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Uridylyltransferases modify free monosaccharides to form UDP-glycans, which can be modified by a variety of processes (methylation, phosphorylation, sulphation, acetylation, etc.). Model for the proposed mechanisms of O-glycosylation in bacteria. In this review we will focus on the different pathways involved in O-glycosylation and roles proposed for this modification, which is increasingly being detected in many bacterial species. This system is primarily utilized to decorate flagellar structural proteins and adhesins, and has been heavily studied in the past two decades (Logan, 2006 Dell et al., 2010). During OTase-independent O-glycosylation, glycosyltransferases sequentially transfer individual monosaccharides to a protein acceptor from their nucleotide-activated forms. This pathway is reminiscent of the N-glycosylation process that occurs in the endoplasmic reticulum of eukaryotes and in the periplasm of bacteria such as Campylobacter jejuni, and also shares some similarities with the synthesis of lipopolysaccharide (LPS) in Gram-negative bacteria (Aebi et al., 1996 Linton et al., 2005 Hug and Feldman, 2011).
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Glycosyltransferases subsequently attach additional monosaccharides to the first sugar residue on Und-PP and, when the carbohydrate is completed, the Und-PP linked glycan is flipped to the periplasmic face, where an OTase transfers the carbohydrate to selected serine or threonine residues in acceptor proteins. The pathway is initiated by a specialized initiating glycosyltransferase that attaches a nucleotide-activated monosaccharide to an undecaprenolphosphate (Und-P) lipid carrier on the inner face of the plasma membrane (Hug and Feldman, 2011). OTase-dependent O-glycosylation has not been described in eukaryotes. OTases are enzymes that transfer a preassembled glycan en bloc to a protein acceptor. In this review, we will focus on O-glycosylation in bacteria, which we propose to be classified based on the utilization of an oligosaccharyltransferase (OTase Fig. In the most common cases of glycosylation, carbohydrates are covalently attached to asparagine residues ( N-glycosylation), or to serine or threonine residues ( O-glycosylation). Since then, protein glycosylation systems have been identified in all forms of life, and are required for various functions. Initially identified in 1938 (Neuberger, 1938), glycosylation was thought of as a solely eukaryotic mechanism until the identification of glycoproteins in Halobacterium and Clostridium (Sleytr, 1975 Mescher and Strominger, 1976). Protein glycosylation, the covalent attachment of carbohydrates to amino acids, is the most abundant post-translational modification in nature, with more than two-thirds of all eukaryotic proteins predicted to be glycosylated (Apweiler et al., 1999). Likely, the O-glycosylation pathways we currently know constitute just the tip of the iceberg of a still largely uncharacterized bacterial glycosylation world. Technological advances in MS and NMR will facilitate the discovery of novel glycosylation systems. Furthermore, O-glycosylation systems are promising targets for antibiotic development. These recombinant glycoproteins can be exploited for vaccines and diagnostics of bacterial infections. Exploiting glycosylation machineries it is now possible to generate glycoconjugates made of different proteins attached to polysaccharides derived from LPS or capsule biosynthesis. Both systems play key roles in pathogenesis. This pathway is employed for glycosylation of flagella and autotransporters.
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OTase-independent glycosylation refers to the pathway in which glycosyltransferases sequentially add monosaccharides onto the target proteins. Multiple proteins, including the pilins, are glycosylated using this mechanism. OTase-dependent glycosylation utilizes an oligosaccharide synthesized on a lipid carrier that is transferred to proteins en bloc by an OTase. We propose a classification of O-glycosylation based on the requirement of an oligosaccharyltransferase (OTase). This review focuses on summarizing the structural diversity, the various pathways and the physiological roles of this post-translational protein modification. However in the recent years multiple O-glycosylation mechanisms have been identified in bacterial species from the most diverse genera, including various important human pathogens. Protein glycosylation was once considered as an eccentricity of a few bacteria.