© 1999 Macmillan Magazines Ltd

component dioxygen, O2) as the oxidant,and non-toxic solvents such as water orsupercritical carbon dioxide. Unfortunately,air oxidations are intrinsically non-selectiveand difficult to control2. Such chain reac-tions involving free radicals are either tooslow (for example, ageing of human tissue,plastics or metal surfaces) or too fast (forexample, combustion) to be useful, except infuel technology. In addition, the metals inhomogeneous catalysts tend to precipitatefrom water as catalytically inactive and insol-uble metal oxides.

On the biological front, ADHOC-99offered insight into the mode of action ofboth the soluble3 and membrane-bound4

forms of methane monooxygenase (sMMOand pMMO respectively). MMO catalysesthe dioxygen oxidation of methane — themost abundant hydrocarbon and the prin-cipal constituent of natural gas — tomethanol, a process of both fundamentaland practical interest. The underlying attrac-tion lies in the challenge of selectively cleav-ing the robust C–H bonds of methane andaccumulating high enough yields ofmethanol product, because methanol ismuch more reactive in air than methaneitself. From a practical outlook, methanolhas almost the same energy content asmethane, but unlike methane it is a relativelysafe liquid that is inexpensive to transportand store. It was established that the wellstudied Fe-containing sMMO (H. Dalton,Univ. Warwick) and the less understood Cu-containing pMMO (S. Chen, Caltech,Pasadena and Academia Sinica, Taipei) sharemany features. Both exert their remarkableselectivities in part by restricting the size ofmolecules that can reach the metal-contain-ing active sites. In both cases shape-specifichydrophobic sites further increase selectivityby favouring binding of methane over themethanol product. Complex interactionsbetween multiple protein components ofboth sMMO and pMMO modulate the flowof electrons to the metal-containing activesites and regulate overall reactivity.

Biological enzymes can also be moreeffective (that is, greener and more selectivein chemical synthesis) than traditional inor-ganic reagents. Oxidases or microorganismsthat contain these enzymes have been used toselectively produce a variety of chiral organicproducts of commercial value to pharma-ceutical and other industries, such as chiralalcohols (W. Adam, Univ. Würzburg). Suchbiocatalytic oxidations will play a growingrole in the greening of technologies that aredependent on this chemistry.

On the synthetic front, lessons learnedfrom studies of oxidation enzymes, as well asstructure–activity information from oxida-tions catalysed by soluble metal complexeshave contributed to the production ofpromising new oxidation catalysts. Onegroup managed to immobilize (on modified

The ability to selectively transformorganic and biological materials byremoving electrons and/or adding oxy-

gen atoms (oxidation) is pervasive in biologyand critical to the pharmaceutical, petro-chemical, agricultural and other industries.Unfortunately, the catalysts available todayfor homogeneous oxidation (reactions inwhich both catalyst and reactants are in thesame phase) are generally far from ideal. Theoxidation catalysts of the future will need tobe highly selective, environmentally benignand highly stable (Fig. 1). Chemists and bio-chemists recently gathered at the ADHOC-99 meeting to report progress in under-standing these factors and in developing newhomogeneous oxidation catalysts*. Investi-gations into exquisitely selective biologicalcatalysts, such as the oxidase and oxygenaseenzymes, and many metal-containing syn-thetic catalysts are guiding us towards theideal oxidation catalyst.

Selectivity in catalytic oxidations, as withother chemical transformations, is crucial.

Generating the desired product in low yieldwastes money and natural resources and lowselectivity means toxic or otherwise unac-ceptable by-products1. In general, catalyststability often dictates economic viability.‘Green’ oxidation requires the use of hydro-gen peroxide or ideally air (or its reactive

news and views

436 NATURE | VOL 401 | 30 SEPTEMBER 1999 | www.nature.com

Homogeneous catalysis

Controlled green oxidationCraig L. Hill

Selective

Green oxidant (O2 or H2O2) solvent (H2O or CO2)

Stable oxidative hydrolytic

Idealoxidationcatalyst

Figure 1 The ideal homogeneous oxidationcatalyst. The oxidation catalysts of the futurewill have to display all three of these inter-related attributes.

*Seventh International Symposium on Dioxygen Activation and

Homogeneous Catalytic Oxidation (ADHOC-99), York, UK, 19–23

July 1999.

nalling through the erythropoietin receptorand activation of death receptors seem tocompete through GATA-1 to elicit one ofseveral possible cell fates — differentiation,reversible maturation arrest or death.

The findings of DeMaria and colleaguesemphasize the close relationship betweencell-survival and cell-maturation pro-grammes, as well as the intricate balancebetween positive and negative influences inregulating cellular processes (Fig. 1). Weknow that external cues — such as exposureto FasL or erythropoietin — impinge on theinternal programmes controlled by GATA-1to modulate cellular maturation. But ques-tions remain. First, what is the physiologicalrole of the negative control of red-blood-cellformation through death receptors? To date,observations have been limited to culturesystems. Nonetheless, the increased levels ofsoluble FasL found in people with anaemia,and the excessive activation of death-recep-tor pathways in chronic disease, suggestpotential biological relevance11.

