## TPP-Dependent Enzymes **Key Point:** Thiamine pyrophosphate (TPP) is an essential cofactor for enzymes catalyzing oxidative decarboxylation and non-oxidative carbon transfer reactions. ### TPP-Requiring Enzyme Complexes | Enzyme | Reaction Type | Substrate | | --- | --- | --- | | Pyruvate dehydrogenase | Oxidative decarboxylation | Pyruvate → Acetyl-CoA | | α-Ketoglutarate dehydrogenase | Oxidative decarboxylation | α-Ketoglutarate → Succinyl-CoA | | Transketolase | Non-oxidative carbon transfer | Pentose phosphate pathway | | Branched-chain α-ketoacid dehydrogenase | Oxidative decarboxylation | Branched-chain amino acids | ### Succinate Dehydrogenase — The Exception Succinate dehydrogenase catalyzes the oxidation of succinate to fumarate in the TCA cycle and is part of Complex II of the electron transport chain. This enzyme requires **FAD (flavin adenine dinucleotide)**, not TPP, as its prosthetic group. **High-Yield:** The three major TPP-dependent enzyme complexes are the **pyruvate dehydrogenase complex, α-ketoglutarate dehydrogenase complex, and transketolase**. Remember: TPP is used for **decarboxylation and carbon transfer**, while FAD is used for **oxidation-reduction** reactions. **Mnemonic:** **PDT** = Pyruvate Dehydrogenase, α-Ketoglutarate dehydrogenase, Transketolase — all require TPP.
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