Substrate-level phosphorylation in the Krebs citric acid cycle:

• Succinyl-CoA synthetase (also termed succinate thiokinase) couples the cleavage of the high-energy thioester bond of succinyl-CoA to the phosphorylation of a nucleoside diphosphate ($GDP + P_i \to GTP$ in animals, which readily interconverts to ATP), representing the sole substrate-level phosphorylation step in the citric acid cycle.
• Succinate dehydrogenase oxidizes succinate to fumarate while reducing $FAD$ to $FADH_2$.
• Fumarase stereospecifically hydrates fumarate to L-malate.
• Malate dehydrogenase oxidizes L-malate to oxaloacetate, producing $NADH$.

Enzymes of the oxidative phase of the pentose phosphate pathway (PPP):

• Alcohol dehydrogenase is a fermentative/hepatic enzyme reducing $NAD^+$ or oxidizing ethanol.
• 6-phosphogluconate dehydrogenase catalyzes the oxidative decarboxylation of 6-phosphogluconate to ribulose-5-phosphate with the simultaneous reduction of $\text{NADP}^+$ to $\text{NADPH} + \text{H}^+$ and release of $\text{CO}_2$ (the other $\text{NADP}^+$-reducing enzyme being glucose-6-phosphate dehydrogenase, G6PD).
• Lactate dehydrogenase interconverts pyruvate and lactate using $NADH/NAD^+$.
• Succinate dehydrogenase uses $FAD$ as prosthetic electron acceptor.

Hypoxia adaptation and glycolytic reprogramming in cancer (Warburg effect):

• Assertion (A) is true: Hypoxia-inducible factor 1 (HIF-1) is stabilized in tumor cells, where it transcriptionally upregulates genes encoding nearly all glycolytic enzymes and high-affinity glucose transporters (GLUT1 and GLUT3), enabling high glycolytic flux to generate ATP.
• Reason (R) is true: As solid tumors rapidly expand, they outgrow their existing blood microvasculature, creating severe intratumoral oxygen deprivation (hypoxia, $pO_2 \lt 10\text{--}15\text{ mmHg}$).
• Because growing in this hypoxic microenvironment prevents normal prolyl hydroxylase-mediated degradation of HIF-1$\alpha$, the resulting stabilization of HIF-1 is precisely driven by the tumor's hypoxic condition, making Reason (R) the direct cause and explanation of Assertion (A).