33Lipid vesicles as possible intermediates in the origin of lifePier Luigi LuisiU, Peter Walde and Thomas Oberholzer() (Lipid vesicles liposomes are closed structures in which at)least one lipid bilayer separates an aqueous innercompartment from the bulk external aqueous medium, as inmembranes of contemporary biological cells. Lipid vesicleshave therefore been considered as possible cell precursorsduring the prebiological era on Earth. Recently, it has beenshown that lipid vesicles form spontaneously. Furthermore,it has been demonstrated that thermodynamically controlledpeptide binding to and controlled polymerization reactionson vesicles are possible, thus leading to an increase in themolecular complexity of lipid vesicles. This may have beenrelevant during the prebiological evolution.AddressInstitut fur Polymere, ETH-Zentrum, Universitatstrasse 6, Zurich¨¨¨CH-8092, SwitzerlandUE-mail: [email protected] Opinion in Colloid & Interface Science 1999, 4:33]39Electronic identifier: 1359-0294-004-00033Q 1999 Elsevier Science Ltd. All rights reserved. ISSN 1359-0294IntroductionResearch on the origin of life is based on the elemen-tary idea of molecular evolution, as proposed by Oparinwxmany years ago 1,2 . According to this approach, lifeoriginated from inanimate matter via a spontaneousincrease of molecular complexity and specificity. Anumber of books have been written on this subject; forwx wxexample by Bernal 3 , Miller and Orgel 4 , Fox andwx wx wx wxDose 5 , Folsome 6 , Dyson 7 , and De Duve 8 . Weknow that cellular life was already present on our planetŽat least 3,500 million years ago see the fossil recordswx.reported by Schopf 9,10 , and that the Earth is ; 4,500million years old. It is also assumed that the chemicaltransformations of organic molecules into, for example,amino acids or nucleotides, cannot have started earlierthan 3,900 million years ago, when the Earth was suf-ficiently cold and equilibrated. What happened betweenŽthis time and the time of the earlier fossils ; 400.million years is not known } and the reconstruction ofthis pathway leading to the transition to life is theinterest of researchers engaged in the study of theorigin of life.A current view of the origin of life, which is popularizedin a number of biology and biochemistry textbooks, seewx wxfor example 11]14 , is based on the RNA-world 15]17 .It is proposed that RNA is the key prime molecularentity, since it is endowed with both self-replicationand catalytic properties } the so-called ribozymeswx18,19 . Once a self-replicating RNA family is made,because of its capability to mutate in the process ofself-replication, ribozymes could be formed which mightsubsequently catalyze the formation of proteic enzymesand DNA. This scenario is very appealing, and in facthas gained a large measure of acceptance within thescientific community. There is a problem, however, inthat this scenario is based on the spontaneous, prebioticformation of an RNA ‘family’; namely a highly stereo-regular polymer with a specific sequence. Textbooksassume that this specific RNA family can form sponta-neously from the prebiotic soup, but this, at the presentstate of our knowledge, is highly improbable. Calcula-tions of the probability of such an event occurring havebeen presented in the literature, and, for example,wxJoyce and Orgel 20 define this view as ‘the molecularbiologist’s dream’, while showing how unrealistic thisdream is. This point is discussed in other papers as wellwx21 .Perhaps there are other, until now, undiscovered pat-terns by which a stereoregular specific RNA sequence} or some other suitable precursor } may sponta-neously come into existence, and it would indeed bevery appealing if this was to be the case. However, forthe time being, it is right and reasonable to look foralternative scenarios relevant to the origin of life.The general principles of the application ofsurfactant aggregates in investigating theorigin of lifeOne main alternative avenue is to look for ‘sponta-neous’ processes, such as self-assembly and hy-drophobic interactions, and to consider the level ofmolecular complexity and specificity which can bereached on this basis. The general belief underlyingthis research is that in prebiotic times only very simplereactions would have been operative.On this basis, it is appropriate to look at self-assemblyas an initial step for the construction of an organizedsystem which demonstrates molecular complexity andwhich will have a useful function. Lipid vesicles } andby lipids we mean in this context simple as well ascomplex amphiphiles, for example carboxylic acid soapsand phospholipids } have been considered as precur-wxsors of the living cell 22]25 . Connected with this isthe idea that compartmentalization is an important prin-ciple for the development of life on Earth since it leadswxto the first working cellular unit 25 . Pioneering work inwxthis field has been carried out by Deamer et al. 26,27who also showed that boundary structures are formedSelf-assembly34‘Bottom-up’ and ‘top-down’ approaches in research on the origin.from organic compounds extracted from the Murchisonwxmeteorite 28,29 .Starting from small molecules, the procedure by whichone advances up the ladder of molecular complexity toŽ.eventually a minimal living cell, can be defined as the‘bottom-up’ approach; see Figure 1. In this experimen-tal set up, one cannot use enzymes or nucleic acids, asthese macromolecules, by definition, did not exist at thestart of evolution.Still using lipid vesicles as the central theme, we canalso define a ‘top-down’ approach. The rationale is thefollowing: even the simplest modern living cell containsseveral hundred proteins and several hundred familiesof nucleic acids. Presumably, this high complexity is theresult of cellular evolution; when cellular life started,the cell did not require such a large number of enzymesor nucleic acids to function. This brings us to thequestion ‘what is the minimum number of enzymes andnucleic acids which a cell requires to still perform theŽmost basic functions of life basically self-maintenance.and reproduction ?’ This question can in principle beinvestigated by transforming a simple liposome into afunctional biological cell, by sequential addition of en-zymes and nucleic acids. We will return to this approachlater on in this article.The ‘bottom-up’ approachEarly work on self-replication of surfactant assemblieswas carried
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