Self-organizing biochemical cycles

Abstract

I examine the plausibility of theories that postulate the development of complex chemical organization without requiring the replication of genetic polymers such as RNA. One conclusion is that theories that involve the organization of complex, small-molecule metabolic cycles such as the reductive citric acid cycle on mineral surfaces make unreasonable assumptions about the catalytic properties of minerals and the ability of minerals to organize sequences of disparate reactions. Another conclusion is that data in the Beilstein Handbook of Organic Chemistry that have been claimed to support the hypothesis that the reductive citric acid cycle originated as a self-organized cycle can more plausibly be interpreted in a different way.

Figure 1

The reductive (reverse) citric acid cycle adapted from the scheme in Morowitz et al. (13). The nature of the chemical steps is indicated as follows: 1a, introduction of CO₂ and reduction to give an α-ketoacid; 1b, introduction of CO₂ to give a β-ketoacid; 2a, reduction of a carbonyl group; 2b, reduction of a double bond; 3, reversible hydration/dehydration; and 4, cleavage of citrate to acetate and oxaloacetate.

Figure 2

A simple example of molecular replication in a nonbiological system (17).

Figure 3

The simplest hypothetical autocatalytic formose reaction cycle. In each turn of the cycle, a glycolaldehyde molecule facilitates the synthesis of a second glycolaldehyde molecule from two formaldehyde molecules. The direct formation of glycolaldehyde from formaldehyde does not occur. The stereochemistry of the asymmetric carbon atoms (marked with an asterisk in the diagram) is not specified. The side reaction leading to a branched-chain aldehyde is one of the many reactions that tend to complicate the cycle or divert molecules from it.