Seeing a carbon-monoxide molecule escaping from the clutches of the Myoglobin protein

In a stunning tour-de-force experiment (pdf), the ejection of carbon dioxide (CO) from a molecule of myoglobin is directly measured using time-resolved X-ray crystallogaphy.

Myoglobin, also known as the hydrogen atom of protein structures, normally binds and transports oxygen molecules (O) around blood-stream. The actual binding to O is done by a heme group trapped in the heart of the myoglobin structure. One of the remarkable things is that by itself, a heme molecule will prefer to bind to CO rather than O. But when the heme is trapped in myoglobin, the myoglobin structure ejects the CO from the heme if it happens to bind.

This following movie is a direct measurement of the ejection of CO from the heme in a molecule of myoglobin. Incredibly sensitive – the movie occurs in 0.00000316 s of real time – time-resolved X-ray crystallography is used to directly observe changes in the electron density of the CO, the heme and myoglobin:

The horizontal blob in the middle is the heme group, and the head sticking out of the top of the heme group is the carbon monoxide molecule.

The color scheme is thus: white corresponds to the electron density that is seen throughout the whole 3.16 micro-seconds of simulation. The purple that remains is the trace of the original electron density. The green is the movement of electron density into the final state. What is remarkable is the amount of movement (as indicated by the change of electron density) that comes from the carbon monoxide molecule escaping from the grip of the heme group. You are directly witnessing protein chemistry in action.