Gut Bacteria Metabolism: Main << Cellular energy, ATP and phosphorylation >> Oxidation and reduction: Redox reactions

All living cells require energy to function. This energy should ideally be self-contained and in a form that is readily available, interconvertible and universally useful. Its use should also be efficient and not wasteful.

In order to accomodate these requirements, life settled on a universal energy currency in the form of the biomolecule, adenosine triphosphate, or ATP. Transferable energy is contained within the phosphate ester bonds and heat energy is released when these bonds are hydrolysed (Figure 1). Under physiological conditions, ATP is biologically unstable due to its rapid use and turnover, but surprisingly, it is chemically stable, with a half life > 1 year in artificial seawater (Hulett1970).

Figure 1. Hydrolysis of adenosine triphosphate (ATP) to adenosine diphosphate (ADP) produces about 30 kilojoules of energy per mole of ATP. A similar amount of energy is liberated if ADP undergoes further hydrolysis to adenosine monophosphate (AMP).

Hydrolysing the phosphate esters directly would be wasteful unless the intention is to deliberately generate heat. Rather, the energy is used, usually with the help of enzyme catalysts, to drive chemical reactions that don't occur spontaneously under normal physiological conditions (endogenic reactions). Although various amounts of energy is lost (as heat) during these reactions, a single ATP can potentially facilitate a cascade of chemical transformations before the energy derived from that ATP molecule is exhausted.

The high energy status (potential energy) of ATP is sacrificed either to raise the potential energy of other molecules (chemical work), to move molecules against a gradient (transport work) or to power the mechanisms involved in movement, such as flagella proteins (mechanical work). This is most often achieved through transfer of a phosphate group (phosphorylation step) to a heteroatom (O, S, N) of the target substrate or protein. ATP is said to have high phosphate transfer potential, but it doesn't have the highest (Table 1), which means ATP can easily be regenerated by those that do, such as phosphoenol pyruvate (PEP).

ATP phosphorylation of proteins (via kinases) often induces a conformational shift in its structure to one that would be energetically unfavourable under normal circumstances. Phosphorylation may activate enzymes, allow ions to be transported or protect the protein from degradation. The process can be readily reversed by enzyme-mediated hydrolysis (phosphatases) of the phosphate group.

Table 1. Important phosphorylated intermediates. PEP and 1,3-BPG are generated during glycolysis.

Phosphorylated Compound	ΔG°' of Phosphate hydrolysis (kJ/mol)	Phosphate Transfer Potential	Phosphorylated Structure
Phosphoenol pyruvate (PEP)	-62	62
1,3-Bisphosphoglycerate (1,3-BPG)	-49	49
ATP → ADP	-31	31
Glucose-6-phosphate (Gluc-6-P)	-14	14
Glycerol-1-phosphate (Glyc-1-P)	-9	9

Although ATP is the most important energy currency, other nucleotide triphosphates are used during protein synthesis (GTP), lipid synthesis (CTP) and glycan synthesis (UTP). All these energy-carrying molecules need to be regenerated after they've transferred their phosphate groups. This occurs through a variety of energy generating metabolic pathways when carbohydrates, fatty acids or amino acids are oxidised and the energy generated is used to reduce a series of electron carriers via an electron transport chain (see Oxidation and reductdion: Redox reactions).