Science in Christian Perspective
Hemoglobin Structure and the
Biogenesis of Proteins.
Part II. Significance of
Protein Structure to the Biogenesis of Life
GORDON C. MILLS
Dept. of Human Biological Chemistry and Genetics
University of Texas Medical Branch Galveston, Texas
From: JASA 27 (June 1975): 79-82.
In Part I (Journal ASA, March 1975), the author has reviewed structure-function
relationships for the hemoglobin molecule, with particular emphasis
on the properties
of mutant hemoglobins of man, In Part II, the relation of these
studies to theories
for the biogenesis of life is discussed. It is concluded that protein function
is a consequence of a unique arrangement of the individual amino
acids, and that
this arrangement could not be achieved by chance. It is noted that
data on protein
stability is not in accord with views that proteins might survive for extremely
long periods of time. Adverse effects of inactive protein molecules on cellular
function are also noted. The data strongly favor the view that
"design"
or "intelligence" must be involved in the beginning of life.
The review of properties of abnormal hemoglobins suggests several aspects that
are significant in relation to theories for the biogenesis of life. A
tenet advanced by various workers, and especially by Fox and his coworkers', has been that a
random combination of amino acids would produce a protein with minimal, but adequate function. Presumably, this protein, in the absence of degradative
organisms, would remain functional until all other proteins, nucleic
acids, etc.,
necessary for primitive life were also produced. Among the implications of this
hypothesis are:
(1) The function of a protein as a catalyst, oxygen carrier, etc., is
not uniquely
dependent upon amino acid sequence. Also numerous bonds other than the peptide
bond linking the amino group and the C1-carboxyl group could he tolerated. For
example, peptide bonds might utilize the amino group of lysine, the C4-carboxyl
group of aspartic acid or the C5--carboxyl group of glutamic acid.
(2) In the absence of organisms or enzymes produced by organisms, a
protein molecule
would retain functional activity for long periods of time (thousands
of years?).
(3) All of the functionally inactive protein molecules in the protein
"soup"
would not adversely affect the action of those proteins that did have
functional
activity.
(4) Minimal functional activity of the various essential proteins would be all
that would he necessary to sustain life.
(5) Out of all the randomly produced protein molecules, one is
selected and ultimately
serves as the archetypal globin chain, the precursor of the various polypeptide
subunits of globin for all living organisms which utilize a heme protein as an
oxygen carrier.
Protein Function and Amino Acid Sequence
Let us consider the relation of protein function to amino acid sequence. Is it
plausible to suggest that a random arrangement of amino acids would produce a
protein that would function in the transport of oxygen as does hemoglobin? The
studies summarized earlier indicate that the polypeptide chains making up the
hemoglobin molecule are indeed unique, and that the properties of
these polypeptides
are a consequence of their amino acid sequence. A recent tabulation indicated
that 41% (49 out of 112) of the known hemoglobin mutants have
impaired functions2.
For mutants where the amino acid substitution involved an internal portion of
the molecule or contacts between subunits, 89% (41 of 40) had
impaired function.
It seems likely that the means utilized for detection of mutant
hemoglobins would
introduce some bias toward detecting hemoglobins with abnormal
function. Nevertheless,
the conclusion is clear that a single amino acid change in the
hemoglobin molecule
will, in a high percentage of eases, result in diminished funetiono.
A unique structure for hemoglobin is also indicated if one examines the known
variations in amino acids at each of the positions of the beta, gamma and delta
chains of the vertebrate hemoglobins3. There are 40 positions in the
poly peptide
chain where there may he 2 alternative amino acids, 23 positions with
3 alternatives,
13 positions with 4 alternatives, 11 positions with 5, 5 positions
with 6, 3 positions
with 7, and 1 position with 8. Amino acids in the remaining 50
positions are identical
in the species studied. Theoretically, there could he 20 possible alternatives
for each position in the molecule. As hemoglobins from additional species of vertebrates
are studied,
the number of alternatives for amino acids at each position of the polypeptide
chain will undoubtedly be increased somewhat. Nevertheless, the trend
of the data
clearly supports the view that there can he only limited replacement
of amino
acids without loss of function.
The structure of the polypeptide chains of the hemoglobin molecule is a consequence of design and not simply of chance combinations of amino acids. Murray Eden and George Wald also conclude that chance combinations of chance mutations cannot explain the unique structure of the hemoglobin molecule.
It is also important to note that every peptide linkage in the globin
poly peptide
chain involves the a-amino group and the C1-carboxyl group. The e-amino group
of lysine, the C4-carboxyl of aspartic acid, and the Q5-carboxyl of
glutamic acid
are not involved in peptide linkages even though these amino acids
occur frequently
in the molecule (for the human beta chain: aspartic acid, 7 times;
glutamic acid,
8 times; and lysine, 11 times). Also, every optically active amino acid in the
globin polypeptide chain is of the L- configuration. This implies
optical purity
of the starting materials and no raeemization during the formation of the amino
acid polymer. These characteristics of the hemoglobin molecule are not found in
the proteinoid produced from amino acids by thermal polymerization1.