Second, the proposed main site at whichcaspases cleave human GATA-1 is not con-served in the mouse sequence. Does thisindicate that other sites are used as well, orthat there are species-specific differences?Third, does caspase-mediated cleavage tar-get critical nuclear regulatory proteins in

other types of differentiated cell? For exam-ple, might GATA-3 be a substrate in T cells?Caspase-induced death has been likened12 toa “well planned and executed military opera-tion”. In this context, using the inactivationof GATA-1 by caspases to negatively regulatethe formation of red blood cells is analogousto using a laser-guided smart bomb —a tactical strike at the command centre ofthe differentiation pathway. ■

Stuart H. Orkin is in the Division of Hematology,Children’s Hospital, Harvard Medical School and theHoward Hughes Medical Institute, 300 LongwoodAvenue, Boston, Massachusetts 02115, USA.e-mail: orkin@rascal.med.harvard.eduMitchell J. Weiss is at Ontogeny Inc., 45 MoultonStreet, Cambridge, Massachusetts 02138, USA.

1. DeMaria, R. et al. Nature 401, 489–493 (1999).2. Silva, M. et al. Blood 88, 1576–1582 (1996).3. Gregory, T. et al. Blood 94, 87–96 (1999).4. Orkin, S. H. Curr. Opin. Genet. Dev. 6, 597–602 (1996).5. Weiss, M. J., Keller, G. & Orkin, S. H. Genes Dev. 8, 1184–1197

(1994).6. Weiss, M. J. & Orkin, S. H. Proc. Natl Acad. Sci. USA 92,

9623–9627 (1995).7. McDevitt, M. A., Shivdasani, R. A., Fujiwara, Y., Yang, H.

& Orkin, S. H. Proc. Natl Acad. Sci. USA 94, 6781–6785 (1997).

8. Nagata, S. Cell 8, 355–365 (1997).9. Dai, C.-H., Price, J. O., Brunner, T. & Krantz, S. B. Blood 91,

1235–1242 (1998).10.DeMaria, R. et al. Blood 93, 796–803 (1999).11.Hasegawa, D. et al. Blood 91, 2793–2799 (1998).12.Thornberry, N. A. & Lazebnik, Y. Science 281, 1312–1316

(1998).

© 1999 Macmillan Magazines Ltd

comprises a polypeptide chain called globin,associated with protohaem IX — a planarcomplex of iron and protoporphyrin IX.These structural units form tetramers in ver-tebrate red blood cells, but in invertebratesthe combinations are more diverse, rangingfrom the monomeric haemoglobin of pro-tozoa to the gigantic annelid extracellularhaemoglobin which has 144 subunits. Thephysiological function of haemoglobin isattributed to its ability to bind dioxygenreversibly, depending on the partial pressureof oxygen (PO2). To function as an oxygencarrier, the oxygen affinity of haemoglobin(which is measured by the reciprocal of PO2

at 50% saturation) must be comparable toPO2 in the environment.

The haemoglobin found in the peri-enteric fluid of Ascaris worms has one ofthe highest observed affinities for oxygen.However, this tight binding makes Ascarishaemoglobin unsuitable as an oxygen carri-er. So what is its physiological function?Based on biochemical experiments, Min-ning et al.4 now propose that Ascaris haemo-globin is an NO-dependent ‘deoxygenase’,which uses endogenously produced NO asa substrate to detoxify oxygen. They havecome up with a model that includes morethan ten steps, proceeding through an inter-

news and views

NATURE | VOL 401 | 30 SEPTEMBER 1999 | www.nature.com 437

Haemoglobin is well known for its func-tion in the vascular system of animals,transporting oxygen from the lungs or

gills to peripheral tissues. It also aids, bothdirectly and indirectly, the transport of car-bon dioxide and regulates the pH of blood.Three years ago, a fourth function for mam-malian haemoglobin was proposed1 byJ. S. Stamler and his collaborators — theysuggested that the allosteric mechanism ofhaemoglobin is involved in the nitric oxide(NO)-mediated regulation of blood flow.Haemoglobin is a model allosteric protein2,yet it has no catalytic activity, leading the lateJeffries Wyman (a prominent protein scien-tist) to dub it an “honorary enzyme”2,3.Now, a report by Minning et al.4 on page 497of this issue effectively removes that hon-orific by showing that haemoglobin from aparasitic nematode, Ascaris suum, is a trueenzyme, serving as an NO-activated deoxy-genase.

