs) might disrupt biofilm persistence .

Knowledge Gaps and Future Directions

• Unresolved Questions:

  • Does H. influenzae SdaA exist, or is serine catabolism mediated by alternative enzymes (e.g., threonine dehydratases)?

  • How does SdaA interact with SdaC in serine import and utilization?