Isolation and characterization of sialate lyase from pig kidney

Molecular Characterization of a Novel N-Acetylneuraminate Lyase from a Deep-Sea Symbiotic Mycoplasma

N-acetylneuraminic acid (Neu5Ac) based novel pharmaceutical agents and diagnostic reagents are highly required in medical fields. However, N-acetylneuraminate lyase（NAL）for Neu5Ac synthesis is not applicable for industry due to its low catalytic efficiency. In this study, we biochemically characterized a deep-sea NAL enzyme (abbreviated form: MyNal) from a symbiotic Mycoplasma inhabiting the stomach of a deep-sea isopod, Bathynomus jamesi. Enzyme kinetic studies of MyNal showed that it exhibited a very low K_m for both cleavage and synthesis activities compared to previously described NALs. Though it favors the cleavage process, MyNal out-competes the known NALs with respect to the efficiency of Neu5Ac synthesis and exhibits the highest k_cat/K_m values. High expression levels of recombinant MyNal could be achieved (9.56 mol L⁻¹ culture) with a stable activity in a wide pH (5.0–9.0) and temperature (40–60 °C) range. All these features indicated that the deep-sea NAL has potential in the industrial production of Neu5Ac. Furthermore, we found that the amino acid 189 of MyNal (equivalent to Phe190 in Escherichia coli NAL), located in the sugar-binding domain, GX189DE, was also involved in conferring its enzymatic features. Therefore, the results of this study improved our understanding of the NALs from different environments and provided a model for protein engineering of NAL for biosynthesis of Neu5Ac.

Harnessing cancer cell metabolism for theranostic applications using metabolic glycoengineering of sialic acid in breast cancer as a pioneering example

Abnormal cell surface display of sialic acids - a family of unusual 9-carbon sugars - is widely recognized as distinguishing feature of many types of cancer. Sialoglycans, however, typically cannot be identified with sufficiently high reproducibility and sensitivity to serve as clinically accepted biomarkers and similarly, almost all efforts to exploit cancer-specific differences in sialylation signatures for therapy remain in early stage development. In this report we provide an overview of important facets of glycosylation that contribute to cancer in general with a focus on breast cancer as an example of malignant disease characterized by aberrant sialylation. We then describe how cancer cells experience nutrient deprivation during oncogenesis and discuss how the resulting metabolic reprogramming, which endows breast cancer cells with the ability to obtain nutrients during scarcity, constitutes an "Achilles' heel" that we believe can be exploited by metabolic glycoengineering (MGE) strategies to develop new diagnostic methods and therapeutic approaches. In particular, we hypothesize that adaptations made by breast cancer cells that allow them to efficiently scavenge sialic acid during times of nutrient deprivation renders them vulnerable to MGE, which refers to the use of exogenously-supplied, non-natural monosaccharide analogues to modulate targeted aspects of glycosylation in living cells and animals. In specific, once non-natural sialosides are incorporated into the cancer "sialome" they can be exploited as epitopes for immunotherapy or as chemical tags for targeted delivery of imaging or therapeutic agents selectively to tumors.

Preferential Accumulation of 14C-N-Glycolylneuraminic Acid over 14C-N-Acetylneuraminic Acid in the Rat Brain after Tail Vein Injection

The two main molecular species of sialic acid existing in nature are N-acetylneuraminic acid (Neu5Ac) and N-glycolylneuraminic acid (Neu5Gc). Neu5Ac is abundant in mammalian brains and plays crucial roles in many neural functions. In contrast, Neu5Gc is present only at a trace level in vertebrate brains. The brain-specific suppression of Neu5Gc synthesis, which is a common feature in mammals, suggests that Neu5Gc has toxicity against brain functions. However, in vivo kinetics of Neu5Gc in the whole body, especially in the brain, has not been studied in sufficient detail. To determine the in vivo kinetics of Neu5Gc, ¹⁴C-Neu5Gc was enzymatically synthesized and injected into rat tail veins. Although most of ¹⁴C-Neu5Gc was excreted in urine, a small amount of ¹⁴C-Neu5Gc was detected in the brain. Brain autoradiography indicated that ¹⁴C-Neu5Gc was accumulated predominantly in the hippocampus. ¹⁴C-Neu5Gc transferred into the brain was incorporated into gangliosides including GM1, GD1a, GD1b, GT1b and GQ1b. Reduction of ¹⁴C-Neu5Gc after intracerebroventricular infusion was slower than that of ¹⁴C-Neu5Ac in the brain and hippocampus. The results suggest that Neu5Gc is transferred from blood into the brain across the blood brain barrier and accumulates in the brain more preferentially than does Neu5Ac.

