CHEMICAL & ENGINEERING
NEWS
OF THE WEEK
SEPTEMBER 1, 2003 - EDITED BY WILLIAM G. SCHULZ & BETHANY HALFORD
GLYCOCHEMISTRY
YEAST PRODUCE HUMANLIKE PROTEINS Genetic engineering advance yields sugar patterns needed for drugs
A
FTER LONG-STAND-
ing efforts by several groups, a research team has succeeded in genetically engineering yeast to produce proteins with sugar groups attached in patterns similar to those needed for therapeutic use. The glycosylation work was carried out by associate professor of biochemical engineeringTillmanU. Gerngross of D a r t m o u t h College and coworkers there and at GlycoFi Inc., Lebanon, N.H. [Science, 301, 1244(2003)}. A number of scientists who specialize in protein glycosylation say the study is an important advance. But they note that the glycosylated proteins reported by the team are still more simplistic than many human glycoproteins. Specific carbohydrate sequences often must be added to newly biosynthesized proteins for the proteins to become biologically active. Most therapeutic proteins, including the anemia agent erythropoietin and the anti-infection and antitumor drug interferon, are currently produced in mammalian cells, which are capable of glycosylating them in a humanlike manner. But mammalian cell systems tend to produce low protein yields, require long fermentation times, and glycosylate proteins heterogeneously Many biotech companies would prefer to produce glycoproteins in simpler microbial systems such as yeast. But yeast produce glycosylation patterns HTTP://WWW.CEN-ONLINE.ORG
different from those in humans. The resulting glycoproteins are immunogenic and unsuitable for therapeutic use. Now, Gerngross and coworkers have genetically engineered yeast to express humanized and homogeneously glycosylated proteins. The researchers eliminate the normal glycosylation pathway in yeast, replace it with a combinatorial library of eukaryotic catalytic domains, and select for yeast mutants best able to produce proteins with a desired glycosylation pattern. Such humanized glycoproteins could be produced in industrialscale yeast expression systems, Gerngross says. The technique could lead to the production of therapeutic proteins that are less expensive and more potent than current products. It also could aid structure-function studies of proteins with different glycosylation patterns. "The establishment of a yeast cell line incorporating a limited humanlike glycosylation pathway represents a major technological breakthrough and provides the opportunity to manufacture safe, effective, and relatively inexpensive therapeutics on a large scale," comments Pauline Rudd of the Glycobiology Institute at the University of Oxford. "Several groups have been attempting to remodel the carbohydrate structures in fungal organisms for more than a decade," says professor KelleyW Moremen ofthe Complex Carbohydrate Re-
search Center at the University of Georgia, Athens. The combinatorial approach enabled the Dartmouth-GlycoFi group to overcome the obstacles, and the resulting technique is "a great advancement to the field." Nevertheless, he says, "several more steps need to be engineered into the fungal host before they will generate structures that truly resemble mature human glycoforms." Molecular biology professor James C. Paulson of Scripps Research Institute says, ' T h e Dartmouth and GlycoFi groups have done a terrific job of manipulating yeast to produce a truncated N-linked carbohydrate." However, he adds that "there is still a long way to go. N-Glycans of most therapeutic glycoproteins contain additional sugars, such as galactose and sialic acid." GlycoFi yeast geneticist Stephen R. Hamilton says that levels of glycosylation vary on different proteins and that "not all require fully sialylatedglycans." And Gerngross notes that he and his colleagues are currendy working "on the remaining steps to obtain terminally sialylated proteins in yeast."—STU BORMAN
IN BRIEF: ACS ELECTIONS Candidates' statements for the fall 2003 ACS national elections are posted on the following website: http:// chemistry.org/election. Statements will appear in the Sept. 15 issue of C&EN.
YEAST MANIPULATORS GlycoFi's Hamilton (left) is the lead author on the Science paper, and Dartmouth's Gerngross (right) is the corresponding author; Pichia pastoris (shown) is the yeast strain genetically transformed by the Dartmouth-GlycoFi group. C&EN
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