February 2011

Ruma V. Banerjee and Stephen W. Ragsdale: deciphering sulfur and carbon metabolism

One day, he received a call from Rowena Matthews, a colleague and fellow microbial biochemist at the University of Michigan. The two had conversed previously at a microbiology conference, where she described her frustration at not being able to obtain redox measurements for a particular enzyme because the cobalt-containing cofactor had such a low redox potential.

“I had told her I’m sure I could get the measurements because my lab was working on a corrinoid/iron-sulfur protein with a similarly very negative redox potential, but we had managed to develop some techniques to overcome that, so she should let me know if she wanted any assistance,” recalls Ragsdale.

In the phone call, Matthews took Ragsdale up on his offer and mentioned that she had just the perfect person, a bright and talented postdoctoral fellow, to come to Ragsdale’s lab and collaborate on this effort.

In Michigan, Ruma Banerjee was packing her bags for a trip. She had been working with Matthews for about a year and a half on methionine synthase, which adds a methyl group to homocysteine to complete the biosynthesis of the amino acid methionine.

Her initial experiments had gone quite well – Banerjee had managed to clone and characterize the gene from this enzyme in E. coli; however, her kinetic studies were stalling, as she was unable to analyze the transition of the vitamin B12 (cobalamin) cofactor from cobalt (II) to cobalt (I) during the reaction.

Matthews had arranged for Banerjee to do some measurements with a colleague in Milwaukee, Stephen Ragsdale. He had developed a new type of cell for spectroelectrochemical analysis (a method allowing spectroscopic detection of changes in oxidation state) that could analyze even minute changes.

After arriving, Banerjee and Ragsdale spent the next several days carrying out a host of experiments on methionine synthase. “It was intense,” she recalls. “We did all these electrochemical and spectroscopy studies, and were getting great data.

“At the same time, there was definitely more than spectroelectrical chemistry going on in that lab.”

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