Characterization of a novel N-acetylneuraminic acid lyase favoring industrial N-acetylneuraminic acid synthesis

N-Acetylneuraminic acid lyase (NAL, E.C. number 4.1.3.3) is a Class I aldolase that catalyzes the reversible aldol cleavage of N-acetylneuraminic acid (Neu5Ac) from pyruvate and N-acetyl-D-mannosamine (ManNAc). Due to the equilibrium favoring Neu5Ac cleavage, the enzyme catalyzes the rate-limiting step of two biocatalytic reactions producing Neu5Ac in industry. We report the biochemical characterization of a novel NAL from a “GRAS” (General recognized as safe) strain C. glutamicum ATCC 13032 (CgNal). Compared to all previously reported NALs, CgNal exhibited the lowest k_cat/K_m value for Neu5Ac and highest k_cat/K_m values for ManNAc and pyruvate, which makes CgNal favor Neu5Ac synthesis the most. The recombinant CgNal reached the highest expression level (480 mg/L culture), and the highest reported yield of Neu5Ac was achieved (194 g/L, 0.63 M). All these unique properties make CgNal a promising biocatalyst for industrial Neu5Ac biosynthesis. Additionally, although showing the best Neu5Ac synthesis activity among the NAL family, CgNal is more related to dihydrodipicolinate synthase (DHDPS) by phylogenetic analysis. The activities of CgNal towards both NAL's and DHDPS' substrates are fairly high, which indicates CgNal a bi-functional enzyme. The sequence analysis suggests that CgNal might have adopted a unique set of residues for substrates recognition.

UDP-GlcNAc 2-Epimerase/ManNAc Kinase (GNE): A Master Regulator of Sialic Acid Synthesis

UDP-N-acetylglucosamine 2-epimerase/N-acetylmannosamine kinase is the key enzyme of sialic acid biosynthesis in vertebrates. It catalyzes the first two steps of the cytosolic formation of CMP-N-acetylneuraminic acid from UDP-N-acetylglucosamine. In this review we give an overview of structure, biochemistry, and genetics of the bifunctional enzyme and its complex regulation. Furthermore, we will focus on diseases related to UDP-N-acetylglucosamine 2-epimerase/N-acetylmannosamine kinase.

Metabolism of vertebrate amino sugars with N-glycolyl groups: elucidating the intracellular fate of the non-human sialic acid N-glycolylneuraminic acid

The two major mammalian sialic acids are N-acetylneuraminic acid and N-glycolylneuraminic acid (Neu5Gc). The only known biosynthetic pathway generating Neu5Gc is the conversion of CMP-N-acetylneuraminic acid into CMP-Neu5Gc, which is catalyzed by the CMP-Neu5Ac hydroxylase enzyme. Given the irreversible nature of this reaction, there must be pathways for elimination or degradation of Neu5Gc, which would allow animal cells to adjust Neu5Gc levels to their needs. Although humans are incapable of synthesizing Neu5Gc due to an inactivated CMAH gene, exogenous Neu5Gc from dietary sources can be metabolically incorporated into tissues in the face of an anti-Neu5Gc antibody response. However, the metabolic turnover of Neu5Gc, which apparently prevents human cells from continued accumulation of this immunoreactive sialic acid, has not yet been elucidated. In this study, we show that pre-loaded Neu5Gc is eliminated from human cells over time, and we propose a conceivable Neu5Gc-degrading pathway based on the well studied metabolism of N-acetylhexosamines. We demonstrate that murine tissue cytosolic extracts harbor the enzymatic machinery to sequentially convert Neu5Gc into N-glycolylmannosamine, N-glycolylglucosamine, and N-glycolylglucosamine 6-phosphate, whereupon irreversible de-N-glycolylation of the latter results in the ubiquitous metabolites glycolate and glucosamine 6-phosphate. We substantiate this finding by demonstrating activity of recombinant human enzymes in vitro and by studying the fate of radiolabeled pathway intermediates in cultured human cells, suggesting that this pathway likely occurs in vivo. Finally, we demonstrate that the proposed degradative pathway is partially reversible, showing that N-glycolylmannosamine and N-glycolylglucosamine (but not glycolate) can serve as precursors for biosynthesis of endogenous Neu5Gc.

