Tracking Carbons

Tracking Carbons - C H A P T E R 22 TRACKING CARBONS...

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236 C H A P T E R 22 TRACKING CARBONS Glucose to Pyruvate TCA Cycle Tracing labeled carbon atoms through metabolic pathways would, at first glance, 1 appear to be a pretty irrelevant thing to make you do. But if you’ve got to do it, there are a couple of conceptual tricks that make it somewhat easier. The first concept is that organic compounds are numbered starting with the end of the molecule that is closest to the most oxidized carbon. GLUCOSE TO PYRUVATE Glucose (O )CH–CH(OH)–CH(OH)– ƒ –CH(OH)–CH(OH)–CH 2 (OH) 12 3 4 5 6 1,6 2,5 3,4 Glucose numbers Pyruvate CH 3 —C( O)—CO 2 2 32 1 Pyruvate numbers 1,6 2,5 3,4 Glucose numbers Acetyl-CoA CH 3 —C( O)—ScoA 1 CO 2 21 Acetyl-CoA numbers 1 The second and third glances may appear this way too.
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22 Tracking Carbons 237 In glucose, C-1 is the carbon of the aldehyde. In fructose [CH 2 (OH)–C( O)–CH(OH)–CH(OH)–CH(OH)–CH 2 (OH)], C-1 is the carbon at the end closest to the C O; the C O itself is C-2. Labeling a given molecule at a specific carbon atom means that an isotopically labeled carbon atom ( 14 C or 13 C) has been introduced at a given position in the starting molecule. For example, the statement that glucose has been labeled at C-2 means that only carbon number 2 of glu- cose has been tagged with a 14 C or 13 C carbon atom. The other carbon atoms are not labeled (they are 12 C). The problem is to determine where this label ends up after the glucose is metabolized to some other com- pound. Tracing the carbons of glucose to pyruvate gets complicated when fructose 1,6-bisphosphate (FBP) is cleaved into two 3-carbon fragments, glyceraldehyde 3-phosphate (G3P) and dihydroxyacetone phosphate (DHAP). The numbers of the original carbons of glucose are indicated by the superscripts next to the carbon. The guilty party is the triose phosphate isomerase (TIM) reaction that interconverts DHAP and G3P. To be converted to pyruvate, the DHAP first has to be converted to G3P. TIM just moves the carbonyl group between the two carbons that don’t have phosphate attached. TIM doesn’t touch the phosphate. So, if the DHAP is labeled at the carbon that has
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This note was uploaded on 04/09/2008 for the course BIO SCI 98 taught by Professor Goulding during the Spring '08 term at UC Irvine.

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Tracking Carbons - C H A P T E R 22 TRACKING CARBONS...

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