NASA Astrobiology Institute (NAI)

1. Early Metabolic Pathways

   Project Investigators:

   Other Project Members

   Andrew Pohorille (Project Investigator)

   Jack Szostak (Project Investigator)

   Janos Lanyi (Project Investigator)

   David Deamer (Collaborator)

   Michael Wilson (Research Scientist)

   Anthony Keefe (Postdoc)

   Dimitris Stassinopoulos (Postdoc)

   Michael New (Research Scientist)

   Leonid Brown (Postdoc)

   Astrobiology Roadmap Objectives:

   • Objective 2: Develop and test plausible pathways by which ancient counterparts of membrane systems, proteins and nucleic acid were synthesized from simpler precursors and assembled into protocells.
   • Objective 3: Replicating, catalytic systems capable of evolution, and construct laboratory models of metabolism in primitive living systems.

   Project Progress

   The main, long-term goals of this work have been (a) to develop protein enzymes representative of those that existed on the early earth, (b) to couple their catalytic activity in model protocells to external sources of energy and nutrients, and © to determine conditions, under which such protocellular systems can evolve using theoretical modeling.
   
   The selection of four new adenosine triphosphate (ATP)-binding proteins from a six trillion random polypeptides has been completed. The sequences of these proteins are not related to any known biological proteins, and as such they are the first truly novel proteins of non-genomic origin. Deletion studies revealed that the minimal binding unit is less than 50 amino acids long and, thus, is the smallest known ATP-binding protein. The results demonstrate that the method of in vitro selection of proteins is successful and general, and therefore can be used to select proteins with different activities. The method also provides a unique tool to study early evolution of proteins.
   
   A simple bioenergetics system consisting of bacteriorhodopsin and ATP synthetase incorporated into phospholipid liposomes was built and its ability to synthesize ATP in response to light was demonstrated and optimized. Further, ATP generation was coupled to synthesis of acetyl coenzyme A. This represents a simple system in which environmental energy drives an essential metabolic reaction.
   
   It was demonstrated that mixed short chain monocarboxylic acids and alcohols can form stable membranous vesicles which retain macromolecules such as DNA. Thus they represent plausible models for early membranes. It was further shown that simple membranes can be sufficiently permeable to nutrients to support template-directed synthesis of RNA. This result demonstrates that once genetic material has been entrapped within the boundaries of a protocell, replication is possible using external sources of nutrients and energy.
   
   A simple model of the early evolution of protocells in the absence of genome was investigated in detail. It was shown that a community of protocells containing proteins only (specifically, proteins capable of both forming and breaking peptide bonds) can increase its catalytic potential under a wide range of assumptions about the distribution of catalytic activities in random proteins. This suggests that non-genomic evolution may be a robust phenomenon.
   

Publications