Metabolism of vertebrate amino sugars with N-glycolyl groups: intracellular β-O-linked N-glycolylglucosamine (GlcNGc), UDP-GlcNGc, and the biochemical and structural rationale for the substrate tolerance of β-O-linked β-N-acetylglucosaminidase

The O-GlcNAc modification involves the attachment of single β-O-linked N-acetylglucosamine residues to serine and threonine residues of nucleocytoplasmic proteins. Interestingly, previous biochemical and structural studies have shown that O-GlcNAcase (OGA), the enzyme that removes O-GlcNAc from proteins, has an active site pocket that tolerates various N-acyl groups in addition to the N-acetyl group of GlcNAc. The remarkable sequence and structural conservation of residues comprising this pocket suggest functional importance. We hypothesized this pocket enables processing of metabolic variants of O-GlcNAc that could be formed due to inaccuracy within the metabolic machinery of the hexosamine biosynthetic pathway. In the accompanying paper (Bergfeld, A. K., Pearce, O. M., Diaz, S. L., Pham, T., and Varki, A. (2012) J. Biol. Chem. 287, 28865-28881), N-glycolylglucosamine (GlcNGc) was shown to be a catabolite of NeuNGc. Here, we show that the hexosamine salvage pathway can convert GlcNGc to UDP-GlcNGc, which is then used to modify proteins with O-GlcNGc. The kinetics of incorporation and removal of O-GlcNGc in cells occur in a dynamic manner on a time frame similar to that of O-GlcNAc. Enzymatic activity of O-GlcNAcase (OGA) toward a GlcNGc glycoside reveals OGA can process glycolyl-containing substrates fairly efficiently. A bacterial homolog (BtGH84) of OGA, from a human gut symbiont, also processes O-GlcNGc substrates, and the structure of this enzyme bound to a GlcNGc-derived species reveals the molecular basis for tolerance and binding of GlcNGc. Together, these results demonstrate that analogs of GlcNAc, such as GlcNGc, are metabolically viable species and that the conserved active site pocket of OGA likely evolved to enable processing of mis-incorporated analogs of O-GlcNAc and thereby prevent their accumulation. Such plasticity in carbohydrate processing enzymes may be a general feature arising from inaccuracy in hexosamine metabolic pathways.

Characterization of a sialate pyruvate-lyase in the cytosol of human erythrocytes

Sialate pyruvate-lyases, also known as sialate aldolases (EC 4.1.3.3), reversibly catalyse the cleavage of free N-acetylneuraminic acids to form pyruvate and N-acetylmannosamine. These enzymes are widely distributed and are present in numerous pro- and eukaryotic cells, in which they are localized only in the cytosol. They play an important role in the regulation of sialic acid metabolism by controlling the intracellular concentration of sialic acids of biosynthetic or exogenous origin, thus preventing the accumulation of toxic levels of this sugar. Application of an original colorimetric micromethod for N-acetylmannosamine determination, as well as the use of [4,5,6,7,8,9-14C]N-acetylneuraminic acid, led us to evidence a cytosolic neuraminate aldolase activity in human red blood cells (RBCs) and then to define the main characteristics of this enzyme: Michaelis-Menten type, K(m:) 1.4 +/- 0.05 mM, optimal pH: 7.6 +/- 0.2, optimal temperature: 70 +/- 2 degrees C, inhibition by heavy metals: Ag(+) and Hg(++). These enzyme parameters are close to those of the bacterial and mammalian aldolases described up to now. At the moment, the presence of sialate pyruvate-lyase in the cytosol of red blood cells remains an enigma.

