AMER. ZOOL., 20:5-6 (1980)
Respiratory Pigments: Overview1
J . BONAVENTURA
Marine Biomedical Center, Duke University Marine Laboratory,
Beaufort, North Carolina 28516
AND
S. C. WOOD
Department of Physiology, University of New Mexico,
School of Medicine, Albuquerque, New Mexico 87131
Respiratory proteins process the materials involved in aerobic respiration; e.g.,
oxygen, carbon dioxide, protons and electrons. There are two basic types, the gas
transporters (hemoglobins, myoglobin,
hemerythrin, etc.), and electron transporters (cytochromes). Why are these proteins
of interest? Perhaps it bears repeating that
the electron transporting proteins act at
the base of the pyramid of all of biology in
the stepwise process which generates ATP
from ADP and high energy compounds.
Complementing these are the transporters
of oxygen. Without them, the electron
transport chain in oxygen-poor tissues
would stop for lack of a terminal electron
acceptor, and large multicellular organisms could not exist in their present form.
Hemoglobins and myoglobins were the
first studied oxygen transporting proteins.
This occurred probably because of their
universality in vertebrates and their wide
distribution in other groups of organisms.
Hemoglobin has become a prototype, representing a protein whose functional properties are under metabolic control. It is
probably the most extensively studied protein in existence. Its structure is known to
atomic dimensions, and the structural basis
of its functional properties is consequently
amenable to residue by residue analysis.
The molecular mechanisms underlying its
function are more thoroughly understood
than for any other protein. Generalizations based on hemoglobin studies have
been made concerning enzymes, other biological macromolecules, and even cells.
From comparative studies of hemoglobins have come many ideas, examples and
demonstrations of how molecular control
of protein function is effected. In this symposium, "Respiratory Pigments," researchers from the United States and Europe
described "the state of the art" in the area
of oxygen transporting proteins. This
large group of proteins was discussed in
detail and the audience was brought into
contact with much of the current thinking
about these proteins. Additionally, large
numbers of unanswered questions were
posed. These questions reveal not only the
interests of the symposium speakers, but
also probably forecast the areas where new
knowledge in this field will be sought for
the next few years. The following questions, grouped in classes according to different kinds of proteins, illustrate these
probable research directions.
MAJOR UNRESOLVED QUESTIONS
Hemocyanins
1. Is there any degree of homology between hemocyanins of arthropods and
molluscs?
2. Is there a physiological significance to
the subunit diversity in hemocyanins
(or is the diversity present for assembly aspects alone)?
3. What is the relationship between what
is seen in vitro (in terms of molecular
control) and the in vivo function of
hemocyanin?
4. Is the messenger coding for molluscan
hemocyanins a message coding for a
400,000
dalton, or a 50,000 dalton
1
From the Symposium on Respiratory Pigments prepolypeptide?
sented at the Annual Meeting of the American So5. What is the molecular mechanism
ciety of Zoologists, 27-30 December 1978, at Richwhich underlies the modulation of hemond, Virginia.
J . BONAVENTURA AND S. C. WOOD
mocyanin oxygen affinity by anion
binding?
6. Can the differences in the oxygen affinities and chloride sensitivities of
electrophoretically distinct subunits of
arthropod hemoglobins be correlated
with the different roles they play in
assembly of the oligomers found in
vivo?
7. What is the correlation between AH
for oxygen binding (under physiological pH) and the habitat?
8. Are there organic phosphate analogs
in animals possessing hemocyanins?
Hemerythrins
1. Is the oxygen affinity of hemerythrin
subject to allosteric modulation?
2. What is the functional role for occurrence of hemerythrins as oligomeric
proteins?
3. What is the functional role (if any) of
polymorphic hemerythrins occurring
in individual animals?
Invertebrate hemoglobins
1. What is the mode of synthesis of subunits in hemoglobin showing multidomain structures?
2. What is the role, if any, of carbohydrate in structure, function or polymeric assembly?
3. Why is there one heme per 24,000
grams of protein instead of one heme
per 17,500 grams of protein?
4. Is there a 1:1 ratio of the number of
polypeptide chains to the number of
hemes?
5. What is the molecular nature of the
effect of divalent cations on the structure of these hemoglobins?
6. What is the nature of the subunit surfaces which favor subunit-subunit
binding?
7. In lower organisms, is the "burden" of
adaptation borne by molecules instead
of organs?
2.
3.
4.
5.
6.
7.
8.
9.
10.
11.
12.
13.
14.
Vertebrate hemoglobins
1. Energy metabolism of the red blood
cell; how does it support circulation
15.
and what is its function in oxygen
transport?
What is the meaning of the very high
ATP in the red blood cells of Squamata?
What is the mechanism of formation
and the significance of extremely high
ATP in red blood cells of early embryos?
Is there a combination of organic
phosphate requirements in teleosts?
Is the level of inositol penta phosphate
manipulable in avian red cells or in
other cells where it is found?
To what extent do electrostatic interactions govern the pH sensitivity of
oxygen binding by vertebrate hemoglobins?
How do the structural properties of
a2y32 tetramers allow for interactions
that do not occur in /34 tetramers, and
how important are the functional differences between the two types of
chains?
How does the indirect effect (via the
Donnan distribution of hydrogen ions
and intracellular pH) interact with the
direct (allosteric) effect of organic
phosphates on oxygen affinity?
What is the physiological significance
of low oxygen affinity of blood in
many reptiles in view of the variable
intracardiac shunt and resulting low
arterial saturation?
How can the intraerythrocyte enzymes
that are sensitive to ATP (e.g., PFK)
operate in the presence of such large
ATP concentrations?
What is the molecular basis for thermal and pH insensitivity?
What is the structural basis for the
Root Effect?
Is there any physiological significance
to the genetic variability of hemoglobins?
What is the role of genetic background for metabolic enzymes (e.g.,
LDH, Hb, PFK) in the regulation of
O2-Hb affinity and natural selection?
What, if any, is the role of multiple
hemoglobins within fish erythrocytes?