By Carl Price and Ellen Reardon, originally published in Let’s Talk Plants! May 2008, Issue No. 164. Reprinted May 2022.
Springtime is the greenest time of the year. It is the time when plants accumulate high concentrations of chlorophyll, the green pigments of leaves, and the chlorophyll absorbs sunlight in the initial step that fuels photosynthesis. As important as chlorophyll is, and as vividly as it catches our eyes, it is not the only component in the process of photosynthesis. The chloroplasts, the tiny, green organelles that are the engines of photosynthesis, also contain dozens of enzymes that catalyze the many steps needed to convert carbon dioxide into sugars and starch. The first of these steps, the conversion of carbon dioxide to a six-carbon sugar, involves the world’s most abundant protein: Rubisco or ribulose-bisphosphate carboxylase. Its role in the most important process of life on this planet, and the tales of its discovery, make a fascinating story.
ISOLATION OF FRACTION I PROTEIN
From the end of World War II until 1950, Sam Wildman was working as a Research Fellow in the laboratory of James Bonner at Caltech. Bonner asked Wildman to study the soluble proteins of green leaves. The methods for analyzing proteins in those days were very primitive, the most common being the differential precipitation of proteins with different concentrations of ammonium sulfate. Wildman found that about half of the total soluble protein of the leaf precipitated in a single, non-green fraction at about 35-percent saturation. To facilitate bookkeeping, Wildman called the material “Fraction I.”
By a stroke of luck, Linus Pauling, the Chemistry Chairman in the building next door, was assembling analytical devices that had been invented in Sweden and were almost unknown elsewhere. One of these was a 30-foot-long device invented by Arne Tiselius that could analyze proteins by their electric charge. The procedure, now routine, is called electrophoresis. Wildman was invited to examine his leaf proteins in the Tiselius device and found Fraction I to be a single compound.
A second device in Pauling’s laboratory was an analytical ultracentrifuge, invented by another Swede, Theodor Svedberg, which enabled one to measure the mass (size) of particles. In 1949 Wildman’s Fraction I protein was analyzed in the ultracentrifuge and was seen to be a single species with the phenomenal molecular weight of 600,000, about 10 times larger than hemoglobin. While Fraction I protein was clearly seen to be a major component of the chloroplast, no one in 1949 had any idea about its function.
THE PATH OF CARBON DIOXIDE FIXATION IN PHOTOSYNTHESIS
During World War II, the University of California at Berkeley became one of the sites for research on radioisotopes for nuclear weapons. A by-product of these studies was the production of a radioactive isotope of carbon, 14C, with an exceptionally long-life span. After the war, Melvin Calvin assembled a laboratory at Berkeley to exploit the unique properties of 14C to analyze the path of carbon dioxide fixation in photosynthesis. By 1954 Calvin, with Andrew Benson and James Bassham, had worked out the steps in the process, which is now called the "Calvin-Benson Cycle" pathway. Andrew Benson lived in La Jolla and was a retired professor at UCSD and Scripps Oceanographic.
It’s important to note that Calvin and Benson had identified the chemical intermediates in the pathway but had no information on the enzyme or enzymes that catalyze the steps in the pathway. They identified the first step as the combination of carbon dioxide with a five-carbon sugar phosphate, ribulose-bisphosphate, to make a six-carbon compound. This was followed immediately by the splitting of the six-carbon compound into two molecules of 3-phosphoglyceric acid.
Two years later, in 1956, Bernard Horecker at the National Institutes of Health in Bethesda, Maryland, isolated an enzyme from spinach leaves that would catalyze this reaction. It came to be known as ribulose-bisphosphate carboxylase or Rubisco. By analytical ultracentrifugation Horecker and associates found the enzyme to have a molecular weight of 600,000, and it quickly became evident that Fraction I protein is Rubisco.
RUBISCO IS THE KING
Further studies showed that, not only is Rubisco an exceptionally large protein, it occurs with only small modifications in all photosynthetic organisms, from photosynthetic bacteria to algae and on to higher plants. Because Rubisco is a relatively inefficient enzyme, the amounts of Rubisco in plants are extremely large, corresponding to a quarter of the total protein of leaves.
It is the world’s most abundant protein.