Sialic acids recycling

A simplified one-pot two-step enzymatic approach was used by James Paulson s group to produce sialoside derivatives (23). Instead of recycling the CMP-sialic acid, the sialyltransferase donors with modifications at C-5 or C-9 position of NeuSAc were enzymatically synthesized from ManNAc or its C-2 or C-6 modified derivatives, pyruvate, and CTP using a sialic acid aldolase and a CMP-Neu5Ac synthetase/sialyltransferase fusion protein. After removing the protein by membrane filtration, the filtrate containing produced CMP-sialic acid... 

Three enzyme systems are required for the catabolism of sialic acids 0-acetylesterases, sialidases and lyases. Furthermore, a sialic acid transporter has been described (section 10.5), which carries liberated sialic acids from lysosomes into the cytosol, where they are either degraded by the lyase or recycled after activation with CTP (section 8.3). Sialic acid permeases provide bacteria with sialic acids for nutritional purposes (section 9.4). 

NANA (sialic acid) residues are lost from the serum proteins. This change signals their removal from the circulation and their eventual degradation. An asialoglycoprotein receptor on the liver cell surface binds such proteins, and the receptor— hgand complex is endocytosed and transported to the lysosomes. The amino acids from the degraded protein are then recycled within the liver. 

Immobilized enzymes are often more stable than the soluble counterparts and can be recycled. They are also removable from a reaction mixture. PHA synthase fusions with the enzyme of interest were successfully developed and enabled production of recombinant enzyme already attached to a support material (PHA) in one step. The fl-galactosidase LacZ [21] was the first immobilized enzyme followed by alpha-amylase, organophosphohydrolase, and enzymes involved in sialic acid synthesis [76-78]. These demonstrated applicability PHA-immobilized enzymes in food processing, biomass conversion, bioremediation, and fine chemical synthesis. 

Although 0-acetyl modifications of sialic acids can be enzymatically removed (see Section 4.2), no mechanism for the dehydroxylation of Neu5Gc is known. Neu5Gc from recycled glycoconjugates may thus augment the hydroxylase-mediated Neu5Gc biosynthesis (Muchmore et al., 1989). 

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