P/O ratio

**Definition:** P/O ratio = moles of inorganic phosphate (Pi) incorporated into ADP (to make ATP) per atom of oxygen consumed (½ O2). Each atom of O reduced to H2O accepts 2 electrons, so the P/O ratio is effectively ATP yield per 2 electrons passed down the ETC.

**Values (modern consensus):** - NADH (enters at Complex I): ~2.5 ATP. Protons pumped: ~10 H+ (Complexes I, III, IV). - FADH2 (enters at Complex II or glycerol-3-phosphate dehydrogenase, bypassing Complex I): ~1.5 ATP. Protons pumped: ~6 H+ (Complexes III, IV). - Older textbooks: 3 and 2, respectively. - Why the fractions: about 4 H+ are needed per ATP (≈3 H+ through the F0F1 ATP synthase plus ~1 H+ for Pi/ADP/ATP transport across the inner mitochondrial membrane). 10 H+/4 ≈ 2.5 and 6 H+/4 = 1.5. - Succinate oxidation (Complex II): P/O ≈ 1.5. - Ascorbate/cytochrome c (entering at Complex IV): P/O ≈ 1 (the 2 e- pass only through Complex IV).

**Pathophysiology/mechanism:** - Chemiosmotic theory (Mitchell): the ETC pumps H+ into the intermembrane space, creating a proton-motive force (electrochemical gradient). ATP synthase (Complex V) uses this gradient to make ATP. - The P/O ratio measures how tightly oxidation (O2 consumption) is coupled to phosphorylation (ATP synthesis). Respiratory control: O2 consumption is limited by ADP availability. Adding ADP increases O2 use (state 3).

**Clinical significance/high-yield associations:** - **Uncouplers** (2,4-dinitrophenol, aspirin overdose, thermogenin/UCP-1 in brown fat, thyroid hormone excess) dissipate the proton gradient. O2 consumption continues or increases, but ATP synthesis falls, so the P/O ratio decreases. Energy is released as heat, causing hyperthermia, sweating, and tachycardia. Salicylate toxicity: fever, tinnitus, anion gap metabolic acidosis with respiratory alkalosis. - **ATP synthase inhibitors** (oligomycin): block the proton channel, so the gradient builds up and the ETC halts. O2 consumption and ATP synthesis both fall. - **ETC inhibitors:** rotenone and amobarbital (Complex I), antimycin A (Complex III), cyanide, CO, azide, and H2S (Complex IV). They stop electron flow, so O2 consumption stops and ATP synthesis falls. - Uncouplers vs. inhibitors: uncouplers lower the P/O ratio, whereas inhibitors block electron transport or phosphorylation. Uncouplers do not inhibit ATP synthase directly. - Malate-aspartate shuttle (heart, liver) delivers cytosolic NADH to the matrix as NADH (≈2.5 ATP). Glycerol-3-phosphate shuttle (skeletal muscle, brain) delivers it as FADH2 (≈1.5 ATP). - Total ATP per glucose with complete oxidation: ~30-32 (modern) vs. 36-38 (older), reflecting the revised P/O ratios.

**Test tip:** If a question gives O2 consumption and ATP production in a mitochondrial preparation, a low P/O ratio with high O2 consumption indicates uncoupling (e.g., DNP). Low O2 consumption and low ATP indicates ETC or ATP synthase inhibition.