Читать книгу Human Metabolism - Keith N. Frayn - Страница 26

Chemical energy transduction is the process of transferring energy between different forms, and involves two main biochemical energy carrier types: (i) ATP (and also, less ubiquitously, GTP and creatine phosphate) which carries energy in the form of ‘high energy’ phosphate groups and (ii) NADH, NADPH, and FADH₂, which carry energy in the form of an electron (actually, as discussed above, a hydrogen atom with an extra electron – a hydride ion: H⁻). The energy status of the cell may be quantified with either of these systems – hence, (i) the ‘energy charge’ (phosphorylation potential) of the cell is an index of the degree of ATP phosphorylation (the amount of AMP that is phosphorylated to ADP and ATP: = [ATP] + [½ADP] ÷ [ATP] + [ADP] + [AMP]), and (ii) the ‘redox potential’ of the cell denotes the degree of reduction of NAD⁺ (i.e. the NADH:NAD⁺ ratio) and NADP⁺ (NADPH:NADP⁺). Besides carrying energy ‘down’ (catabolism) or ‘up’ (anabolism) metabolic pathways (Figure 1.12), the energy charge/phosphorylation potential and redox potential are major regulators of metabolic pathways to ensure appropriate energy provision.