Iron versus Copper II. Principles and Applications in Bioinorganic Chemistry Ei-lchim Ochiai Juniata College, Huntingdon, PA 16652
In the previous articles (1,2) (ref 1 is to be designated as long period of time, however, probably until about 2.2 billion years ago, the free oxygen level of the atmosphere remained the first of this series), we examined some basic orincioles in the hioselection of elements. We invoked there-four fundavery low as a result of the presence of oxygen sinks. Historimental rules governina selection, rules related to: 1) an elecally the most important oxygen sink was the vast quantity of Fe(I1) in the ocean. I t is believed that the ancient ocean ment's basic fitness for a given task, 2) its abundance, 3) its contained a much higher level of iron than does today's efficiency, and 4) evolutionary pressure. Using these, we ocean. Free oxygen, as soon as it was produced, reacted with attempted to rationalize why a specific element would have been chosen for a specific biological function. We found we this Fe(II), oxiding i t to Fe(III), the iron then being depositcould gain considerahle understanding of such enzymes and ed in the form of oxides, either magnetite Fea04or hematite Fe2Oa.In fact, the majority of the accessible iron ore on the proteins as superoxide dismutases, cobalt-dependent BIZcoenzvme and molvbdenum enzvmes u s i n ~onlv those basic oresent earth is found in the form of so-called "banded iron rules along with knowledge of the chekcalproperties of formation" (BIF), consisting mainly of alternate layers of the elements. We wish now to expand this kind of exercise auartzand of ironoxides. It ia noteworthv that H l F r ~ a s e dto and apply the principles to several interesting specific cases form rather abruptly, about 1.8 bil1ion"years ago (see the in the ensuing several articles. Our first case studv examines figure). the relationship between iron and copper. The first life forms on earth are thought to have emerged Iron and copper are the two most familiar metals in our no later than 3.5 billion years ago (5); these first organisms life. Even the most casual observers will have noticed some are hypothesized to have been heterotrophic bacteria incaof their distinctive characteristics. Iron and its alloys are pable of producing their own food by photosynthesis. Photosynthetic sulfur bacteria soon developed, but even these did used widely, hut a serious problem is their tendency to rust. On the other hand, copper is much more resistant to rusting not use water for photosynthesis. The first water-decomposand hence has found use in water piping systems. This parine. .. oxveen-releasing " ...ahotosvnthetic oreanisms were cvanobacteria or their ancestors; precisely when they appeared is ticular difference is due to their disparate electrochemical nut yet known with certainty. As indicated in the figure, the (oxidation-reduction) .orooerties. The ooint can be demon. strated by consulting reduction pntentials r E h ) data. The rate oideposition of RIF increased markedly toward the end of its onjduction aeriud. This increase is verv likelv to have red~~riion ontentiills of thr svstem Fe(ITI)/Fe(ll)and Fetll) rianobara t neutral p~ are relatively cow ( ~ e ~ ~ & h & n a t i t & ' e ( ~ ~ ) : ' ~ ;been due ro the pioliferation of mygen-prod~ring = -0.2 V, Fe(II)/Fe(O) = -0.44 V); this means that the teria. It seems, plausible, therefore, that cyanobucteria first evolved sometime Iwtween 3.0 and 2.5 billion years ago and oxidation of Fe(0) to Fe(I1) and then to Fe(II1) is relatively easy in thermodynamic terms. Copper, on the other hand, is that their population increased gradually at first, leading tu the onset uf BIF formation. The cyanobarteria population difficult to oxidize because it has relatively high reduction potentials (Cu(II)/Cu(O) = +0.34 V, Cu(II)/Cu2S = +0.2 V). This basic difference is reflected in manv other asDects of the behaviors of the two elements, including aspects with hioloaical - and environmental significance. We will oresentlv Stroto-hund examine some important examples.
