Chemical & Engineering
NEWS MARCH 20, 1967
The Chemical World This Week Enzymes unravel cell wall Biochemists at the University of Wisconsin have determined the structure of cell-wall peptidoglycan in several bacteria by enzymic degradation methods. Using Staphylococcus aureus and other bacteria, Dr. Jack C. Strominger and coworkers Donald J. Tipper, Jerald C. Ensign, Jean-Marie Ghuysen, and Walther Katz in Wisconsin's pharmacology and bacteriology departments have isolated and characterized the polysaccharide and polypeptide units comprising the cell wall's main structure [Biochem., 6, 906, 921, 930 ( 1 9 6 7 ) ] . The cell-wall substrate in most bacteria is a polymer—peptidoglycan— made of a polysaccharide substituted by peptide units. These units are cross-linked to a variable extent subsequent to polysaccharide polymerization. Cleavage of the polysaccharide to disaccharides leaves a polymer, in this case a glycopeptide, in which disaccharide units are attached to a polypeptide. Similar breakdown of the polypeptide chains gives mainly the polysaccharide. Both these processes are essential to successful cell-wall structure determination, and both occur usefully only when highly selective enzymes are used to break each kind of molecular link. As in other fields of research, this latest advance stems from at least one happenstance. During his early bacteria work, Dr. Strominger knew how to use a peptide-active enzyme (peptidase) if a suitable one were obtainable. Almost simultaneously, coworker Ensign, then a student at the University of Illinois, came u p with the peptidase, but didn't know how to use it. At a lecture by Dr. Strominger, they learned of their mutual dilemma. Using the enzyme, and others acquired later, the Wisconsin scientists have now managed to rupture molecular links with a high degree of selectivity. Glycopeptide isolated from bacterial cell walls contains equimolar amounts of the hexosamines, N-acetylglucosamine and N-acetylmuramic acid, the Wisconsin group finds. Also, the disaccharide, N-acetylglucosaminyl-Nacetylmuramic acid, is /?-l,4-linked. Another glycopeptide, obtained using
TA
TA
TA
X -Y-X -Y-x - Y - x - Y - X - Y - X - Y - X - Y -X-fY-X-fYX-Y-x/Y-x/y--x|Y--X-lY-X-|Y--XfY~
•
X = acetylglucosamine Y = •
=
L-alanyl-D-isoglutaminyl-L-lysyl-D-alanine
acetylmuramic acid
= pentaglycine bridges
TA-P = teichoic acid antigen attached to polysaccharide
Schematic of S. aureus cell wall From at least one happenstance
a different enzyme on cell walls of the same bacterium, yields the isomeric disaccharide, N-acetylmuramyl-AT-acetylglucosamine—also p-1,4-linked. Therefore, the Wisconsin team concludes, polysaccharide in the peptidoglycan derives from alternating £-1,4linked residues of the two hexosamines. Peptide-free polysaccharide chains, recovered with the aid of another enzyme, have lengths averaging 19 to 25 units, but may be as short as 12 and as long as 70 to 100 units. In the peptide structure studies, the absence of a glutamic acid carboxyl end-group and presence of ammonia hinted at the existence of an a-amide. Dr. Tipper then micro-modified an ammo-acid analysis method to use it on amides. When applied to isolated peptides, the method gave results confirming the amide theory. In other words, the Wisconsin team finds that glutamic acid is not a cell-wall constituent, as previously suspected. But its a-amide, isoglutamine, is. This is the first report of naturally occurring isoglutamine, Dr. Strominger believes. Three other amino-acid residues— D- and L-alanine and L-lysine—are linked with isoglutamine to form the polypeptide's recurring structural unit, the biochemists find. They then confirmed identification and sequence of the amino acids by synthesis.
Government studies genetic effects of common chemicals The Federal Government is considering measures designed to protect the public from possible genetic damage caused by common environmental chemicals. The ultimate goals would be to check, routinely, the damage such compounds might do to lower organisms and follow up the suspicious ones with checks on humans. Targeted for study is the complete range of chemical products, including pesticides, food additives, and drugs, as well as products that turn up as air and water pollutants. Currently, the Government monitors these chemicals only to determine acute toxicity. These developments are far from the policy stage. But action has been taken at the highest levels. At the moment, Dr. James A. Shannon, director of the National Institutes of Health, is studying a report that outlines the kinds of studies needed to determine subtle interactions between chemicals and chromosomes, down to the level of deoxyribonucleic acid. The report was prepared by Dr. James F. Crow, chairman of the University of Wisconsin's genetics department. It resulted from a special meeting held among geneticists last September at MARCH 20, 1967 C&EN 19
