Pro to Gly (P219G) in a silent glycosylation site results in complete glycosylation in tissue plasminogen activator

Identification, characterization, and engineering of glycosylation in thrombolyticsa

Cardiovascular diseases, such as myocardial infarction, ischemic stroke, and pulmonary embolism, are the most common causes of disability and death worldwide. Blood clot hydrolysis by thrombolytic enzymes and thrombectomy are key clinical interventions. The most widely used thrombolytic enzyme is alteplase, which has been used in clinical practice since 1986. Another clinically used thrombolytic protein is tenecteplase, which has modified epitopes and engineered glycosylation sites, suggesting that carbohydrate modification in thrombolytic enzymes is a viable strategy for their improvement. This comprehensive review summarizes current knowledge on computational and experimental identification of glycosylation sites and glycan identity, together with methods used for their reengineering. Practical examples from previous studies focus on modification of glycosylations in thrombolytics, e.g., alteplase, tenecteplase, reteplase, urokinase, saruplase, and desmoteplase. Collected clinical data on these glycoproteins demonstrate the great potential of this engineering strategy. Outstanding combinatorics originating from multiple glycosylation sites and the vast variety of covalently attached glycan species can be addressed by directed evolution or rational design. Directed evolution pipelines would benefit from more efficient cell-free expression and high-throughput screening assays, while rational design must employ structure prediction by machine learning and in silico characterization by supercomputing. Perspectives on challenges and opportunities for improvement of thrombolytic enzymes by engineering and evolution of protein glycosylation are provided.

Structural requirements for additional N-linked carbohydrate on recombinant human erythropoietin

N-Linked glycosylation is a post-translational event whereby carbohydrates are added to secreted proteins at the consensus sequence Asn-Xaa-Ser/Thr, where Xaa is any amino acid except proline. Some consensus sequences in secreted proteins are not glycosylated, indicating that consensus sequences are necessary but not sufficient for glycosylation. In order to understand the structural rules for N-linked glycosylation, we introduced N-linked consensus sequences by site-directed mutagenesis into the polypeptide chain of the recombinant human erythropoietin molecule. Some regions of the polypeptide chain supported N-linked glycosylation more effectively than others. N-Linked glycosylation was inhibited by an adjacent proline suggesting that sequence context of a consensus sequence could affect glycosylation. One N-linked consensus sequence (Asn123-Thr125) introduced into a position close to the existing O-glycosylation site (Ser126) had an additional O-linked carbohydrate chain and not an additional N-linked carbohydrate chain suggesting that structural requirements in this region favored O-glycosylation over N-glycosylation. The presence of a consensus sequence on the protein surface of the folded molecule did not appear to be a prerequisite for oligosaccharide addition. However, it was noted that recombinant human erythropoietin analogs that were hyperglycosylated at sites that were normally buried had altered protein structures. This suggests that carbohydrate addition precedes polypeptide folding.

The oligosaccharyltransferase complex from yeast

N-Glycosylation of eukaryotic secretory and membrane-bound proteins is an essential and highly conserved protein modification. The key step of this pathway is the en bloc transfer of the high mannose core oligosaccharide Glc3Man9GlcNAc2 from the lipid carrier dolichyl phosphate to selected Asn-X-Ser/Thr sequences of nascent polypeptide chains during their translocation across the endoplasmic reticulum membrane. The reaction is catalysed by the enzyme oligosaccharyltransferase (OST). Recent biochemical and molecular genetic studies in yeast have yielded novel insights into this enzyme with multiple tasks. Nine proteins have been shown to be OST components. These are assembled into a heterooligomeric membrane-bound complex and are required for optimal expression of OST activity in vivo in wild type cells. In accord with the evolutionary conservation of core N-glycosylation, there are significant homologies between the protein sequences of OST subunits from yeast and higher eukaryotes, and OST complexes from different sources show a similar organisation as well.

Substitution of asparagine residues in Aspergillus awamori glucoamylase by site-directed mutagenesis to eliminate N-glycosylation and inactivation by deamidation

Aspergillus awamori glucoamylase is a secreted glycoprotein containing N-linked carbohydrate recognition sites at Asn-171, Asn-182 and Asn-395. Site-directed mutagenesis was performed at Asn-182 and Asn-395 to determine whether these residues were N-glycosylated by Saccharomyces cerevisiae, to investigate the function of any glycans linked to them, and to determine the effect of their deamidation on glucoamylase thermostability. Asn-171 and Asn-395, but not Asn-182, were N-glycosylated. Deletion of the glycan N-linked to Asn-395 did not affect specific activity, but greatly decreased enzyme secretion and thermostability. The mutant lacking the N-glycan linked to Asn-395 was synthesized very slowly, and was more associated with cell membrane components and susceptible to proteinase degradation than were wild-type or other mutant glucoamylases. Its secreted form was 30-fold less thermostable than wild-type enzyme at pH 4.5. Replacement of Asn-182 by Gln to eliminate deamidation at this site did not change glucoamylase specific activity or thermostability, while replacement by Asp decreased specific activity about 25%, but increased thermostability moderately at pH 4.5 below 70 degrees C. Both mutations of Asn-182 increased glucoamylase production.

Glycosylation

The majority of candidate recombinant therapeutics are glycoproteins. Four aspects of glycobiology are requisite if the full potential of such reagents is to be realised: an understanding of glycan biosynthesis and its regulation, rapid and sensitive oligosaccharide analytical techniques, determination of structure-function relationships in protein-linked carbohydrate; and manufacturing systems where product glycosylation is manipulable and consistent. The past year has seen significant progress in all of these areas, exemplified by the approval of glycosylation-engineered glucocerebrosidase for clinical use in replacement therapy of Gaucher's disease.