Tricarboxylic Acid Cycle
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Each turn of the TCA cycle eliminates 2 carbons in consecutive (oxidative) decrapoxylation steps:
IDH: isocitrate → ketoglutarate* + CO₂
KGDH: ketoglutarate* → succinyl(CoA) + CO₂
A peculiarity of the TCA cycle is that succinate and (its product) fumarate are symmetrical molecules (⇈). Since this property applies to both of them, we can tell that succinate dehydrogenase modifies succinate evenly.
The next reaction is the conversion of fumarate to malate, where asymmetry appears. Even though fumarate is symmetrical, it contains newer or older atoms throughout the molecule. Depending on which side of fumarate is primarily changed after fumarase, the carbon stay in the cycle will differ.
This gives an idea:
Alterations in Cytosolic and Mitochondrial [U- 13 C]-Glucose Metabolism in a Chronic Epilepsy Mouse Model
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I've adapted it for ease of tracing and until completion:
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Therefore, the original carbons (from a given acetyl group) aren't eliminated straight away. They remain intact on the 1st turn, and the chances of their complete elimination appear to be:
50% on the 3rd turn
25% on the 4th turn
12.5% on the 5th turn
6.25% on the 6th turn
...
*Ketoglutarates:
2-oxoglutarate = a-ketoglutarate
3-oxoglutarate = b-ketoglutarate
Alpha refers to the second carbon because the first is the crapoxyl group, that's disconsidered in counting (similar to beta-oxidation).
Urine Organic Acids as Potential Biomarkers for Autism-Spectrum Disorder in Chinese Children
"[..]3-oxoglutarate, a common metabolite of yeast and fungi (Thomas et al., 2010; MacFabe et al., 2011; Kocovska et al., 2012), was significantly lower in children with autism. The low concentrations of both carboxycitric acid and 3-oxoglutarate that we observed in urine from autistic patients could be due to increased uptake of these compounds across the blood-brain barrier of the brain. Our results are consistent with previous studies that showed anti-fungal treatments for children with autism can effectively reduce the amounts of corresponding organic acid indicators (Cobb and Cobb, 2010), and suggests that gastrointestinal yeast could provide a basis for dietary adjustments such as gluten/casein-free diets that are important for children’s nervous system development and could mitigate autism symptoms. 3-oxoglutarate in urine is associated with the presence of harmful gut flora such as Candida albicans (Schmidt, 1994)."
It's fine to omit the 'alpha' prefix from ketoglutarate for the same reason that we only need to clarify which Paris we're going to when it's not the one in France.
