"Isn't it hard to decide at forks?"
Kana stared at the metabolic pathway diagram. Countless arrows intertwined complexly.
Rei answered. "Cells constantly decide. Which pathway to use."
"How do they decide?"
"Supply and demand," Toma interjected. "Same as economics."
Rei supplemented. "If energy needed, catabolic pathway. If storing, anabolic pathway."
Kana asked. "Who decides?"
"Enzyme activity," Milia explained. "Especially rate-limiting step enzymes."
"Rate-limiting step?"
"Slowest reaction. This determines overall speed."
Rei pointed to the diagram. "In glycolysis, phosphofructokinase."
"This is a branch point?"
"Yes. Here determines whether ATP is made or glycogen synthesized."
Toma proposed an experiment. "Let's change ATP concentration."
Rei prepared solution. "At high ATP concentration, phosphofructokinase is inhibited."
"Inhibited?" Kana asked.
"Negative feedback. Product suppresses enzyme."
Toma added. "If there's lots of ATP, no need to make more."
"Rational," Kana understood.
Rei showed another molecule. "Conversely, AMP activates the enzyme."
"AMP?"
"When ATP is used, becomes AMP. Signal of energy shortage."
Milia drew a diagram. "Allosteric regulation. Binds to different site on enzyme, changes activity."
"Binding sites besides the keyhole?"
"Yes. Regulatory site. Binding here changes enzyme shape."
Toma said excitedly. "When shape changes, active site changes too."
"So affinity with substrate changes," Rei continued.
Kana drew in her notebook. "What about citric acid cycle?"
"Isocitrate dehydrogenase is important," Milia answered.
"This is regulated too?"
"ATP, NADH inhibit. ADP, NAD⁺ activate."
Rei organized. "If energy abundant, cycle slows. If lacking, accelerates."
Kana asked. "Other branch points?"
"Pyruvate," Toma answered. "End of glycolysis, but also a beginning."
"What do you mean?"
Rei explained. "Pyruvate has three paths. Acetyl-CoA, lactate, oxaloacetate."
"Three?"
"If oxygen present, acetyl-CoA to citric acid cycle."
Milia continued. "If no oxygen, lactic acid fermentation."
"When muscles get tired?"
"Yes. Emergency measure to make energy even without oxygen."
Rei added. "For gluconeogenesis, via oxaloacetate to glucose."
Kana was impressed. "Three directions from one molecule."
"Metabolic flexibility," Milia expressed.
Toma asked. "Then how is the choice determined?"
"Enzyme expression, localization, post-translational modification," Rei listed.
"Complex..."
"But principle is the same. Adjust supply according to demand."
Kana looked at the diagram again. "Metabolism is a precise transportation network."
Milia nodded. "There are traffic lights. Enzymes control."
Rei said quietly. "And avoid congestion, transport substances efficiently."
Toma laughed. "Cells are excellent traffic controllers."
Kana wrote in her notebook. "Branching points in metabolic pathways—cellular decision-making."
The three gazed at the metabolic map. Countless choices support life.