The studies cited are in accord with the view that the structure of
the polypeptide
chains of the hemoglobin molecule is a consequence of design and not simply of
chance combinations of amino acids. Any plausible theory for the biogenesis of
proteins must provide an explanation for the ordered sequence of amino acids in
the protein molecule. Murray Eden and George Wald have briefly considered the
relation of hemoglobin structure to evolutionary theory4. They also
conclude that
chance combinations or chance mutations cannot explain the unique structure of
the hemoglobin molecule.
Protein Stability
A second aspect of the properties of hemoglobin is that related to
protein stability.
As noted above, proponents of the random combination theory for the formation
of proteins presume that a protein molecule will retain functional ability for
long periods of time. When compared to many enzymes, hemoglobin would
he considered
a relatively stable protein. Nevertheless, hemoglobin A in solution gradually
is oxidized to methemoglobin. If the heme is removed, the globiu
becomes increasingly
unstable and precipitates. Within the red cell there are protective
enzyme systems
that maintain hemoglobin in the functional state. Methemoglobin reductase is continually involved in reducing methemoglobin
(ferric iron)
to ferrohemoglobin5. Enzymes linked to glutathione peroxidase are essential for
the protection of hemoglobin from oxidative breakdown°. In the
intact metabolically
active red cell, normal hemoglobin retains its functional ability for the life
span of the cell (ca. 120 days). However, when the hemoglobin is abnormal, or
when there is an abnormality in one of the enzymes involved in the protection
of hemoglobin, a pathologic condition often results. The unstable hemoglobins
illustrate this point very well. The amino acid modification causes instability
in the molecule, the globin precipitates within the red cell, and the
precipitate
often attaches to the membrane7. With some mutants, the consequences
of this globin
precipitation within the cell are very severe (e.g., Hb Sabine)8,9. There are
also a wide variety of drug-induced hemolytic anemias10, where a defect in one
enzyme in the protective enzyme sequence permits hemoglobin damage
and precipitation
within the cell. In another type of hemolytic anemia, the formation of either
beta or alpha chains within the maturing erythrocyte is impaired2. Under these
circumstances, there is a formation of a hemoglobin from four like polypeptide
chains; i.e., Hb a4, HI) H (b4) or Mb Barts (g4). These hemoglobins
are functionally
abnormal and are unstable. Hb H and Hb a are quite deleterious to the cell due
to intracellular precipitation and fib Barts appears to be lethal to the fetus.
This indicates that not only must a tetrameric structure be formed, but that we
must have the correct subunits in the tetramer.
The studies summarized above clearly show that even normal hemoglobin is only
moderately stable. In the absence of bacteria or enzymes, it would
still deteriorate
within several months. If the hemoglobin is modified by substitution of certain
amino acids (Part I, Table 2), it becomes unstable. As noted above,
the stability
of hemoglobin does not appear unusual when compared with other
proteins. Consequently,
studies of hemoglobin stability are not consistent with hypotheses
for biogenesis
of protein which appear to require a protein to remain functionally active for
very long periods of time, while all other essential components for life were
being formed.
If one does permit the intervention of intelligence or design, one could make reasonable speculations, but with no apparent way of knowing whether the hypotheses are valid.
Effect of Inactive Protein
Another tenet of the theory for the biogenesis of proteins is that all of the
totally inactive proteins (99.9% of the total ?) would have no adverse affect
on those that do have biological activity. In the hemoglobin mutant, Hb Sabine,
the abnormal protein in solution within the red cell constitutes 10 to 15% of
the total hemoglobin. Possibly an equivalent amount is precipitated within the
cell. No defective enzymes have been demonstrated in these cells and the amount
of normal hemoglobin within the cells appears to be adequate for oxygen transport. Nevertheless, the life span of the red cells is
markedly shortened and this appears to be due to precipitation of the unstable
hemoglobin within the cell. These studies provide experimental evidence that a
living cell cannot tolerate large amounts of nonfunctional
intracellular protein,
especially if the latter tends to precipitate and bind to membranes.
Minimal Protein Function and Life
The implication that minimal function of a protein would he all that would be
necessary to sustain life is difficult to prove or to disprove. With enzymes,
the enzymic activity in most eases is greater than is necessary for
normal metabolism,
and one could argue that diminished activity would not necessarily be
deleterious
to the cell. However, there are a wide variety of known genetic disorders due
to enzyme deficiencies. In many of these cases, the mutant enzyme retains 5% or
more of normal activity, yet this reduced activity is not sufficient
to maintain
cellular metabolism. If one enzyme with markedly reduced function can cause the
severe consequences noted in these genetic disorders, it does not seem possible
that a cell could survive with all proteins exhibiting only minimal function.
