The effect of a methyl group on structure and function: Serine vs. threonine glycosylation and phosphorylation

Abstract

A variety of glycan structures cover the surface of all cells and are involved in myriad biological processes, including but not limited to, cell adhesion and communication, protein quality control, signal transduction and metabolism, while also being intimately involved in innate and adaptive immune functions. Immune surveillance and responses to foreign carbohydrate antigens, such as capsular polysaccharides on bacteria and surface protein glycosylation of viruses, are the basis of microbial clearance, and most antimicrobial vaccines target these structures. In addition, aberrant glycans on tumors called Tumor-Associated Carbohydrate Antigens (TACAs) elicit immune responses to cancer, and TACAs have been used in the design of many antitumor vaccine constructs. A majority of mammalian TACAs are derived from what are referred to as mucin-type O-linked glycans on cell-surface proteins and are linked to the protein backbone through the hydroxyl group of either serine or threonine residues. A small group of structural studies that have compared mono- and oligosaccharides attached to each of these residues have shown that there are distinct differences in conformational preferences assumed by glycans attached to either “unmethylated” serine or ß-methylated threonine. This suggests that the linkage point of antigenic glycans will affect their presentation to the immune system as well as to various carbohydrate binding molecules (e.g., lectins). This short review, followed by our hypothesis, will examine this possibility and extend the concept to the presentation of glycans on surfaces and in assay systems where recognition of glycans by proteins and other binding partners can be defined by different attachment points that allow for a range of conformational presentations.

An Inherent Difference between Serine and Threonine Phosphorylation: Phosphothreonine Strongly Prefers a Highly Ordered, Compact, Cyclic Conformation

“…The low population of conformations with negative values of χ 2 is not surprising for pThr, as these conformations would induce significant steric repulsion between the large phosphate group and the γ-CH 3 group and/or the protein main chain. The apparent preference for an eclipsed conformation was not expected for pThr, although a similar interaction has been observed by NMR in glycosylated Thr. − The greater length of the O–P bond (1.62 Å) compared to the C–H bond (1.09 Å) is expected to reduce, but not eliminate, the steric cost of an eclipsing interaction (H···P distances of 2.5–2.6 Å were observed, which are substantially less than the 3.0 Å sum of the van der Waals radii of P and H).…”

“…In contrast, the structural effects of phosphorylation at Ser were relatively more modest. While dianionic pSer can adopt a structure with all of the features of dianionic pThr, including an eclipsed C−H/O−P bond (χ 2 ∼ +120 or −120°), the absence of a γ-methyl group in pSer results in greater inherent conformational heterogeneity, 61 including a much wider range of conformations at ϕ, ψ, χ 1 , and χ 2 and a substantially reduced likelihood of an intraresidue phosphate−amide hydrogen bond. In addition, as was observed with pThr, the structural effects of pSer were more modest in the monoanionic state.…”

Orthogonal Translation for Site-Specific Installation of Post-translational Modifications

“…298 The structure and function of serine phosphorylation and threonine phosphorylation also differ from each other. 299 Because of the similarity between Sep and phosphothreonine (pThr), the established Sep-OTS is a good start for pThr-OTS development. To achieve this goal, there are two concerns to be addressed first.…”

“…The abundance of serine phosphorylation in nature is about 4.5-fold that of threonine phosphorylation . The structure and function of serine phosphorylation and threonine phosphorylation also differ from each other …”

Potassium Channels, Glucose Metabolism and Glycosylation in Cancer Cells

“…Sugar structures on the cell surface form a complex layer called the glycocalyx [161]. Glycans attached to macromolecules play crucial roles in cellular processes and intercellular interactions [162]. The alteration in the glycosylation pattern is closely related to physiological and pathological changes that occur in the cell [15].…”