Profile of Bob B. Buchanan PROFILE

PROFILE Profile of Bob B. Buchanan PROFILE Paul Gabrielsen, Science Writer On a clear June day in 1975, Bob Buchanan and two anaerobic fermentation pro- Oslo colleagues pulled up water samples from a lake cesses responsible for their in Norway, examining the microorganisms growing at beverages. Another friend at various depths. “When we got to 6 meters,” Buchanan Duke was Melinda Speas, a recalls, “there was a band of Chlorobium growing,” graduate student in zoology. referring to a genus of green bacteria (now called Their friendship was later Chlorobaculum) that belongs to a unique class of or- rekindled in California and they ganisms called photolithotrophs. These bacteria re- married in 1965. quire sunlight for photosynthesis but obtain cellular After earning his doctorate reductants from sulfur compounds rather than water. in 1962, Buchanan secured a The bacteria buck the traditional chemical pathways postdoctoral fellowship at the for energy production and carbon fixation and instead University of California, Berkeley, use pathways discovered by Buchanan, a member of where he would spend the next the National Academy of Sciences and emeritus pro- 55 years. He hoped to work fessor at the University of California, Berkeley. with renowned microbiologist H. A. Barker, but because From Appalachia to the Laboratory Barker was on sabbatical leave Buchanan was born in 1937 in Richmond, Virginia. at the time, Buchanan instead Fearing for the family’s safety during World War II, joined the group of Barker’s Buchanan’s mother took him and his two older sisters former student, Jesse Rabinowitz. to a family farm in Southwest Virginia, near the Ap- Buchanan and Rabinowitz crys- Bob B. Buchanan. Image courtesy of Bob B. palachian town of Glade Spring, while his father tallized the recently discov- Buchanan. remained in Richmond to work. Farm life shaped ered ferredoxin protein from Buchanan’s early years. He cared for pet lambs as a child Clostridium species (2). Watch- and put himself through college by growing beans and ing the protein crystals grow, Buchanan says, was milking cows. “By hand!” he says. “almost like witnessing the birth of a child.” The pub- Buchanan attended nearby Emory and Henry Col- lication resulting from their work remains his most highly lege, where he took a bacteriology class from zoolo- cited paper. Working on ferredoxin set the course for gist Lee T. Douglas. Under Douglas’ mentorship, Buchanan’s research career. “One can trace the thread Buchanan’s interest in microorganisms blossomed. “I from then to today,” he says. remember seeing a culture of Bacillus subtilis, a bacterium that can grow with or without oxygen, stained Ferredoxin with crystal violet under a simple microscope,” By the time Buchanan completed his postdoctoral Buchanan says. “The cells were so brilliant; that’s what fellowship in 1963, researchers knew that ferredoxin, I wanted to work with.” He was fascinated by the discovered the year before (3), carried highly ener- concept of life without oxygen. getic electrons and participated in fundamental bi- ological processes (4, 5). At the time, biologists knew From Duke to Berkeley of one process by which aerobic organisms produced Buchanan moved to Duke University in 1958 for his energy: the Krebs or citric acid cycle. Working with graduate work. He studied Actinomyces,facultative Daniel Arnon, an expert in photosynthesis, and post- anaerobes known to infect cattle and humans (1). doctoral scholar Michael Evans, Buchanan set out to Buchanan characterized a new species collected from determine whether the energetic electrons carried by an infected human tear duct. The highly infectious iso- ferredoxin could reverse the cycle. late, named Actinomyces propinicus, differed from “Textbooks said the cycle was irreversible,” known species in cell wall composition and in the for- Buchanan says, “but it turned out that ferredoxin mation of propionic acid as a fermentation product. It could do the job.” In the citric acid cycle, glucose was later reclassified into the genus Proprionibacterium. breaks down into pyruvate, which, in turn, breaks While at Duke, Buchanan and his friends in the down into acetyl-CoA and carbon dioxide. Buchanan humanities departments frequented a beer cellar in and his colleagues had earlier reversed the reaction, Chapel Hill, North Carolina, where he explained the combining acetyl-CoA, carbon dioxide, and reduced www.pnas.org/cgi/doi/10.1073/pnas.1714970114 PNAS Early Edition | 1of3 Downloaded by guest on October 1, 2021 ferredoxin to generate pyruvate (6). They further re- 1990s, Buchanan asked whether thioredoxin partici- versed another reaction, the breakdown of α-keto- pated in plant functions outside the chloroplast. He fo- glutarate. With the participation of ferredoxin and cused on the process of seed germination. “Seeds and three enzymes, the citric acid cycle could be fully re- redox had never gotten together before but since I had versed (7), fixing carbon dioxide and synthesizing never really studied plants formally, I could ask simple