By its original definition, the termhaemoglobin is used for the proteins con-tained in blood, haemolymph or coelomicfluid. However, proteins in other, non-circu-lating fluids or in single-celled organismsmay also have this name. Haemoglobin iswidely distributed among prokaryotes, uni-cellular eukaryotes, plants and animals. It

Physiology

The haemoglobin enzymeKiyohiro Imai

silica) soluble manganese complexes used forperoxide bleaching5 so that they are recoveredmore easily after the reaction (D. De Vos,Katholieke Univ. Leuven). They also reportedeffective new homogeneous oxidations,including olefin epoxidations, catalysed bythese manganese complexes. Another groupsynthesized titanium-containing silicate cagemolecules that represent structural and func-tional analogues of TS-1, the titanium-con-taining zeolite that has become one of themost commercially significant catalysts forgreen peroxide oxidation of organic molec-ules6. Reactions of these soluble cage com-pounds with peroxides and alkenes con-tribute to the growing evidence that the activesites in TS-1 involve a tripodal centre,(SiO)3Ti(OH), in which the titanium isbound by three oxygen atoms of the microp-orous solid (M. Crocker, Shell Res., Amster-dam). Also reported at the meeting was a suc-cessful air-based oxidation that was a greenversion of the Wacker reaction — one of thelargest homogeneous catalytic processesinvolving dioxygen oxidation of ethylene toacetaldehyde7. In this reaction, chelatingdiamine–palladium complexes catalyse theoxidation of olefins to ketones without usingtwo undesirable ingredients currently

play of the haem iron and several aminoacids of the protein moiety (the haem pock-et; see Box 1, overleaf).

The physiological role of Ascaris haemo-globin was previously proposed to be a ferri-haemoprotein reductase activity in the squa-lene epoxidation reaction, a pathway forsterol biosynthesis5. But this is now in doubtbecause sterols are essential nutrients, so it isunlikely that they are synthesized de novo inAscaris6. Instead, Minning and colleaguessuggest that, in contrast to most otherhaemoglobins, Ascaris haemoglobin’s physi-ological function is not to supply oxygenbut to eliminate it. The deoxygenase activityremoves oxygen toxicity by keeping tissueshighly anaerobic. This makes sense becauseAscaris worms inhabit the anaerobicintestines of their host and have a fully anaer-obic mitochondrial oxidation pathway thatis poisoned by even trace amounts of oxy-gen6. A similar role as an oxygen scavengerhas been proposed7 for leghaemoglobin,which is found in the root nodules oflegumes. This haemoglobin has a very highaffinity for oxygen, keeping the symbioticnitrogen-fixing bacterium found in the rootnodules anaerobic and protecting the nitro-gen-fixation enzyme system from oxidation.However, unlike Ascaris haemoglobin,leghaemoglobin is not a sink of oxygen —that is, the oxygen is trapped but not metabo-lized.

The globin–haem complex can be adapt-ed to various biological functions throughthe design of the haem pocket. By replacingonly a few amino-acid residues, the oxygenaffinity can be varied up to 27,000-fold, andthe reversible oxygen-binding ability is evenreplaced by catalytic activities involving O2

metabolism. Take the report by Lebioda etal.8 on page 445 of this issue. These authorshave found that a dehaloperoxidase isolatedfrom Amphitrite ornata, a terebellid poly-chaete, evolved from a globin. It still has thethree-dimensional ‘globin fold’ (a character-istic of haemoglobin), and it retains its abili-ty to bind oxygen. But whereas this enzymeevolved from a globin, the recently discov-ered indoleamine dioxygenase-like myoglo-bins are haemoproteins that have attainedthe ability to bind oxygen reversibly throughconvergent evolution from indoleaminedioxygenase9.

Despite a tremendous number of studies,all the mysteries of haemoglobin have not yetbeen solved. Future work will extend ourknowledge in two main directions. The firstinvolves haemoglobin’s reactions with NO,such as its S-nitrosylation action (exploredby Stamler’s group)1,4 and its function as anegative allosteric effector by ligating haem,as studied by Yonetani’s group10. The secondis likely to encompass the origin and evolu-tion of globin, including the functionaldiversity of the haemoglobins in variousorganisms. ■

required by the Wacker process — copperand chloride (R. Sheldon, Delft Univ. Tech.).

Significant steps have been made inunderstanding and controlling selectivity inhomogeneous oxidation reactions and inproducing high-value oxidized organicmaterials with minimal environmental cost.But catalyst stability is generally ignored andit directly effects both selectivity and greenoperation (Fig. 1). If a catalyst is damagedduring operation (turnover) it often trans-forms to a species that remains active (con-suming substrate and resources) but one thatnow generates useless or harmful products.In this way catalyst damage can undo care-fully crafted gains in other areas. All threeintersecting requirements must be con-sidered in the future to produce the idealhomogeneous oxidation catalyst. ■

Craig L. Hill is in the Department of Chemistry,Emory University, Atlanta, Georgia 30322, USA. e-mail: chill@emory.edu

1. Sheldon, R. A. J. Chem. Tech. Biotechnol. 68, 381–388 (1997).

2. Hill, C. L. & Weinstock, I. A. Nature 388, 332–333 (1997).

3. Wallar, B. J. & Lipscomb, J. D. Chem. Rev. 96, 2625–2657 (1996).

4. Elliott, S. J. et al. J. Am. Chem. Soc. 119, 9949–9955 (1997).

5. Hage, R. et al. Nature 364, 637–639 (1994).

6. Clerici, M. G. Stud. Surf. Sci. Catal. 78, 21–33 (1993).

7. Parshall, G. W. & Ittel, S. D. Homogeneous Catalysis 2nd edn

(Wiley, New York, 1992).