Characterization of the N-acetylneuraminic acid synthase (NeuB) from the psychrophilic fish pathogen Moritella viscosa

• The psychrophilic fish pathogenic bacterium Moritella viscosa produces sialic acids.
• Presence of sialic acids among fish pathogens points toward an important function.
• The N-acetylneuraminic acid synthase NeuB from M. viscosa was characterized.
• The enzyme shows typical cold adapted features.
• This efficient enzyme can be further explored for production of sialic acids and derivates.

Moritella viscosa is a Gram-negative psychrophilic bacterium that causes winter ulcer disease in Atlantic salmon and cod. Its genome reveals that it possesses the ability to synthesize sialic acids. Indeed, sialic acid can be isolated from the bacterium and when analyzed using HPLC-MS/MS, the presence of N-acetylneuraminic acid was confirmed. Thus, the N-acetylneuraminic acid synthase NeuB from M. viscosa (MvNeuB) was recombinantly produced and characterized. The optimum pH and temperature for MvNeuB activity are 7.5 and 30 °C, respectively. The KM for N-acetylmannosamine and phosphoenolpyruvate is 18 ± 5 and 0.8 ± 0.2 mM, respectively. The kcat value (∼225 min−1) for both N-acetylmannosamine and phosphoenolpyruvate is the highest turnover number found for an enzyme in this class until the date. A calorimetric study of MvNeuB shows that the enzyme has a two-step transition peak probably reflecting the two domains these proteins consist of. MvNeuB is less stable at higher temperature and has a high catalytic activity at lower temperature compared to mesophilic counterparts. Enzymes from psychrophilic organisms are generally cold adapted meaning they can maintain adequate function near the freezing point of water. Cold adapted enzymes are catalytically more efficient at lower temperature and are more thermo-labile compared to their mesophilic counterparts. MvNeuB is a typical cold adapted enzyme and could be further explored for production of sialic acids and derivates at low temperatures.

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