Now in a paper just out in the Journal of the American Chemical Society, Withers, David Kwan, Jayachandran Kizhakkedathu (Centre for Blood Research and UBC Chem dept.) and others describe the development of an improved enzyme that takes us a step closer to having universal blood.
Blood comes in four major types: A, B, AB and O. The difference between them lies in the sugar structures that festoon the surface of red blood cells. Both blood type A and B have the same core sugar structure as blood type O, but differ in an additional sugar at the tip of the sugar structure. Type A has an N-acetylgalactosamine residue. Type B has a galactose residue. Type AB has a mix of both residues. The moiety carrying the additional residue can be tacked onto the core structure in various ways, giving rise to subtypes of A and B blood.
The additional residue presents trouble during blood transfusions: It can trigger life-threatening immune responses. Type A people can’t take type B blood; type B people can’t take in type A blood. Type AB can’t take either A or B. Only type O blood can be freely shared without the fear of immune responses.
The idea from the 1980s has been to use enzymes to remove the moieties with the terminal N-acetylgalactosamine or the galactose residues to leave the core sugar structure on red blood cells, just like in type O blood. But to date, sugar hydrolases have not been sufficiently efficient to make the idea practical.
So Withers’ group, which had some success in engineering different classes of sugar enzymes, tackled the creation of more efficient sugar hydrolases by directed evolution. In directed evolution, researchers carry out iterative rounds of mutations on a gene to ultimately produce a protein that performs better than the original gene product.
Kwan, Withers and the rest of the team carried out directed evolution on the family 98 glycoside hydrolase from a strain of Streptococcus pneumoniae. Kwan explains that the structure of the enzyme is known, which helped the investigators design their variants.
Kwan adds that the enzyme also is good at cleaving most A and B subtypes with the exception of a few A subtypes. The investigators decided to engineer the enzyme so it had better activity against those A subtypes. By directed evolution, the investigators got an engineered enzyme with a 170-fold greater efficiency than the original enzyme.
However, the engineered enzyme still doesn’t remove every single moiety with the N-acetylgalactosamine. Withers says that the immune system is sensitive enough to small amounts of the moiety to start an immune response. He says, “Before our enzyme can be used clinically, further improvements by directed evolution will be necessary to effect complete removal” of all moieties with the terminal N-acetylgalactosamine residues.
The investigators are now looking to tackle the remaining subtypes of type A blood that the S. pneumoniae hydrolase struggles to cut. Withers says, “Given our success so far, we are optimistic that this will work.”