It's not accurate to say there's one major enzyme involved in the Krebs cycle. The Krebs cycle, also known as the citric acid cycle, is a complex series of reactions with numerous enzymes playing crucial roles.

However, some enzymes are particularly important for specific steps and are often considered "key players":

* Citrate Synthase: Catalyzes the first step, combining acetyl-CoA with oxaloacetate to form citrate.

* Isocitrate Dehydrogenase: Catalyzes the oxidative decarboxylation of isocitrate to α-ketoglutarate, generating the first NADH.

* α-Ketoglutarate Dehydrogenase Complex: This complex catalyzes the oxidative decarboxylation of α-ketoglutarate to succinyl-CoA, generating the second NADH and releasing CO2.

* Succinyl-CoA Synthetase: Catalyzes the conversion of succinyl-CoA to succinate, generating GTP.

* Succinate Dehydrogenase: This enzyme is embedded in the inner mitochondrial membrane and catalyzes the oxidation of succinate to fumarate, generating FADH2.

* Fumarase: Catalyzes the hydration of fumarate to malate.

* Malate Dehydrogenase: Catalyzes the oxidation of malate to oxaloacetate, generating the third NADH.

Therefore, depending on what you're specifically interested in, different enzymes can be considered "major" in the Krebs cycle.

It's also worth noting that the entire cycle is highly regulated, with many enzymes being influenced by allosteric regulation and feedback mechanisms.