First substrate of Kreb&;s cycle is

Acetyl CoA enters the cycle and is completely oxidized. During this process, energy is trapped. The sources of acetyl CoA are shown in Figure 18.1. Pyruvate derived from glycolysis is oxidatively decarboxylated to acetyl CoA by pyruvate dehydrogenase (Fig. 9.22). This is the link between the TCA cycle and glycolysis. The pyruvate dehydrogenase reaction occurs in the mitochondria. Pyruvate with the help of a carrier can enter the mitochondria from the cytoplasm. The acetyl CoA derived from beta-oxidation is formed in the mitochondria itself. All the enzymes of citric acid cycle are located inside the mitochondria.Ref: MN Chatterjea Textbook of Medical Biochemistry, 8th edition, page no: 216