Nadh Fadh

Expressed as P/O ratio the above ratios become: P

Consider the TCA cycle, which generates 3 NADH + H+, 1 FADH + H+, and 1 ATP. According to the general rule the expected ATP production in the TCA cycle becomes:

From NADH + H+ From FADH + H+ Substrate-level phosphorylation Total

12 mol ATP per turn of the cycle

Each turn of the cycle releases 2 mol of CO2, consumes 2 mol of O2 or 4 atoms of O, and releases 8 electrons. Given the classical concept that each turn of the cycle produces 12 mol of ATP as above, the classical P/O ratio for the entire TCA cycle becomes:

The above general simplification, however, does not accommodate the variety of arrangements that may exist in membranes for electron transport phosphoryla-tion. Therefore a more detailed or mechanistic calculation can be based on the chemiosmotic theory (Fig. 9.4).

According to this theory electrons are passed from donors such as NADH to redox receptors in respiratory complexes within the inner membrane of mitochondria or the plasma membrane of bacterial cells. The redox receptors are connected in a series of couples within up to four of these complexes. Reduction of some redox receptors draws an H+ ion from the cytoplasm (n phase of the membrane). As it flips to the reduced state, the H+ is extruded to the periplasmic space (p phase), thereby increasing the H+ concentration in the periplasm. As this process continues a gradient of H+ develops across the membrane with a high

FIGURE 9.4 Schematic outline of the functional arrangement of components of the electron transport chain in the inner membrane according to the chemiosmotic theory. The "loop," "Q-cycle," and "proton pump" mechanisms for H+ exclusion are represented. Not all parts are always present. This would be typical of many aerobic chemoorganotrophic soil microorganisms. For further details see Gottschalk (1986) and Nicholls and Ferguson (1992).

FIGURE 9.4 Schematic outline of the functional arrangement of components of the electron transport chain in the inner membrane according to the chemiosmotic theory. The "loop," "Q-cycle," and "proton pump" mechanisms for H+ exclusion are represented. Not all parts are always present. This would be typical of many aerobic chemoorganotrophic soil microorganisms. For further details see Gottschalk (1986) and Nicholls and Ferguson (1992).

concentration in the p phase compared to the n phase. Consequently an electrical potential, or proton motive force (AP), is developed to drive H+ from the p to the n phase through the ATP synthase complex. (Think of all those lonely expelled H+ ions anxious to get back to their mates.) The energy released in this process is used to form ATP from ADP. Consequently the production of ATP per mole of O2 consumed or per 2e~ released is determined first by the number of redox couples in the membrane, which determines the moles of H+ extruded to the p phase, and second by the number of moles of H+ that must be driven from the p to the n phase through the ATP synthase complex to produce a mole of ATP. The product of these two ratios determines the actual P/O ratio:

The electron transport chain contains up to four complexes involved in H+ extrusion and electron transport. The number of such complexes through which the electrons flow for a particular electron source determines the value of the H+/2e~ ratio as schematically represented in Fig. 9.4. The H+/2e~ ratio appears to vary from 10 for NADH oxidized by O2 to 6 for FADH oxidized by O2 (Nicholls and Ferguson, 1992). Considering that the ATP/H+ ratio appears often to be close to 4 (Nicholls and Ferguson, 1992) we can calculate the P/O ratio using Eq. (1). Consequently the P/O ratio may be:

— =-X-= 10 X - = 2.5 for NADH + H+ oxidized by O2 and

— = — X AT- = 6 X - = 1.5 for FADH + H+ oxidized by O2.

The ATP produced per turn of the TCA cycle can now be calculated using these values derived from the more mechanistic chemiosmotic theory. Specifically we now get 3 X 2.5 = 7.5 mol of ATP from NADH + H+, 1 X 1.5 = 1.5 mol ATP from FADH + H+, and 1 mol of ATP from substrate-level phosphorylation. Given the values above from the chemiosmotic theory each turn of the cycle produces 7.5 + 1.5 + 1 = 10 mol of ATP per turn of the TCA cycle. The more detailed P/O ratio for the entire TCA cycle now becomes:

Worm Farming

Worm Farming

Do You Want To Learn More About Green Living That Can Save You Money? Discover How To Create A Worm Farm From Scratch! Recycling has caught on with a more people as the years go by. Well, now theres another way to recycle that may seem unconventional at first, but it can save you money down the road.

Get My Free Ebook


Post a comment