The tricarboxylic acid cycle (Krebs cycle, citric acid cycle) is a focal end point for the oxidation of carbohydrate, fat and amino acids via acetyl coenzyme A. Pyruvate is converted to acetyl coenzyme A by the pyruvate dehydrogenase complex. The reactions of the TCA cycle generate carbon dioxide, reduced NAD, reduced FAD and GTP* There are negative and positive controls for the TCA cycle *Guanosine triphosphate (GTP) is a guanine nucleotide containing three phosphate groups esterified to the sugar moiety. GTP functions as a carrier of phosphates and pyrophosphates involved in channeling chemical energy into specific biosynthetic pathways.

2. • The tricarboxylic acid cycle (Krebs cycle, citric acid
cycle) is a focal end point for the oxidation of
carbohydrate, fat and amino acids via acetyl
coenzyme A.
• Pyruvate is converted to acetyl coenzyme A by the
pyruvate dehydrogenase complex.
• The reactions of the TCA cycle generate carbon
dioxide, reduced NAD, reduced FAD and GTP*
• There are negative and positive controls for the TCA
cycle
• *Guanosine triphosphate (GTP) is a guanine nucleotide containing three
phosphate groups esterified to the sugar moiety. GTP functions as a
carrier of phosphates and pyrophosphates involved in channeling
chemical energy into specific biosynthetic pathways.

5. Kreb s Cycle

8. Kreb’s Cycle

10. Succinyl CoA synthetase
Fumarate hydratase

13. COMPETING REACTIONS
• Intermediate compounds in glycolysis
and the TCA cycle can be used in many
metabolic pathways
• Diversion of intermediates creates a
need for reactions that will maintain the
level of oxaloacetate.

15. ANAPLEROSIS (filling up)
• One of the most important reactions for the
production of oxaloacetate is the reaction
catalyzed by pyruvate carboxylase.
• High levels of pyruvate carboxylase for
gluconeogenesis occur in liver and kidney
but other cells have lower activities of
pyruvate carboxylase to maintain the
concentration of oxaloacetate.
Anaplerotic reactions (from the Greek ἀνά= 'up' and πληρόω= 'to fill') are
chemical reactions that form intermediates of a metabolic pathway. Examples
of such are found in the citric acid cycle (TCA cycle). The TCA cycle is a hub of
metabolism, with central importance in both energy production and
biosynthesis.

16. REGULATION OF PYRUVATE DEHYDROGENASE AND THE
TCA CYCLE
Enzyme Inhibitor Activator
PyruvateDH ATP
NADH
Acetyl CoA
Citrate synthase ATP
IsocitrateDH ATP ADP
NADH
-Ketoglutarate DH NADH
Succinyl CoA

19. SUGGESTED READING
• Lippincott’s Biochemistry, 6th edition,
pages 109-115