The sialate-pyruvate lyase from pig kidney. Elucidation of the primary structure and expression of recombinant enzyme activity

The first complete primary structure of a mammalian sialate-pyruvate lyase, namely of the enzyme from porcine kidney, was elucidated by a combination of different PCR techniques followed by sequencing of the resulting fragments. The primers used were either deduced from four porcine lyase peptides or from an alignment of human and mouse expressed sequence tags (ESTs), which were found to be homologous to already known microbial lyase sequences, and cDNA alone or after ligation with a plasmid vector served as a template. The lyase primary structure consists of 319 amino acids with a calculated protein molecular mass of approximately 35 kDa, which fits well to the value determined for the native enzyme. The porcine lyase sequence made it possible to assemble several ESTs from mouse and man in order to obtain the complete putative lyase genes. The three mammalian sequences reveal a high degree of homology both on the nucleotide (83% of the nucleotides are identical between all three sequences) and on the amino-acid level (72% of the amino acids are identical between all three sequences), and thus form a tightly related group. In contrast, the identity between the lyase primary structures from pig kidney and the microbial enzyme from Clostridium perfringens is much less pronounced (25%). Thirty-one amino acids were found to be absolutely conserved in all lyase sequences. Among them are two amino acids (lysine 173 and tyrosine 143 in the porcine lyase) that are most important for the catalytic reaction. After expression cloning, recombinant enzyme activity was expressed in Escherichia coli BL21(DE3)pLysS, which confirms the identity of the cloned sequence and verifies one of the putative human and murine sequences. After SDS/PAGE of a cell extract of the expression clone, a band of 35kDa was stained on the gel.

The sialate pyruvate-lyase from pig kidney: purification, properties and genetic relationship

For further insight into the structural relationship between mammalian and microbial sialate pyruvate-lyases, the enzyme from pig kidney was purified to homogeneity from the tissue homogenate by a heat precipitation step followed by anion exchange and Hydrophobic Interaction Chromatography or native gel electrophoresis, respectively. The pure enzyme preparation exhibited an about 1000-fold increase of specific activity compared to the supernatant after the first centrifugation and revealed a single band at 34-37 kDa after SDS-PAGE, which represents the monomeric form of the protein. While the native enzyme seems to be a trimer according to the molecular weight obtained by gel filtration (108 kDa), crosslinking with dimethylpimelimidate suggests it to be a tetramer. The lyase is optimally active at about 75 degrees C and in the pH range of 7.6 to 8.0 and belongs to the class I-aldolases, due to its non-requirement of metal ions and the presence of lysine as the main functional residue in its catalytic centre. These data are similar to those obtained with bacterial lyases. However, peptide fragments of this enzyme show less similarity to primary lyase structures of microbia than to those derived from expressed sequence tags of mammals.

Aberrant metabolic sialylation of recombinant proteins expressed in Chinese hamster ovary cells in high productivity cultures

The incorporation of sialic acid into therapeutic recombinant glycoprotein expressed in Chinese hamster ovary (CHO) cells during growth in large bioreactors (10 l) has been monitored under high productivity conditions induced by the presence of sodium butyrate. Samples of the bioreactor culture (approximately 4 x 10(6) cells) were labeled with 3H-N-acetylmannosamine, a metabolic precursor of sialic acid. After 24 h, the recombinant glycoprotein, an immunoadhesion chimeric molecule, was purified and the amount of sialic acid incorporated was determined as radioactive counts. The labeling profile of the protein over the course of the culture was compared with the sialic acid content of the molecule as determined by direct chemical analysis. Early in the culture, the two methods of analysis gave a similar sialylation profile. However, after sodium butyrate was included in the culture, the metabolically incorporated sialic acid rapidly and dramatically decreased to near undetectable levels. In contrast, sialic acid content of the protein, as determined by chemical analysis, decreased only moderately and gradually over the culture period, from a maximum of 6.1 to about 5. 0 mol sialic acid/mole of protein after 10 days in culture. These results suggest that butyrate may enhance reutilization of existing glycoproteins in the culture, generating sialic acid for biosynthesis through lysosomal degradation and thereby bypassing de novo biosynthesis.