Abstract

A critical feature of the biological function of heme proteins is the direct coupling of protein motion to the process of binding exogenous ligands to the heme. In carbonmonoxymyoglobin (MbCO), a substantial, specific conformational relaxation is associated with the transition from the ligated to the unligated form of the protein. The analogous tertiary structural changes of the monomer heme subunits of hemoglobin ultimately lead to the R→T quaternary structural transition, the allosteric control mechanism of O₂ binding efficiency [1]. We have studied these processes on the earliest timescales, using picosecond, time-resolved infrared (TRIR) spectroscopy. It has long been known that infrared spectra in the amide region are sensitive to protein secondary conformation [2]. Recent advances in equipment and techniques have permitted researchers to quantitatively predict secondary structures from infrared spectra [3,4], particularly in the amide I region [4]. Therefore, it is now possible to study protein motion in time-resolved experiments on dynamics and function. The ligation reactions of small molecules such as CO with the heme site of Mb exemplify the mechanisms available to O₂. CO is an ideal candidate for initial time-resolved IR experiments in the amide I region because it is easily photolyzed, little geminate recombination [5], and the structure of both MbCO and unligated Mb have been studied by crystallographic methods [6]. TRIR has already been applied to the stretching vibrations of the bound and free CO ligand [7,8]; dynamics of the protein, however, have yet to be probed by TRIR spectroscopy of the protein vibrations. Here we report results on the motions of the protein in response to ligation reactions, probed in the amide I region centered about 1650 cm^-1.

© 1994 Optical Society of America