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Evolution of the Atmosphere and Sedlmeniary Ore Formailon Before we proceed further, however, we need to discuss briefly how the redox state (expressed by Eh) of the terrestrial environment might have changed over the course of geological time. TheEh value of a water hody (such as an ocean) depends on a number of factors; important among these are the oxygen pressure of the atmosphere in contact with it, the conceniratibn level of reducing-materials such as organic matter, and the depth. The most influential factor is the atmospheric oxygen. The present atmosphere contains as much as 0.2 atm of free oxygen (dioxygen), but this has not been the case over all of earth's history. Rather, the oxygen level has changed drastically. The exact temporal profile of the evolution of atmospheric oxygen has not been determined unequivocally (3, 4). I t seems certain, however, that the atmosphere on the primitive earth (4.6-3.0 billion years ago) was virtually devoid of free oxygen. Free oxygen was only gradually produced and added to the atmosphere, first by the radiolysis of water vapor by ultraviolet rays from the sun and later by the photosynthetic decomposition of water by organisms. For a
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3.0
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The age profileof (a) geological (magmatic)activity. (b) BIF fmtion,and (c)stratabound (sedimentary)and porphyritic (igneous)copper ore (production in 1974):Gyago = Giga (billion)yearsago: Gpy = Giga years horn the creation of me earth. (Adoptedfrom (a, b) J. Eichlsr,J. In "Handbook of Strataboundend Stratiform Oredeoosits": K. H. Wolfs. Ed.: Elsevier: 1976: Vol 7. oo . . 157-201: ICI Bowen. R.. Gulall aka. A. Capper: ts Geolagl and Economm ' dalstead. 1977 )FromOch~a~. E- . in 'Cosmocnernostry andthe O18gmof L fe '; Ponnamperurn. C .Ed : Readel: 1983: p 235
increase seems to have accelerated around 2.2-1.8 billion years ago, and there resulted a vast quantity of BIF, which may have exhausted the supply of readily available Fe(I1). If this is so, it means that by tbat point the atmospheric oxygen level must bave become sieoificant and the ocean's EIlevel bave become relatively high. Let us designate this period (up to 1.8 billion vears ago) as the "iron era". the time when iron was abundant in the ocean. Interestingly, throughout this entire period no significant amount of sedimentary copper ore had deposited (see the figure). As copper is a difficult element to oxidize, it would have remained in its native metallic state or as the very insoluble cuprous sulfides a t a time when the Eh level was as low as it must have been in the iron era. Therefore, throughout the iron era, the ocean was very likely devoid of soluble conoer .. ion. Onlv when Eh became hieh enough " .(>+0.2 V).. . i.e., when the atmospheric oxygen level became high enough, could copper become solubilized as Cu(I1) (or perhaps CuClk'). Sedimentation mostly as cuprous sulfides would again bave occurred when the Cu(I1) moved into a reducing environment such as deeper levels of the ocean, leading ti what we have come to regard as major ore deposits. This process seems to have become significant about 1.7 billion years ago, to have peaked about 1.0 billion years ago, and then to have declined (see the figure); let us designate this period (1.7-0.4 billion years ago) as the "copper era".
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Phylogeny of Iron and Copper
Iron is essential tovirtually all organisms and is utilized in a very large number of ways. As far as reduction potential is concerned, iron in its many forms can encompass the entire biologically significant range: from about -0.5 V to about +0.6 V. The lower portion is covered by iron-sulfur clusters as those found in bacterial (Fe& and plant (Fe&) ferredoxins. The reduction potentials of iron-porphyrin (heme) proteins range from a low of -0.4 V (in some anaerobic bacterial cytochrome cs, as well as catalase and cytochrome P-450; see part I11 of this series for an explanation) to a high of +0.5 V (cytochrome a, as). In the intermediate range fall the reduction notentials of hemoelobin and mvoelobin. " " There is also a ;ariety of proteins containing so-called nonheme iron. Examples include hemervthrin (an oxveen-carrv.ing protein found in some species df sea worms), dioxygenases, and superoxide dismutase whose reduction potentials are expected to fall in the mid range, along with ribonucleotide reductase. Dlstrlbution of Cower Enzvmes and Proteins Distribution Enzyme or protein
