In the 1960s, two Australian scientists, M.D. Hatch and C.R. Slack described a new pathway for carbon fixation in photosynthesis. The Hatch-Slack pathway describes a biochemical system in which carbon is first incorporated into the four-carbon molecule, oxaloacetic acid. Earlier, in the 1950s, an American scientist, Melvin Calvin, uncovered a chemical pathway for photosynthetic carbon fixation that came to be called the Calvin Cycle. In his scheme, carbon was first fixed into the three carbon compound, 3-phosphoglyceric acid (PGA). He was awarded the Nobel Prize in for this discovery in 1961. For a time the Calvin Cycle was thought to be the only carbon fixation pathway. Hatch and Slack's surprising discovery proved otherwise. Their efforts were aided by the work of G.O. Burr and his colleagues at the Sugar Cane Research Institute in Hawaii who reported that PGA was not the first photosynthetic product in sugarcane plants, a species that is especially efficient in photosynthesis.
The experimental procedure for discovery of early products involved exposing photosynthesizing plant material for a very brief time to carbon dioxide enriched with radioactive 14C. The plant cells were then extracted and analyzed by chromatography to determine what chemicals had incorporated radioactivity carbon from the carbon dioxide. Burr's laboratory found that PGA was not the first compound to become radioactive, but it remained for Hatch and Slack to sort out details of the newly discovered pathway.
The two fixation mechanisms are sometimes called the C3 and C4 pathways based on the number of carbons in the initial product. Each pathway has its own unique catalyzing enzyme and acceptor molecule. The molecule that accepts the carbon in the C4 pathway is a three-carbon compound called phosphoenolpyruvate (PEP) and the catalyzing enzyme is PEP carboxylase. The acceptor molecule for the C3 pathway is a five-carbon sugar called Ribulose 1,5 bisphosphate (RuBP), which splits into 2 molecules of PGA after adding a carbon, and the enzyme is RuBP carboxylase. The two carboxylating enzymes use different forms of carbon dioxide as substrate. RuBP carboxylase incorporates carbon dioxide, whereas PEP carboxylase uses the hydrated bicarbonate ion form. The enzymes also differ in their cellular location. RuBP carboxylase is located in the chloroplast, and PEP carboxylase in the cytoplasm. Sugarcane, and other plants that first fix carbon by the Hatch-Slack pathway also use the Calvin cycle in a curious system that involves the uptake of carbon dioxide twice. The complex double fixation process is commonly found in tropical plants and other plants that are highly efficient in photosynthesis. This efficiency may result from PEP carboxylase's greater affinity for carbon dioxide and its ability to assist the less efficient enzyme.
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