A glucose molecule contains 6 carbon atoms, and each one must be converted into "CO"_2.
Glycolysis first converts the glucose molecule into two three-carbon fragments (pyruvate).
These fragments are then fed into the Tricarboxylic Acid Cycle in a preparatory step.
Preparatory step
The first carbon atom of pyruvate is removed as "CO"_2 when pyruvate is converted into "Acetyl-CoA" by pyruvate dehydrogenase.
underbrace("CH"_3"COCOO"^"-")_color(red)("pyruvate") + "NAD"^(+) + "H-SCoA" stackrelcolor(blue)("pyruvate dehydrogenase"color(white)(ml))(→) "NADH" + "CO"_2 + underbrace("CH"_3"CO-SCoA")_color(red)("Acetyl-CoA")
The Tricarboxylic Acid Cycle
Here's an overview of the Tricarboxylic Acid Cycle.
The remaining two carbons are lost as "CO"_2 in Steps 3 and 4 of the Tricarboxylic Acid Cycle.
Step 3. Isocitrate to α-ketoglutarate
The second molecule of "CO"_2 is formed in this step.
underbrace(stackrel("-")("O")"COH(OH)C"("COO"^("-"))"CH"_2"COO"^("-"))_color(red)("isocitrate")
+"NAD"^(+) stackrelcolor(blue)("isocitrate dehydrogenase"color(white)(ml))(→) underbrace(stackrel("-")("O")"COCOCH"("COO"^("-"))"CH"_2"COO"^("-"))_color(red)("oxaloacetate") + "NADH" + "H"^+
underbrace(stackrel("-")("O")"COCOCH"("COO"^("-"))"CH"_2"COO"^("-"))_color(red)("oxaloacetate") + "H"^"+"stackrelcolor(blue)("isocitrate dehydrogenase"color(white)(ml))(→) underbrace(stackrel("-")("O")"COCOCH"_2"CH"_2"COO"^("-"))_color(red)("α-ketoglutarate") + "CO"_2
Step 4. α-Ketoglutarate to succinyl-CoA
Here's where the third molecule of "CO"_2 is formed.
underbrace(stackrel("-")("O")"COCOCH"_2"CH"_2"COO"^("-"))_color(red)("α-ketoglutarate")+ "CoASH" + "NAD"^"+" stackrelcolor(blue)("α-ketoglutarate dehydrogenase"color(white)(ml))(→) underbrace("CoA-S-COCH"_2"CH"_2"COO"^("-"))_color(red)("succinyl-CoA") + "CO"_2 + "NADH"