Piastocyanin
Prokalyotes Some aerobic bacteria some C Y ~ P bacteria
Stellacyanin Hernocyanin Cyiochrome oxidase (aa3-ty~e) Superoxide dismutase (CuIZn-SOD)
Amine oxidase D-Galactose axidase Ascobate oxidase Laccase Tymsinase Ceruloplasmin (ferroxidase) Do~amin-beta-hvdroxviase
Some aerobic bacteria Pholobacterlum leiognathi Caulobacler wesentus
Eukaryotes
some green algae, some red algae, land plants. Rhus vernicifera, horseradish. Mung bean, etc. invenebrate~ mitochondria
animais, fungi. land plants (not found in most algae and protozoa) fungi, plants, animals Dactyiiurndendroides animals, plants Poiyjmm versicolor. Rhus vmicifera animals animals animals
The reason that iron is so widely used in organisms is twofold. One reason is chemical; iron can accommodate a very wide range of reduction potentials, as outlined just above. The other reason is historical: in the oceans of the primitive earth, where life is believed to have started, iron was plentiful and readily available to organisms. If all the organisms on the earth today are assumed to be derived from the primitive organisms on the early earth, i t would oulv be natural to find widesnread utilization of iron despite thk fact that the element is'no longer readily available on the earth todav. This Doses a nroblem to all oreanisms. Indeed, organisms have had to contrive special methods to secure iron. Thus, some bacteria secrete so-called siderophores (61,compounds that bind Fe(II1) so strongly as to be able to extract iron from such insoluble iron species as Fe(OH)$, FeP04, and even sometimes from the stainless steel containers used as culture vessels. Some plants, too, are known to secrete a kind of siderophore, mugineic acid or avenic acid (7), particularly in alkaline soil. Other organisms, including human beings, try to conserve iron as much a3 possible. Iron in the human body can be lost accidentally through hemorrhaging, but otherwise it isused over and over again. In the earlier stages of organismic evolution. the reva ailiog condition was anoxic; hence, life forms had beeontent with anaerobic metabolism, ~ l ~ c o l v s ai svery , inefficient wav to use the potential energy contained in fobdstuff. ~ c c o r d ingly, the proteins and enzymes involved in anaerobic metabolism were designed to function in the lower portion of the reduction-potential spectrum. As the atmosphere became oxygenic, organisms began to take advantage of the strong oxidizing power of oxygen, thus evolving the more efficient aerobic metabolism. Metabolism continued to move upward in the reduction potential scale with further developments in aerobic metabolism, and proteins and enzymes of higher reduction potentials came to be utilized; this trend resulted in the formation of such proteins as hemoelobin, cytochromes a, b, and c, and dioxygenases. Returning to copper, it appears from the argument outlined earlier that this element would first have become available to organisms about 1.7 billion years ago. Only then could oreanisms beein to utilize it. We would exoect (8). . ,, therefore, that copper would he important only to rhatively "newer" organisms and would be involved in reactions related to high& portion of the reduction potential scale. The fact that the maioritv of the known comer proteins and enzymes are found only in eukaryotes (seethe table) is consistent with this hypothesis. Eukaryotes (organisms consisting of nucleated cells) are believed to be a relatively recent development, emerging sometime around 1.5 billion years ago. A few copper proteins occur in some aerobic prokaryotes (organisms lacking a distinct nucleus, such as bacteria). These include plastocyanin, azurin, terminal oxidase of the mitochondria1 type, and some enzyme systems involved in the reduction of NO; or N?O. Plastocvanin seems to be found in all land planis, some-green and red algae (which are eukaryotes), and in some cyanobacteria. I t may be tbat those cyanobacteria or other bacteria that contain copper proteins developed in the copper era as defined above. Prokarvotic organisms that deveikped before 1.7 billion years ago could not have utilized copper ( 8 ) .I t must be noted, however, that the absence of copper proteins in an organism does not necessarily mean that it developed earlier than 1.7 billion years ago. Organisms could choose not to use copper, even if it were available. One of the most interestine enzvmes to consider in the present context is superoxideudis&utase (SOD). There are three varieties of this enzvme (9): Fe-SOD. Mn-SOD and CuIZn-SOD. The first two-are found in prokaryotes (a) and in the mitochondria of eukaryotic cells (b), whereas the Cul Zn-SOD is found in the cytoplasmic portion of eukaryotic cells (c), and in a very few prokaryotes (d). Facts (a) and (c) Volume 63
