THE CHEMICAL WORLD THIS WEEK Wisconsin's institute for enzyme re search, changes in the shapes of mem cation to the study of ^-thalassemia, a brane proteins drive these energy-re hereditary, ultimately fatal anemia af quiring reactions. flicting mainly children of Mediter Dr. Green told the National Acad ranean stock. More important in the emy of Sciences at its annual meeting long run, the discovery portends an in Washington, D.C., that he and his era when scientists can take a bio coworkers—Dr. John H. Young, Dr. chemical look at protein synthesis in George A. Blondin, and Dr. Garret diseased human cells, see what has Vanderkooi—have been led by their gone wrong, and perhaps even treat studies of mitochondria to propose a diseases by direct attack on the genetic conformational model for energy trans translation system. ductions in membrane systems. Mi Studying protein synthesis at the tochondria are organelles within many molecular level, the team tried to cells, where the final steps of respira make hemoglobin chains in the test tion produce energy from the oxida tube by mixing components isolated tion of carbohydrates to carbon diox from living reticulocytes (immature ide. Some of the energy is used to red blood cells). The reticulocyte is move cations and anions against os ideal for such studies, Dr. Anderson motic pressure, across the mitochon says, because it is essentially a sack drial membranes, from regions of low containing only the cell's genetic trans concentration to regions of higher con lation machinery and because it makes centration. Some of the energy is also only one primary end product, hemo used to make ATP, an energy-rich globin. A few chains were made, but molecule that provides the energy to the rate of production was low. move muscles and make proteins. The team then followed up an ob The energy comes initially from a servation made several years ago by series of electron transfer steps as elec Dr. Richard Schweet and coworkers: trons "fair to lower free energy levels that a protein fraction isolated from through a chain of quinones and che ribosomes seemed to stimulate hemo lated transition metals within the mito globin production. Investigating this chondrial membrane. ATP formation fraction, the team found that it con and ion transport are somehow cou tained at least three distinct factors, pled to this flow of electrons, but a each of which was necessary to initiate molecular mechanism for this process hemoglobin synthesis. By adding the has eluded biochemists for decades. initiation factors to the system, they The missing link between electron were able to make hemoglobin at a transport and ATP formation or ion rapid rate. The team also studied the effects of varying the amounts of other compo nents. They found that if one spe cific group of tRNA molecules was left out, the a and β chains of the hemo globin molecule were made at differ ent rates. This meant that different mRNA molecules can be decoded at different rates, depending on the types of tRNA molecules present. Thus, the N H L I scientists have shown two ways to alter protein pro duction in mammalian cell-free sys tems: by varying the amount of one or more of the initiation factors, or by varying the amount or type of tRNA. Now they are working to determine whether defects in either of these mechanisms might be responsible for causing thalassemia.
transport is energy storage in the membrane itself, Dr. Green says. Free energy released during electron trans port may be conserved as proteins in the membrane change their conforma tions to unstable shapes of higher en ergy. If an energized conformation has a free energy sufficiently greater than the free energy of the protein's thermodynamically stable conforma tion, ATP might be formed as the pro tein relaxes to its stable shape. Dr. Green bases this model on a conformational cycle that he and his colleagues observe in electron micro graphs of mitochondrial membranes during stages of ATP formation from ADP (adenosine diphosphate) and in organic phosphate. Membranes and lipid-protein subunits within them both change from "nonenergized" to "energized" conformations as a result of electron transport. When inorganic phosphate is added, Dr. Green ob serves a third "energized-twisted" con formation. When ADP is finally added to form ATP, the membrane returns to the nonenergized shape. Relating ion transport to changes in protein conformation, Dr. Green sug gests , that a conformation change would rearrange amino acid groups at the protein's surface, exposing some and burying others. In particular, amino acids that bind anions might be shifted. Cations would follow this redistribution, Dr. Green says, even into regions of higher concentration.
MEMBRANES:
High-Energy Shapes Biochemists may be using the wrong suppositions when they try to explain ATP (adenosine triphosphate) forma tion and ion transport across mem branes, using the traditional concepts of chemical reactions in homogeneous solutions. In the view of Dr. David E. Green, codirector of University of
Wisconsin's Green Missing link between electron transport and ATP MAY 11, 1970 C&EN 23
