Recombinant Snail Sialic Acid Aldolase is Promiscuous towards Aliphatic Aldehydes

Aldolases are enzymes that reversibly catalyze the cleavage of carbon-carbon bonds. Here we describe a recombinant sialic acid aldolase originating from the freshwater snail Biomphalaria glabrata (sNPL), and compare its substrate spectrum with a sialic acid aldolase originating from chicken (chNPL). In contrast to vertebrate animals which can synthesize, degrade, and incorporate sialic acids on glycoconjugate ubiquitously, snails (as all mollusks) cannot synthesize sialic acids endogenously, and therefore the biological function and substrate scope of sNPL ought to differ significantly from vertebrate sialic aldolases such as chNPL. sNPL was active towards a series of sialic acid derivatives but was in contrast to chNPL unable to catalyze the cleavage of N-acetylneuraminic acid into N-acetylmannosamine and pyruvate. Interestingly, chNPL and sNPL showed contrasting C4(R)/(S) diastereoselectivity towards the substrates d-mannose and d-galactose in the presence of pyruvate. In addition, sNPL was able to synthesize a series of 4-hydroxy-2-oxoates using the corresponding aliphatic aldehyde substrates in the presence of pyruvate, which could be not achieved by chNPL.