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are consistent with the above areument. One of the two bacterial Cu,Zn-SOD'S so far knuwn is found in a symbiotic orranism. Pt~olobacteriurnleiomnthi: it is beliewd ( 1 0 ) t h i t this protein was obtained 6y a gene transfer from the eukaryotic host (ponyfish) to the prokaryote. In the case of another prokaryotic CuIZn-SOD, that from Caulobacter cresentus CB15, no definitive origin has been delineated (11). Fact (b) is rather intriguing, but is very much consistent with the idea that the mitochondrion, an organelle in eukaryotic cells, originated as a symbiotic aerobic bacterium (12). According to this hypothesis, a pro-eukaryotic cell incorporated, within its confine when i t formed, some prokaryotic organisms as symbionts. Eventually, the aerobic bacteria thus incorporated turned into semiautonomous organelles, mitochondria. Most aerobic bacteria did not develop CutZn-SOD, perhaps because %SOD and Mn-SOD were adequate for the task that needed to he done. I t is interesting to reflect the fact that we carry in our bodies these vestiges of ancient organisms or rather their remnants. An Anthropological Issue Iron is the most widelv used metal todav in develoned society; for that reason the present era is oken calledthe "iron age". Iron production is avery energy-intensive activity and one which requires sophisticated techniques. Copper and its alloy hronze, on the other hand, were much more-widely used in the past than today. The period before the advent of the iron age, several thousand years ago, is often called the "hronze age". As we have seen., comer " .. has a high reduction potential, and thus it is often found in nature even today in the native metallic state and is relatively easy to obtain by reduction of copper ores, though sophisticated techuiaues (electrolvsis) are reauired to refine it. It can thus be infirred that copper metai was readily obtainable by relativelv primitive techniques, which explains the hronze , sequence, age ir&age sequence of Eivilization. ~ h u sthis too, can be explained in terms of basic chemical properties of the two elements. I t may he noted that this-sequence is
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exactly the opposite of the geological one: iron era copper era. This apparent anomaly occurs because biological use of the elements is concerned with their soluble (oxidized) forms, whereas our use of iron and copper is with their solid metallic forms (reduced states). +
Concluding Remark I t has been argued that such seemingly disparate phenomenaas the hioinorganic chemistry, the ore formation, and the technical utilization of iron and copper may all he explainable in terms of the very basic electrochemical behaviors of the two elements. Chemistrv is essentiallv an inductive science in that exand observations are made in perimental data are order to arrive at aeneralizations. This aspect of the science is emphasized, an& rightly so, in present-day chemical education. I t might, however, he useful to try occasionally to apply chemical principles deductively in order to see how much we can explain. The present article and this series as a whole is an attempt to demonstrate the deductive way of thinking as it can be applied to hioinorganic chemical prohlems. Acknowledgment The author wishes to thank W. E. Russey for editing the text. Literature Clted (11 O ~ h i a iE , ~ lJ.Chem.Educ. . 1978,35.631. (21 Ochiat. E-I. Riosysfem 1918,10.329. I l l Holland. H. D. "The Chemical Evolution of the A t m ~ o h e r eand Ocean": Princeton IJniuenify: 1984. (41 Clemmy,H.:Bsdham. N. Geology 1982.10, 141. I51 Schopf, .I. W., Ed. "Earth's Earliest Bimphere-Its Origin and Emlogy"; Princeton University: 1983. 161 Neilands. J. B., Ed. "Microbial Iron Metabolism":Academic: New York. 1974. 171 Suriura,Y.: Nomuto, K. Slruel. Bond. 1984,58,107. 181 Ochiai. E-I. Biosyshms 1983.16.81; Oehisi. E~I. In "CosmoehemisLryand the Origin nf 1.ifr"; Ponnamperuma, C.. Ed.; Reidel: p 235. (91 Fridovich, 1. Ann. Rau. Rioehem. 1915.44. 147. 1101 BannistPr,J. V.;Parkor,M. W . h c N a l . A c d Sri.. USA 1985.8% 149. (11) Steinman, H.M. J.Bio1. Chsm. 1982,257, 10283. 1121 Msrpulis. L. "Symbiosis in Cell Evolution": Freemen 1981.
