substrate-level phosphorylation

**Definition:** ATP (or GTP) is synthesized by enzymatic transfer of a phosphoryl group from a high-energy phosphorylated intermediate directly to ADP (or GDP). It is independent of the proton gradient, ATP synthase (complex V), and oxygen. Contrast with oxidative phosphorylation, which uses the electron transport chain to build a proton-motive force that drives ATP synthase.

**Key reactions (high-yield):** 1. **Glycolysis, phosphoglycerate kinase (PGK):** 1,3-bisphosphoglycerate + ADP -> 3-phosphoglycerate + ATP. This occurs twice per glucose. 2. **Glycolysis, pyruvate kinase:** phosphoenolpyruvate (PEP) + ADP -> pyruvate + ATP. This occurs twice per glucose. PEP has a very high phosphoryl-transfer potential. Pyruvate kinase is activated by fructose-1,6-bisphosphate (feed-forward) and inhibited by ATP, alanine, and glucagon (via phosphorylation, in the liver). 3. **TCA cycle, succinyl-CoA synthetase (succinate thiokinase):** succinyl-CoA + GDP + Pi -> succinate + CoA + GTP. This occurs once per acetyl-CoA. GTP is converted to ATP by nucleoside diphosphate kinase. Some tissues (e.g., liver, kidney) mainly make GTP, and others (e.g., heart, skeletal muscle) mainly make ATP. 4. **Creatine kinase / phosphocreatine:** phosphocreatine + ADP -> creatine + ATP. This is sometimes grouped here as a phosphagen-based phosphoryl transfer in muscle and brain. Adenylate kinase (2 ADP -> ATP + AMP) is similar.

**Net yield:** Glycolysis gives a net of 2 ATP per glucose by substrate-level phosphorylation (4 produced minus 2 consumed by hexokinase and PFK-1). The TCA cycle gives 2 GTP/ATP per glucose (1 per acetyl-CoA).

**Clinical significance:** - **Red blood cells** have no mitochondria and depend entirely on anaerobic glycolysis, so substrate-level phosphorylation is their only ATP source. Pyruvate kinase deficiency causes decreased ATP, rigid RBCs, and extravascular hemolytic anemia (autosomal recessive; echinocytes/"burr cells"; increased 2,3-BPG, which shifts the O2 curve rightward). PGK deficiency is X-linked and causes hemolysis, myopathy, and neurologic findings. - **Hypoxia/ischemia:** Cells shift to anaerobic glycolysis, so substrate-level phosphorylation continues while the ETC fails. Pyruvate is reduced to lactate to regenerate NAD+ and keep glycolysis running, which causes lactic acidosis. - **Arsenate poisoning:** Arsenate replaces inorganic phosphate in the glyceraldehyde-3-phosphate dehydrogenase step. This forms an unstable 1-arseno-3-phosphoglycerate that hydrolyzes spontaneously, so the PGK step is bypassed. Glycolysis proceeds but with no net ATP gain, which uncouples substrate-level phosphorylation. - **Rapid/obligate anaerobes:** Organisms such as *Clostridium* and cells such as RBCs and the lens, cornea, and renal medulla rely heavily on substrate-level phosphorylation. - **Arsenite** (not arsenate) inhibits lipoic acid-containing enzymes (pyruvate dehydrogenase, alpha-ketoglutarate dehydrogenase), which reduces TCA flux and thus substrate-level phosphorylation by succinyl-CoA synthetase.

**Pearls for boards:** - Substrate-level phosphorylation is fast but low-yield, and oxidative phosphorylation is slower but high-yield (about 26-28 of 30-32 ATP per glucose). - It is not inhibited by cyanide, CO, oligomycin, or uncouplers (e.g., 2,4-DNP), because it does not use the ETC or ATP synthase. However, because NADH reoxidation depends on the ETC (or lactate fermentation), prolonged inhibition of the ETC halts the TCA cycle and eventually limits it. - The energy for the phosphoryl transfer comes from the highly negative free energy of hydrolysis of the donor (PEP, 1,3-BPG, and succinyl-CoA thioester all exceed ATP's ΔG of hydrolysis).