Science

How life may have emerged without phosphate,

Among the many unanswered questions on life’s origin, the enigma of how phosphate ended up playing a prominent role in cellular biochemistry has been puzzling scientists for decades. Phosphate is present in a large proportion of known biomolecules. It is an essential component of biochemical energy transduction (most notably through ATP), cofactors such as NADH, and information storage.

Phosphate possesses many attributes that exclusively suit the ion for biotic and prebiotic roles, including stabilizing adenosine triphosphate, deoxyribonucleic acid (DNA), ribonucleic acid (RNA) and contributing to the amphiphilic character of phospholipids that make them ideal components of cell membranes.

Structural phosphate may be inside a cell, "structural to a nucleic acid such as DNA and RNA or a phospholipid. Outside the cell, phosphate may be dissolved in extracellular fluid (ECF) or form structures such as bone and teeth. Bringing phosphate in any form into the cell from a phosphate containing structure or for such a structure and when needed transporting phosphate out of the cell perhaps to a structure is a necessary activity of phosphate homeostasis for that cell.

However, phosphate is geochemically scarce and difficult to access, often serving as the limiting nutrient in a variety of modern ecosystems. Phosphate is found in terrestrial and marine ecosystems, tightly complexed with rocks and minerals, requiring mechanisms for environmental extraction and transport.

The ensuing dilemma of phosphate’s high importance in spite of its poor bioavailability is particularly challenging for early life, as primordial protocells would have needed both a readily available phosphate source and a simple mechanism for early phosphate acquisition. Currently, there is no consensus for a phosphate source in early life, with theories ranging from acid-mediated ion solubilization, high concentrations of reduced phosphorus species in early oceans, or accumulation during late heavy bombardment.

The substitute solution to this problem is that primitive forms of life could have initially emerged and endured without a major dependence on phosphate. Multiple scenarios for early metabolic pathways that do not rely on phosphate have been proposed.

In many of these scenarios, sulfur and iron are conjectured to have fulfilled major catalytic and energetic functions prior to the appearance of phosphate. Most notably, in the thioester world scenario, thioesters are hypothesized to have played a role similar to the one played today by ATP. Thioesters are widespread in modern metabolism, primarily as Coenzyme A (CoA) derivatives (e.g., Acetyl-CoA), and are used as condensing agents, enabling the synthesis of heterogeneous biopolymers.

To obtain insight into the early stages of the evolution of metabolism, Scientists analyzed the biosphere-level collection of all known metabolic reactions.

They show that before the incorporation of phosphate in metabolism, all reactions are thioester-based and How life may have emerged without phosphate, 

Refrence: Remnants of an Ancient Metabolism without Phosphate


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Saturday 04 March 2017