7.2.8 Conclusions,
Myoglobin and hemoglobin have been studied exhaustively since chemists,
biochemists, and biologists realized their common and abiding interest in these
biomolecules. In Section 7.2, a brief review of the major knowledge categories
has been presented. No quantitative information has been presented on
kinetic or thermodynamic aspects of O 2 binding, although much is known.
Reference 9 , for instance, gives an excellent review of quantitative kinetic
and thermodynamic information on myoglobin, hemoglobin, and their model
compounds.
While much has been learned about myoglobin and hemoglobin, many
controversies and uncertainties remain. It is certain that myoglobin stores
dioxygen in muscle tissue, whereas hemoglobin carries dioxygen through the
bloodstream to all parts of the body. We know the primary, secondary, tertiary,
and quaternary structures of the protein matrix with a great deal of certainty
and we have much information about the cooperative nature of dioxygen
binding to hemoglobin. Most known instrumental and analytical techniques
have been used in the study of these important dioxygen carriers. X - ray crys-
tallography, infrared and high - fi eld NMR have yielded much structural infor-
mation but have not been able to unequivocally resolve a major remaining
uncertainty: Why does O 2 preferentially bind to hemes in spite of the known
greater affi nity of Mb and Hb for CO over O 2? Consensus will probably arise
from a blending of several explanations: hydrogen bonding and electrostatic
considerations that favor O 2 coordination and steric considerations that dis-
favor CO coordination. Certainly the questions and answers will make for
interesting future reading.
7.3 Introduction to Cytochromes,
Cytochromes occur in nature in immense variety. These proteins are usually
involved in electron transfer reactions and contain a variety of heme cofactors
that carry the electrons being transferred and serve as catalytic sites for the
redox chemistry. The metalloporphyrin heme cofactors, normally carrying an
iron ion that cycles between the Fe(II) and Fe(III) oxidation states, categorize
cytochromes according to the type of heme – porphyrin ligand they contain.
Some cytochromes listed in the Jena Library of Biological Macromolecules
http://www.imb-jena.de/IMAGE.html are cytochromes a, b, c, f, and P450. The
molecules in this database are those that have structural data listed in the
Protein Data Bank (PDB) or Nucleic Acid Database (NDB). (As an example,
after accessing the website, choose search, Main Search page, click on
SWISS - PROT entry description and enter “ cytochrome c ” in the search box.
In mid - 2006, this search retrieved 714 cytochrome c, cytochrome c oxidase,
cytochrome c peroxidase, cytochrome c 1 , and so on, entries that include refer-
ences to the structural data available.)
INTRODUCTION TO CYTOCHROMES 359