Chemical and chemoenzymatic synthesis of glycoproteins for deciphering functions

Abstract

Glycoproteins are an important class of biomolecules involved in a number of biological recognition processes. However, natural and recombinant glycoproteins are usually produced as mixtures of glycoforms that differ in the structures of the pendent glycans, which are difficult to separate in pure glycoforms. As a result, synthetic homogeneous glycopeptides and glycoproteins have become indispensable probes for detailed structural and functional studies. A number of elegant chemical and biological strategies have been developed for synthetic construction of tailor-made, full-size glycoproteins to address specific biological problems. In this review, we highlight recent advances in chemical and chemoenzymatic synthesis of homogeneous glycoproteins. Selected examples are given to demonstrate the applications of tailor-made, glycan-defined glycoproteins for deciphering glycosylation functions.

Figure 1. Biosynthetic pathways for glycoproteins

(a) the biosynthesis of typical N-linked glycoproteins; (b) the biosynthesis of mucin type O-linked glycoproteins

Figure 2

Major strategies for synthesis of homogeneous glycoproteins

Figure 3. Total chemical synthesis of glycoprotein erythropoietin (EPO)

(a) chemical synthesis of an EPO glycoform carrying a single N-glycan; (b) chemical synthesis of a full-length native EPO glycoform carrying glycans at all the four conserved glycosylation sites.

Figure 4. Synthesis of glycoprotein probes for deciphering the molecular mechanism of lectin-mediated protein quality control

(a) a chemoenzymatic synthesis of several homogeneous glycoforms of ribonuclease B; (b) a chemical synthesis of intentionally misfolded homogeneous glycoforms of interleukin-8.

Figure 5

a chemoenzymatic approach to glycoengineering of IgG antibodies.

Figure 6. Synthesis of mucin glycopeptides as anti-cancer vaccines

(a) a chemical synthesis of MUC1 glycopeptides carrying T and ST glycans at defined glycosylation sites; (b) a chemoenzymatic synthesis of large MUC1 glycopeptides carrying multiple Tn, T, and STn glycans.

Figure 7. Synthetic HIV V1V2 glycopeptides for characterizing the glycan specificity of HIV-neutralizing antibodies

(a) a chemoenzymatic synthesis of V1V2 glycopeptide antigens carrying two distinct N-glycans at the conserved glycosylation sites; (b) a chemical synthesis of V1V2 glycopeptide antigens carrying the same N-glycans at the two conserved N-glycosylation sites.