acetyl-CoA and pyruvate. The team recognized the questions, which may be an advantage,” he says. role of such a cycle in anaerobic environments, such as Buchanan considered the redox state of dried seed the rumen of cattle or the depths of a lake, where it proteins. When quiescent, he supposed, seed proteins would enable organisms to synthesize amino acids would be in a stable oxidized state. Exposing seeds to from organic acids and carbon dioxide. The cycle, now water would reduce the seed proteins, breaking them called the Arnon–Buchanan cycle (8), was found in down to serve as nutrients in germination. In collabo- Chlorobium, the bacterium that Buchanan would later ration with graduate student Tom Johnson and visiting sample in a Norwegian lake. scientist Karoly Kobrehel, Buchanan found that reduced Buchanan’s cycle immediately encountered con- thioredoxin served as a redox signal to promote protein troversy, however, challenging the assertion of his solubilization and activate enzymes functional in ger- Berkeley colleague, Melvin Calvin, that all carbon di- mination (14). oxide was fixed through the Calvin–Benson cycle. Thioredoxin has since been found to regulate en- Acceptance of the Arnon–Buchanan cycle took about zymes throughout biology in bacterial and animal a quarter century (9) and “only then did it appear in cells alike (15). As a cellular signal, thioredoxin has textbooks,” Buchanan says. also found practical application. Overexpression of Since that time, the Arnon–Buchanan cycle has been thioredoxin in barley accelerates germination by found in various extreme environments, such as deep about a day (16). “That doesn’t sound impressive ocean hydrothermal vents. (10) “This cycle has taken unless you’re in the malt business,” Buchanan says. directions I never thought possible,” Buchanan says. Thioredoxins can also mitigate allergens in wheat (17) and may play a role in cancer and Parkinson’s Thioredoxin and Chloroplasts disease research. “In cancer, thioredoxin is the bad Hoping to find more evidence for carbon fixation via guy because it encourages cell division,” he says. the Arnon–Buchanan cycle, Buchanan then turned his “In Parkinson’s, it’sthegoodguybecauseyou attention to the role of ferredoxin in chloroplasts, the want revitalization.” photosynthetic centers of plant cells. Using acetyl- Most recently, Buchanan and colleague Peggy CoA as a reaction substrate with ferredoxin yielded Lemaux have collaborated with a group at McGill Uni- no results. His group then added sugar phosphates as versity in Montreal to study the role of redox regulation substrates and found that fructose 1,6-bisphosphate on the genetic activity of barley. The team identified a increased the uptake of carbon dioxide by chloro- gene (TLP8) that influenced malting quality (18). The plast enzymes in the presence of reduced ferredoxin protein produced by the gene binds to β-glucan poly- and ATP. Tracing the chemical path of fructose uti- saccharides, which are insoluble, and assists the filtering lization, Buchanan found that the enzyme fructose process in beer production. The researchers found that 1,6-bisphosphatase was central to this increase and differential expression of the gene in different barley that the enzyme was activated by reduced ferre- varieties affected β-glucan binding, as did the redox doxin. (11) “There was no indication that this or any state of the protein. other enzyme could be regulated in this manner,” Despite his work on plant and grain biochemistry, he says. Buchanan does not keep a garden or brew his own Part of this discovery was fortuitous. Buchanan had beer. “And I grew up on a farm!” he says. “I’membar- added a small amount of magnesium to the reaction, rassed to say that. I really enjoy what I do too much to which turned out to be a cofactor for the fructose 1,6- dilute my time.” bisphosphatase enzyme in chloroplasts (12). “Had I In 1999, Buchanan was at home and noticed some used a saturated concentration of magnesium,” he difficulty moving his right-side limbs. He went to the says, “we would have missed it.” Buchanan’s findings hospital and learned he was having a stroke that were published in 1967. Ten years later, with post- progressed until he lost almost all mobility on his right doctoral scholars Peter Schürmann and Ricardo side. He says that change in diet and adopting an Wolosiuk, Buchanan found that ferredoxin’spart- exercise regimen, which includes regular swimming, ner in regulating photosynthetic enzyme activity are to thank for his recovery. After years of physical (13) is a protein called thioredoxin. Ferredoxin re- therapy, he regained much of his mobility. duced by light can reduce thioredoxin via an en- Buchanan and his wife have established awards to zyme Buchanan named ferredoxin–thioredoxin reductase. encourage scientific enthusiasm and excellence. At The reduced thioredoxin activates enzymes of the chlo- Wake Forest University, his wife’s alma mater, an roplast, which are deactivated by a different mechanism award honors her father, William E.

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