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Subunits and Building Blocks of Complex Organic Molecules

Virtually all biomolecules are constructed from a limited number of generic subunits or building blocks, the best-known examples being proteins and nucleic acids. Lipids, which are formed from only two basic building blocks, are polymers of either acetate or isopentenyldiphosphate precursors. The final products lack a hydrolyz-able functionality (e.g., peptide linkages) at the point where subunits join, and, unlike other proteins and nucleic acids, lipids cannot be depolymerized.

A classic example of lipids are those that are found in membrane lipid bilayers of bacteria and eukarya and are made up of fatty acids esterified to glycerol. The most common fatty acids are all-acetate products and thus have even carbon numbers (e.g., C14, C16, C18, and C20). Odd-carbon-numbered members, generally synthesized from a non-acetyl starter, exist but are less abundant. Extension of fatty acid chain length proceeds by the addition of further acetate units. Terminating and modifying reactions such as desaturation, reduction, or decarboxylation yield common intermediate-molecular-weight series of products such as the plant and algal waxes made up of even-numbered alcohols (e.g., C26, C28, C30, C32) and odd-numbered hydrocarbons (e.g.,

An additional illustration of the building-block principle is displayed by the terpenoids. These polymers of A3-isopentenyldiphosphate have somewhat more complex origins and much more complex structures (Figure 3.4.1). As a result of isoprenoid biosynthesis and its evolution over geological time, terran life contains an enormous array of complex molecules related through their C5 architecture. The multiplicity of isoprenoid biosynthetic pathways, their distribution across different phylogenetic groups, their requirement, or otherwise, for molecular oxygen, and the types of post-synthesis modification are generally held to provide a powerful biosignature of evolutionary origins. For example, the molecules resulting from the pathway shown in Figure 3.4.1 are highly diagnostic of biosynthesis because, individually, they exhibit many features of biosynthesis (e.g., carbon number, chirality, and subsets of isomers).

Crocetane, 2,6,10-trimethyl-7-(3-methylbutyl)-dodecane, squalene, and biphytane are irregularly branched compounds, whereas phytane, labdane, and kaurane are regular and are constructed from four head-tail linked isoprene units. These compounds also illustrate how different structures can be diagnostic for specific physiologies (phytol and farnesol for photosynthesis, phytane for various archaea, crocetane for methanotrophy) or specific organisms (2,6,10-trimethyl-7-(3-methylbutyl)-dodecane for diatoms; biphytane for crenarchaeota; labdane and kaurane for conifers).

1G. Ourisson and P. Albrecht, "Hopanoids. 1. Geohopanoids: The Most Abundant Natural Products on Earth?," Accounts of Chemical Research 25:398-402, 1992.

Cameras and spectral imagers on previous, continuing, and planned life-detection missions to Mars are capable of identifying structures and objects ranging from the macroscopic to the minuscule that, on Earth, are considered visible signatures for past or present biological activity. Such objects and structures include intact microbes, metazoa and metaphytes, stromatolites, microbial mats, and other large-scale structures composed of aggregates of cells, as well as component parts of multicellular organisms such as cysts, pollen, embryos, organs, and so on. On Earth, these objects are pervasive in surface environments and in the deep subsurface and leave no doubt about how abundant and tenacious life is. Researchers can also, to a degree, visually identify in Earth's sediments a rich fossil life extending in age to more than 2 billion years. So far, no such visible "biological" objects have been convincingly identified on Mars or in martian meteorites. If life exists, or existed in the past, on Mars or other

Triangle Congruence Worksheet

FIGURE 3.4.1 Structures of some regular, irregular, and cyclic C2O (diterpenoid) and C3O (triterpenopid), and C4O (tetra-terpenoid) hydrocarbons that have been identified in sediments and that illustrate a variety of biosynthetic patterns based on repeating five-carbon subunits (after J.M. Hayes, "Fractionation of Carbon and Hydrogen Isotopes in Biosynthetic Processes," Reviews in Mineralogy and Geochemistry 43: 225-277, 2001).

FIGURE 3.4.1 Structures of some regular, irregular, and cyclic C2O (diterpenoid) and C3O (triterpenopid), and C4O (tetra-terpenoid) hydrocarbons that have been identified in sediments and that illustrate a variety of biosynthetic patterns based on repeating five-carbon subunits (after J.M. Hayes, "Fractionation of Carbon and Hydrogen Isotopes in Biosynthetic Processes," Reviews in Mineralogy and Geochemistry 43: 225-277, 2001).

planetary bodies, the evidence has not been forthcoming. In many respects, the search for martian life mirrors the search for the earliest life on Earth and faces similar obstacles. Attempting to reconstruct terran life's history back into deep time, researchers are confronted by the problem of a record made increasingly cryptic by the geochemi-cal and geological processes that continually re-surface Earth and modify the rock record.

Poor preservation and ambiguity about what constitutes a biosignature have confounded the search for visible evidence of early microbial life on Earth38-45 and in the martian meteorite ALH 84001 in particular.46 Related reports, and some of the controversies stemming from them, teach researchers that drawing an inference of biogenicity based on morphology is fraught with difficulties. If the feature being observed is demonstrably synge-netic with the host rock and displays a limited size (length and width) distribution, shows evidence of cellular

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