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In a tour de force in this issue of PNAS, Bourgeois et al. (1) have used 2.2-ns x-ray pulses to observe the motion of carbon monoxide (CO) through myoglobin (Mb) and the relaxation of the protein from 3.2 ns to 3 ms after photodissociation. This work follows the pioneering experiments of Moffat and collaborators (2). It demonstrates how far advances in x-ray sources and computers have moved the field of protein structure determination since the path-breaking work of Kendrew et al. and Perutz (3, 4). The recent breakthrough shows how careful studies of proteins, in particular of Mb, impact many different fields. Mb is a monomeric protein that gives muscle its red color. Thirty years ago the textbook function of Mb, storage of dioxygen at the heme iron, was considered to be simple, fully understood, and consequently boring. Mb was essentially written off as a topic of serious research. Since then, the situation has changed: Mb is no longer fully understood. It plays roles other than O2 storage, serves as a prototype for complex systems, and yields insight into the chemistry and physics of soft matter and of chemical reactions. Mb consists of 153 amino acids that fold into a structure that is ≈3 nm in diameter, as depicted in Fig. 1a. Fig. 1b gives a schematic cross section through Mb that shows the active center: a heme group with a central iron atom. Surrounding the heme group are five cavities, the heme cavity and four cavities denoted by Xe1 to Xe4 (5). The amino acids lining the xenon cavities are much more conserved than other amino acids in mammalian Mb; they are thus likely to be important for function. A major part of Mb is taken up by amino acids that do not appear to …

Myoglobin: The hydrogen atom of biology and a paradigm of complexity