In the citric acid cycle the conversion of isocitrate to α-ketoglutarate and the subsequent conversion of α-ketoglutarat

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In the citric acid cycle the conversion of isocitrate to
α-ketoglutarate and the subsequent conversion of α-ketoglutarate to
succinyl-CoA both involve decarboxylation reactions. Explain why
one process requires the co-factor thiamine pyrophosphate while the
other does not.
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