Meta-Biol

• Pathways

The re-entry of protons into the mitochondrial matrix through Complex V causes conformational changes which result in ATP synthesis. Complex V (ATP synthase) is composed of 3 parts; an F1 catalytic core (approx 5 subunits), an F0 membrane proton channel (approx 9 subunits) and two stalks linking F1 to F0. F1 contains three alpha subunits, three beta subunits, and one each of gamma, delta, and epsilon subunits. Each beta subunit contains an active site for ATP synthesis. F0 has at least 9 subunits (a-g, A6L and F6), with one copy each of subunits b, d and F6.
The mechanism of ATP synthesis by Complex V was predicted by Boyer et al in 1973: ADP and Pi bind to the enzyme resulting in a conformational change. ATP is then synthesized, still bound to the enzyme. Another change in the active site results in the release of free ATP into the matrix. The overall reaction is:
ADP + Pi + H+ + nH+ (intermemb. space) = ATP + H2O + nH+ (matrix)

The metabolism of pyruvate provides one source of acetyl-CoA which enters the citric acid (TCA, tricarboxylic acid) cycle to generate energy and the reducing equivalent NADH. These reducing equivalents are re-oxidized back to NAD+ in the electron transport chain (ETC), coupling this process with the export of protons across the inner mitochondrial membrane. The chemiosmotic gradient created is used to drive ATP synthesis.

Metabolic processes in human cells generate energy through the oxidation of molecules consumed in the diet and mediate the synthesis of diverse essential molecules not taken in the diet as well as the inactivation and elimination of toxic ones generated endogenously or present in the extracellular environment. The processes of energy metabolism can be classified into two groups according to whether they involve carbohydrate-derived or lipid-derived molecules, and within each group it is useful to distinguish processes that mediate the breakdown and oxidation of these molecules to yield energy from ones that mediate their synthesis and storage as internal energy reserves. Synthetic reactions are conveniently grouped by the chemical nature of the end products, such as nucleotides, amino acids and related molecules, and porphyrins. Detoxification reactions (biological oxidations) are likewise conveniently classified by the chemical nature of the toxin.

At the same time, all of these processes are tightly integrated. Intermediates in reactions of energy generation are starting materials for biosyntheses of amino acids and other compounds, broad-specificity oxidoreductase enzymes can be involved in both detoxification reactions and biosyntheses, and hormone-mediated signaling processes function to coordinate the operation of energy-generating and energy-storing reactions and to couple these to other biosynthetic processes.