A recent paper by HMS researchers and their colleagues reports the crystal structure of a pivotal enzyme bound to its substrate protein, revealing a specific interaction in the inflammatory cascade of bronchial asthma. This biological snapshot could facilitate further research toward improved asthma therapies.
Appearing in the Aug. 2 issue of Nature, the work was conducted by K. Frank Austen, the AstraZeneca professor of respiratory and inflammatory diseases at HMS and Brigham and Women’s Hospital, and Yoshihide Kanaoka, HMS assistant professor of medicine at BWH; they collaborated with the Miyano laboratory at RIKEN in Japan. Using X-ray crystallography, the researchers delineated the structure of the enzyme LTC4 synthase, which manufactures asthma-causing cysteinyl leukotrienes (cys-LTs).
“The exciting thing is, the crystal structure really does explain how the enzyme works,” said Austen.
The enzyme’s job is to unite LTA4, an elusive lipid produced by immune cells, with the tripeptide glutathione. When these two compounds are joined, LTA4 becomes the inflammatory compound LTC4.
This action is standard enzyme behavior. What sets LTC4 synthase apart from other glutathione transferases is its remarkable specificity. The enzyme is composed of three identical monomers, with the metabolically abundant glutathione wedged between adjacent units, ready to be attached to the LTA4 when it arrives. An arginine residue of each monomer activates the glutathione for this interaction, a step the researchers did not expect.
The arrangement, said Austen “is not only optimal, it’s astonishing,” for its capacity to synthesize LTC4 efficiently. The LTA4 is trapped in a somewhat hydrophobic area to provide stability during the conjugation with glutathione. The two adjacent monomers play an active role in uniting LTA4 and glutathione, so the enzyme requires a minimum of two monomers to do its job.
But why bother having three monomers when all you need is two? “What the trimer gives you is three effective sites,” said Austen, “because every monomer works on both of its sides.” Think of it as a troupe of three jugglers versus two jugglers. The two jugglers can only get one volley of balls going between them—three jugglers could (theoretically) get three volleys going.
Once LTC4 is produced, it proceeds to interact with specific smooth muscle receptors and induces the inflammation and constriction associated with bronchial asthma. These receptors are also found in several other areas of the body, such as leukocytes, vasculature, heart, spleen, and central nervous system. Thus, while “cys-LTs started with asthma, they may have more implications,” Kanaoka said.
Now that the structure of the zealous enzyme has been exposed, drugs might be created to block its activity, and as a result, diminish the entire constellation of health problems tied to it, including asthma, atherosclerosis, and pulmonary fibrosis.
Meanwhile, Austen and Kanaoka are preparing to determine an even clearer depiction of the enzyme’s structure. “We want to know how it looks in the nuclear membrane,” Kanaoka said.
