A University of Hawaii research team has discovered a new class of myoglobin-like proteins in ancient microorganisms. In animals, myoglobin (found in muscle) and its close relative hemoglobin (the main protein in the red blood cell) play an essential role in oxygen transport and storage. The newly identified proteins may be the evolutionary ancestors of proteins involved in oxygen sensing as well as transport and storage.
The findings, which appear in the Feb. 3 issue of the British journal Nature, add another piece to the puzzle of when and how life arose and evolved on earth.
The research team that made this discovery is headed by Maqsudul Alam, associate professor of microbiology, and Randy Larsen, associate professor of chemistry, both of the University of Hawaii at Manoa's College of Natural Sciences. Their team cloned the genes expressing these proteins, which were then purified and characterized. The newly discovered proteins from the ancient microorganism Halobacterium salinarum and the more widely known bacterium Bacillus subtilis share similar properties. These proteins help sense oxygen, allowing the organism to find a more favorable oxygen environment.
"These proteins may help to understand how sensory systems evolved in higher organisms," said Alam, lead author on the Nature paper. Cells must continually sense their changing environment, interpret the sensation and adapt to new surroundings. In multicellular organisms (including humans), sensing is accomplished using specialized sense organs as well as complex mechanisms to communicate this information to other parts of the organism.
"The presence of oxygen in the earth's atmosphere some 1 to 2 billion years ago was both a blessing and a curse," Alam says. Oxygen is an energy-rich molecule that can provide an energy source for cellular function. However, oxygen can also be highly toxic. The challenge for bacterial cells is to sense, capture and store the oxygen for energy production without suffering the hazardous side effects. The newly discovered heme proteins appear to be responsible for oxygen sensing. Both Halobacterium salinarum and Bacillus subtilis also contain other heme proteins that convert the oxygen into useable energy for the cell.
"This finding advances efforts to trace when and where the evolutionary division between plant, animal and bacteria occurred, as well as how the resulting proteins evolved specific functions in different species," Alam explains. "In addition, it could suggest when and how life began and provide ways to trace the presence of life elsewhere in the Universe."
The research team includes graduate students Shaobin Hou and Wesley Riley of the Department of Microbiology at the University of Hawaii. Other collaborators on this project included Professor George Ordal and graduate students Ece Karatan and Mike Zimmer of the Department of Biochemistry at the University of Illinois, Urbana, and former University of Hawaii Postdoctoral Research Fellow Dmitri Boudko. Funding for the research was provided by a CAREER grant from the National Science Foundation.
The Evolution of Hemoglobin -- a backgrounder
About 3.8 billion years ago, the first organisms appeared on the young planet Earth. The were able to use the water vapor, nitrogen, methane and ammonia that made up Earth's atmosphere for food and energy, probably through a process facilitated or catalyzed by metals such as iron and magnesium.
Between 3.3 and 3.5 billion years ago, cyanobacteria (blue-green algae) appeared. These single-celled organisms had the ability to convert energy from the sun into chemical energy through photosynthesis using hydrogen sulfide (H2S).
Between 1 and 2 billion years ago, some bacteria adapted to use water (H2O) in photosynthesis. Oxygen, which is released as a byproduct of H2O photosynthesis, appeared in Earth's atmosphere.
About 500 million years ago, hemoglobin and myoglobin proteins evolved.
SIGNIFICANCE OF OXYGEN
Oxygen an energy-rich basis of life; it is also toxic to some cell components. With the ability to bind oxygen, molecules known as hemoproteins may have initially played a sequestering role, protecting their host cells from the O2 toxin. Scientists believe the role of these hemoproteins eventually expanded to provide a mechanism for the capture, transport and storage of oxygen used in respiration.
Oxygen-handling proteins in vertebrates, invertebrates, plants, bacteria and some other unicellular organisms are all related. Animal, plant and bacteria hemoglobins are remarkably similar both in structure and in the sequence of amino acids from which they are built. The remarkable similarity suggests that a common ancestor protein was present in an ancestral organism before the evolutionary divergence of eukaryotic (nucleus-containing plant and animal cells) from eubacterial life forms. What has changed is the regulation of their function-when and how they act.
Proof that a common ancestor protein exists (which is suggested by the Nature paper) would allow scientists to explore both when the evolutionary split occurred and how changes in the protein occurred. Researchers study evolution by tracing the amount and sequence of change in the genetic coding for specific traits. In the case of hemoprotein, determination of the evolutionary trail would further investigations both of when the plant/animal and eubacteria split occurred and how changes in regulation generated the different functions that hemoglobins play in diverse species.
Myoglobin, a member of the hemoglobin family found in vertebrates, gives muscles their red color. It stores oxygen for use by muscles.
Hemoglobin is a protein in the blood that gives red blood cells their color. It binds oxygen and delivers it to tissues throughout the body. It is also found in the root nodules of some plants.
Aerotaxis transducers is the name given the new protein found in bacteria and archaea, reflecting that it is involved with air and movement. The myoglobin-like protein contains both a sensory module that detects the presence of oxygen, and a transmitter that signals the organism to respond.
Archaea, which means ancient, is the name given to the kingdom that contains microorganisms that live in harsh environments (including the early Earth). The two other kingdoms are Eukaryot, which includes plants and animals, and Bacteria.
"The Evolution of Hemoglobin" by Ross Hardison, professor in the Department of Biochemistry and Molecular Biology at Pennsylvania State University, published in American Scientist (Vol. 87, March-April 1999)
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