MYOGLOBIN AND HEMOGLOBIN 345
as well as dioxygen binding and release. (Section 2.3.4 discusses the practice
and utility of site - directed mutagenesis.) In late 2006, 381 different data depo-
sitions of X - ray and NMR solution studies for deoxy - , oxy - , carbonmonoxy - ,
nitroso - , and cyano - variants of hemoglobin and its mutant congeners were
available in the PDB.
The Figure 7.2 PDB: 4HHB structure of human deoxyhemoglobin was
refi ned at 1.74 - Å resolution using data collected from a synchrotron X - ray
source. The crystallographic R - factor is 16.0%, or 0.16. The estimated error in
atomic positions is 0.1 Å overall, 0.14 Å for main - chain atoms of internal
segments, and 0.05 Å for the iron atoms. The geometry of the iron – nitrogen
complex closely resembles that of the deoxymyoglobin. In deoxyhemoglobin,
the distances of the iron from the mean plane of N(porphyrin) are 0.40(5) Å
and 0.36(5) Å , respectively, at the alpha and beta heme centers, in contrast to
the corresponding distance of + 0.12(8) Å and − 0.11(8) Å in oxyhemoglobin
(see Tables 7.1 and 7.2 and reference 6 ). The Fe – N Im (proximal histidine N ε 2 )
bond length is 2.12(4) Å and the Fe – N p (porphyrin nitrogen) bond length
is 2.06(2) Å , in good agreement with data presented in Table 7.1 for
deoxyhemoglobin.^5
7.2.1 Myoglobin and Hemoglobin Basics,
Myoglobin (Mb) is a globular monomeric protein containing a single polypep-
tide chain of 160 amino acid residues (MW 17.8 kDa) made up of seven
Figure 7.2 Quaternary structure of deoxyhemoglobin tetramer (PDB: 4HHB). Visual-
ized using The PyMOL Molecular Graphics System and ChemDraw Ultra, version 10.0.
(Printed with permission of Delano Scientifi c, LLC and CambridgeSoft Corporation.)
(See color plate)
α 2 α 1β 2 β 1