Also, one of the primary characteristics of life is that enzymic pathways are
under metabolic control. A portion of this control is exercised by enzymes that
have a control site or sites on the enzyme molecule that are distinct from the
catalytic site. Some type of control seems to be a prerequisite for life. The
postulation of chance formation of protein molecules with enzymic
function implies
that there would he no control of enzymic pathways. This lack of control would
he totally contrary to life as we see it exemplified in living cells
today. Although
some primitive enzymes might serve in metabolic pathways, in other
cases, a more
sophisticated enzyme (e.g., one with narrow substrate specificity, allosteric
sites, etc.) would be required. One might make the comparison of the tetrameric
hemoglobin molecule to a more sophisticated enzyme. The properties of
hemoglobin
are dependent not only upon the oxygen-binding site, but its
physiologic function
is dependent upon the proper interaction between the four subunits
and also upon
the binding of 2, 3-diphosphoglycerate. Although one might consider
the monomeric
myoglobin molecule as a more primitive oxygen carrier than hemoglobin, careful
inspection of the data indicates a complex structure for myoglohin.
Its properties
are dependent upon a region of specific amino acids surrounding the heme, and
also upon having appropriate intrachain contacts between R-groups of
the component
amino acids. Consequently, the hypothesis that only minimal protein
function would
be required for primitive life is not supported by the available experimental
evidence.
Selection and Reproduction of an Archetypal
Globin
The question of how one functionally active protein molecule could be selected
from the tremendous numbers of non-functional molecules and be utilized for the
reproduction of further like molecules is totally
answered. Instead of presuming that proteins were the first molecules that were
formed, it would appear more reasonable to propose that the first
molecules formed were nucleic acids. The transfer of information for amino acid sequence could
then proceed by basepairing of the purine and pyrimidine bases. The
major problem,
of course, is that polynucleotide synthesis requires a protein enzyme
as a catalyst.
Also, the synthesis of the mononucleotide budding blocks (nucleoside
triphosphates)
for polynucleotide synthesis requires enzyme catalysis. Consequently,
if one chooses
to make the presupposition that no "intelligence,, or "design"
is involved in the biogenesis of life, the problems appear to be
insurmountable.
If one does permit the intervention of "intelligence" or
"design",
one could make reasonable speculations, but with no apparent way of
knowing whether
the hypotheses are valid.
One additional aspect of hemoglobin structure that merits attention is the lack
of heterogeneity in the polypeptide chains that is found in a
particular species.
If the different hemoglobins are all derived from some archetypal precursor by
mutations, deletions, chain fusion, etc., a marked heterogeneity in structure
should be evident in every species. A small amount of heterogeneity
has been noted
in the glnbin molecule in recent studies11. This would be consistent with
some evolutionary modification of the pnlvpeptide chains with the
passage of time.
This should he distingnshed however, from the marked heterogeneity expected if
all presently existing glohins were derived from a single archetypal precursor.
The author has discussed this problem in
evolutionary theory in more detail previously in relaton to the structure of cytochrome
c12, and the problem will simply be noted in
this publication.
Footnote
oThere is no indication that any of the mutant hemoglobin'
have superior function when compared with HhA. Consequently, there would he no
reason to suggest that natural selection would, at some time in the
future, establish
one of the mutant hemoglobins as the predominant type.
REFERENCES
Fox, S. W., Harada, K., Krampitz, G., and Mueller, C., Chem. Eng. News 48, No.
8,80 (1970).
2Stamatoyannoupnulos, C., Ann. Rev. Gen. 6, 47 (1972).
5Dayhoff, M. 0., Atlas of Protein Sequence and Structure 1972,
National Biomedical
Research Foundation, Silver Spring, Md., p. D-371/D-372.
4Eden, 81. in "Mathematical Challenges to the Neo-Darwinian Interpretation
of Evolution", P. S. Moorhead and M. M. Kaplan, editors. Wistar Institute
Press, 1967, p. 6-19.
5Jaffe, E. R. and Hsieh, H. S., Sem. Hemat. 8, 417 (1971).
6Mills,
C. C., Tex.
Rep. Biol. Med. 21, 487 (1963).
7Jacob, H. S., Sem Hemat. 7, 341 (1970).
8Mills, C. C., Lcvin W. C. and Alpcrin, J. B., Blood 32, 15 (1968).
9MiIls, C. C., Alperin, J. B., Hill, F. L., and Henderson, R. J., Jr.
Biochem. Med. 5, 212 (1971).
10Beutlcr, F., Pharinacol. Rev. 21, 73 (1969).
11Huisman, T. H. J. and Schroeder, W. A., New Aspects of the
Structure, Function
and Synthesis of Hemoglobins, CRC Press, Cleveland, Ohio, 1971, p. 40.
12Mills, C. C., J. Amer. Sci. Affil. 20, 52 (1968).