Table of Contents
Botulism Bug Says No to Nitric Oxide
Finding provides key to molecule's crucial role in the human cardiovascular
and cerebrovascular systems
Pierre Nioche, Ph.D., seated, and C. S. Raman, Ph.D., examine a protein crystal using a microscope attached to the X-ray diffraction equipment in Raman's lab at the UT Medical School at Houston.
Photo by Scott Merville
A deadly bacterium's defense against a mortal molecular enemy illuminates the origins and structure of a vital protein involved in human cell signaling, University of Texas Medical School at Houston scientists reported in the Nov. 26 issue of the journal Science.
The paper also details how evolution transformed one of nature's simplest molecules, nitric oxide (NO), from a poison to anaerobic bacteria into a beneficial signaling molecule in higher animals. It further offers an explanation for how the decades-old practice of treating meat with sodium nitrite prevents life-threatening food poisoning known as botulism.
Discovering how botulism-causing Clostridium botulinum detects NO sheds light on how this gaseous molecule is recognized by its receptor protein in humans to govern crucial processes in the cardiovascular and cerebrovascular systems, said senior author C. S. Raman, Ph.D., director of the Structural Biology Research Center in the Medical School and assistant professor of biochemistry and molecular biology.
"We started by identifying the protein that the botulism bug uses to detect and evade NO," Raman said. "What we have ultimately shown is how this protein evolved from being part of a bacterial protective mechanism into one that enables humans to benefit from NO."
In human beings, NO binds to a receptor called soluble guanylyl cyclase to make cyclic GMP, a molecule that improves blood flow by relaxing blood vessel walls. Ferid Murad, M.D., Ph.D., professor and chairman of the Department of Integrative Biology and Pharmacology at the Medical School, discovered this signaling process in 1977 when he found that NO makes nitroglycerine beneficial to heart patients by activating guanylyl cyclase and dilating blood vessels. That research earned him the 1998 Nobel Prize for Medicine or Physiology.
Since Murad's early work, NO has been found to govern many other vital biological functions and became the basis for medications that treat erectile dysfunction, pulmonary hypertension and shock.
However, the structural details of soluble guanylyl cyclase have remained elusive for nearly three decades. The protein is extremely difficult to crystallize for structural analysis.
"Raman's approach has been that while we are still trying to get the human enzyme crystallized, let's go to lower forms, generate crystals and collect information that can help us someday with the mammalian enzyme," said Murad, who also directs the Brown Foundation Institute of Molecular Medicine for the Prevention of Human Diseases and holds the John S. Dunn Sr. Distinguished Chair in Physiology and Medicine. "He has succeeded with these important findings."
Using high-resolution X-ray crystallographic methods, Raman's team determined an unusual three-dimensional architecture for the bacterial protein, which has a striking similarity to the human NO receptor.
"This structure - the result of four years of hard work, creative efforts and perseverance - has provded us the knowlege base to attack" the structure of the human NO receptor, Raman said. "If you know the structure of a protein, then you can design molecules that target specific regions on the protein." That may allow the design of drugs to combat vascular disease.
And don't forget meat protection. The research team showed that C. botulinum uses the sensor protein to detect nitric oxide and then to flee its presence. "It's a strange topic for a strict vegetarian who has never touched meat in his life," Raman said.
First author Pierre Nioche, Ph.D., joined Raman's laboratory in 2001 as a postdoctoral fellow and has since become a research assistant professor of biochemistry and molecular biology. Co-authors include: Vladimir Berka, Ph.D., senior research associate, and Ah-Lim Tsai, Ph.D., professor, both of the Medical School Division of Hematology; and from the United Kingdom, Julia Vipond of the Health Protection Agency in Salisbury; and Nigel Minton of the Center of Biomolecular Sciences and Institute of Infection, Immunity and Inflammation, University of Nottingham.
Raman, Murad and Tsai also hold faculty appointments at the UT Graduate School of Biomedical Sciences at Houston.
Raman's research is funded by the Pew Scholar Award, the Robert A. Welch Foundation, and the National Institute of Allergy and Infectious Diseases. He also is the recipient of a Young Investigator Award from the Nitric Oxide Society.
By Scott Merville, Public Affairs