RESEARCH
Second Photosynthesis Mechanism Found Photosynthetic formation of glycolic acid with the help of manganese indicates that plants can fix C02 in two ways A second route for photosynthetic fixation of carbon dioxide apparently exists in plants, according to scientists at Charles F. Kettering Foundation, Yellow Springs, Ohio. Results obtained there indicate that carbon dioxide is directly converted to glycolic acid with the help of manganese. This mechanism, says Kettering's research director, Dr. Howard A. Tanner, exists together with the photosynthetic reaction discovered by University of California's Dr. Melvin Calvin. In the Calvin cycle, carbon dioxide reacts with ribulose diphosphate to form phosphoglyeerie acid. Discovery of the apparent mechanism in autotrophic cultures of various algae and aquatic plants (cultures that
grow on carbon dioxide and light) climaxes Kettering's study of the role of manganese and chlorine in photosynthesis. Previous work done there indicates that autotrophic growth requires traces of both. Main role for manganese seems to be in aiding oxygen production, Dr. Tanner explains. It isn't certain, though, whether manganese produces oxygen directly, or if it increases the rate by raising the efficiency of the photochemical phase. Glycolic acid production takes place during the oxidation-reduction reactions which manganese goes through. Dr. Tanner and his co-workers—Dr. Thomas E. Brown, Dr. Clyde Eyster, and Dr. R. W. Treharne—believe that
Second Photosynthetic Path Joins Calvin Cycle Photochemical Phase
Chemical Phase
Manganese Cycle: TPNH + H+ + HC07+ MnCI+—^ [CHO . ] + TPN+ + MnCI(OH)2 2[CHO • ] ~ + (CHO)2 — • CH2OHCOOH Glyoxal
Glycolic Acid
2 MnCI(OH)2 + 2 H + + CH2OHCOOH —^CHOCOOH + 2 MnC!+ Glycolic Acid 42
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Glyoxylic Acid
+ 4 H20
the metal is alternately oxidized and reduced in a cyclic fashion. Carbon dioxide forms glycolic acid, which goes on to glyoxylic acid, then to sugars and other compounds. No Glycolic Acid Without Mn. The observation that manganese-deficient plants produce very little glycolic acid is what prompted the work leading to the new discovery, says Dr. Tanner. Using heterotrophic (sugar plus darkness) as well as autotrophic (carbon dioxide plus light) cultures with carbon-14 dioxide, the Kettering team finds that most chemical products produced in both normal and manganesedeficient systems are similar. Amounts of alanine, glutamic acid, aspartic acid, sucrose, and phosphate esters are nearly identical. But the amount of glycolic acid depends on the amount of manganese. Since glycolic acid is formed only in the presence of manganese, the Kettering group went after the mechanism by tracing the fate of manganese in the system. The oxidation states of the element are tracked by ESR, electron spin resonance (sometimes called electron paramagnetic resonance). The result: As manganese(II) is oxidized to manganese (III), a free radical appears. Then, when manganic ion is reduced to its original bivalent state, the free radical signal disappears. These studies were done on Chlorella pyrenoidosa. Here's what may be happening chemically during the manganese cycle, says Dr. Tanner. Carbon dioxide (as bicarbonate ion) is reduced to a neutral intermediate, splitting off a hydroxyl ion and oxidizing manganous to manganic ion. The neutral intermediate is the free radical found by ESR and is thought to be a CHO group attached to a larger molecule. The intermediate probably combines with another CHO group to form glyoxal; this rearranges to glycolic acid. The reaction is probably coupled with a phosphate energy transfer to give it a reasonable rate.
Manganic ion, meanwhile, is reduced to manganous ion, perhaps at a more acid location where aerobic oxidation furnishes hydrogen ions to the system, according to Dr. Tanner. Photochemical intermediates for powering the new mechanism are similar to, possibly identical with, those needed for the Calvin cycle. Adenosine triphosphate (ATP) and reduced pyridine nucleotide (TPNPL) are formed in light with the evolution of oxygen. The photophosphorylation and pyridine nucleotide reduction, notes Dr. Tanner, occur independently of manganese. The reactions of the manganese redox cycle are locally irreversible, the Kettering group reports [OJiio } . Sci., 60, 231 ( I 9 6 0 ) ] . That's because the oxidation and reduction of manganese occur at different sites; therefore, at different pH's. And since TPNH is consumed by the reaction, it's irreversibly removed. This helps the whole photochemical system to go forward. Clears Up Puzzle. Existence of a second photosynthetic mechanism clears up some puzzling features of photosynthesis, says Dr. Tanner. For example, he points to the observation (made by several groups, including his own) that the amount of glycolic acid in a given system increases if the carbon dioxide "feed" is cut off. Now, he explains, it's evident that glycolic acid formation is a first order reaction with respect to carbon dioxide concentration. Glycolic acid's oxidation to glyoxylic acid by oxidized manganese would be a second order reaction.
Space Propulsion for the future . . . from the
Amino Acids Shape Polypeptide Structure
Reactors
Los Alamos Scientific Laboratory has the major responsibility for research, development and testing in the AEC-NASA Rover program . . . another of the many investigations at Los Alamos into peacetime uses of nuclear energy.
Structure of units is key to helical, extended, or random forms The nature of the amino acids composing a synthetic polypeptide chain determines molecular conformation of the material, according to Dr. Elkan R. Blout and associates at Children's Cancer Research Foundation and Harvard Medical School. They report that there are at least two classes of natural amino acids: those forming a-helical polypeptide structures and those giving either ($ (extended) or random (nonperiodic) conformations []ACS, 82,3787 ( I 9 6 0 ) ] . Both the extended (pleated-sheet)
family of Nuclear
PHOTO: First field test of a KIWI nuclear propulsion reactor.
For employment information write: Personnel Director Division 60-92
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