September 29, 1999
Researchers Discover Hemoglobin's Enzymatic Nature
The nematode worm Ascaris lumbricoides, an intestinal
parasite that infects one billion people worldwide, has uncommonly
strong hemoglobin that binds to oxygen 25,000-times more tightly than
does human hemoglobin. The affinity of Ascaris hemoglobin for
oxygen is so powerful that many researchers believed the molecule could
not possibly play a role in respiration.
Learning the true function of Ascaris hemoglobin and
understanding how and why the worm's hemoglobin embraces oxygen
molecules so tightly have been great challenges, said Howard Hughes
Medical Institute (HHMI) investigator Jonathan Stamler. "Why would a
molecule that functions as an oxygen carrier bind oxygen so tightly
that the oxygen molecules would never come off?"
“Why would a molecule that functions as an oxygen carrier bind oxygen so tightly that the oxygen molecules would never come off?”
Jonathan S. Stamler
Two collaborating groups of HHMI scientists have ferreted out the
secrets of this oxygen-hungry molecule by using a variety of
biochemical techniques and by taking physical measurements of the
hemoglobin inside the worm. In so doing, they have identified a
biochemical link between the hemoglobins used by primordial bacteria to
detoxify themselves of atmospheric gases and the modern mammalian
hemoglobins that underpin respiration.
The findings not only open a window to understanding how a wide
variety of organisms evolved to utilize or guard against atmospheric
gases such as oxygen and nitric oxide, but they may also provide new
ideas for ways to starve cancerous tumors of oxygen.
In the September 30, 1999, issue of the journal Nature,
research groups led by Stamler, an HHMI investigator at Duke
University, and Daniel
Goldberg, an HHMI investigator at Washington University in St.
Louis, describe how Ascaris hemoglobin acts as an enzyme to
neutralize oxygen, which in high doses is toxic to the worm.
"In humans, hemoglobin delivers oxygen and nitric oxide to tissues,"
said Stamler. According to the new data, however, the nematode
hemoglobin uses nitric oxide to trigger an enzymatic reaction that
actually consumes oxygen. These data suggest that the primary role of
the nematode's hemoglobin is to destroy oxygen. This would make sense,
Stamler explained, because the Ascaris parasite has a low
tolerance for oxygen. "The worms can deal with oxygen, but they don't
really like it," he said.
Even in low-oxygen environments such as the human intestine, where
the worms live, oxygen molecules do seep in. These must be neutralized,
which the worm's hemoglobin does by grabbing and consuming oxygen
molecules in a unique enzymatic reaction driven by nitric oxide.
Using spectroscopic techniques, the scientists studied how the
nematode hemoglobin acts in the presence of different concentrations of
oxygen and hemoglobin. The results of these experiments led the
researchers to propose that Ascaris destroys oxygen via a
10-step chemical reaction.
The key to this process, says Stamler, lies with the exact
positioning of a single sulfur-containing amino acid within the
oxygen-binding pocket of the nematode's hemoglobin. "If this amino acid
is on one side of the pocket, the hemoglobin uses nitric oxide to
destroy oxygen. If it's on the other side of the pocket, as it is in
mammalian hemoglobin, nitric oxide acts as a regulator of oxygen
delivery," explained Stamler.
The discovery places Ascaris hemoglobin at a unique
evolutionary juncture between primitive hemoglobins that evolved in the
first living creatures on the planet—when the Earth's atmosphere
was composed mostly of nitric oxide—and modern hemoglobin found in
mammals and birds.
"In the primordial atmosphere, nitric oxide came before oxygen,"
Stamler said. "It was probably there before any life form, and the
first bacteria needed a way to protect themselves from nitric oxide."
Nitric oxide, like oxygen, can destroy many biologically important
molecules if allowed to roam freely throughout an organism.
These first hemoglobins were likely enzymes that consumed nitric
oxide and had nothing to do with oxygen, Stamler noted. About 450
million years ago, as Earth's atmosphere began to contain more oxygen,
mammalian hemoglobin evolved the ability to carry oxygen molecules to
tissues and cart away the waste gas carbon dioxide.
In recent years, studies from a number of laboratories have
identified a variety of roles for hemoglobin. Stamler, for example, has
demonstrated that when hemoglobin binds to nitric oxide it causes blood
vessels to dilate. But demonstrating that the nematode hemoglobin can
function as an enzyme that catalyzes a series of chemical reactions is
a first.
Moreover, the researchers' discovery of the worm's ability to use
nitric oxide to kick start an enzymatic reaction helps explain the
complicated interplay of hemoglobin and the three gases involved in
respiration, oxygen, carbon dioxide and nitric oxide.
"The worm is evolving the first indications of a respiratory
function involving oxygen concentrations," Stamler said. "The worm is
controlling oxygen concentrations by using nitric oxide just as we do
— only with a different outcome. Instead of regulating its delivery to
tissues, the worm's hemoglobin destroys unneeded oxygen."
Looking beyond the evolutionary implications of the findings,
Stamler is excited by the prospect of using Ascaris hemoglobin
as a molecular scaffolding upon which to design new therapeutic agents
that can starve cancerous tumors of the oxygen they need to survive.
"This is the first enzyme that eliminates oxygen," he said. "The hope
is it could be used therapeutically," much like the so-called
antiangiogenesis agents now being tested for their ability to shut off
the blood supply to tumors.
Other authors of the Nature paper include Dena M. Minning of
HHMI at Washington University; and Andrew J. Grow, Joseph Bonaventura,
Rod Braun and Mark Dewhirst of Duke University.
|
