Question

Alamine is labeled at the alpha carbon with C14. where would the label be found in glucose (on which carbon(s))? where would it be found in ribose? please show the pathway that lead to the answer!

*alanine

Answer 1

Alanine (3C) is an amino acid formed by transamination of its keto acid called pyruvate by the enzyme transaminase. Since, the amino acid is labelled with radioisotope at alpha carbon, that is the Carbon next to the first Carbon (the second carbon of alanine is C14 labelled). There is no change in the carbon chain length, only deamination occurs removing the amino group from alanine, forming pyrivate. Hence, pyruvate is also labelled at second or the alpha carbon.Two pyruvate molecules are required for glucose formation through gluconeogenesis. Pyruvate (3C) leads to formation of 2 molecules of Glyceraldehyde-3-Phosphate (GAP), followed by isomerization of one of them to form Dihydroxyacetone-3-Phosphate (DHAP). The two combine to form Fructose-1,6-bisphosphate, of which 1,2, and 3 Carbon come from DHAP, and 4, 5, and 6 from GAP molecule. Fructose-1,6-bisphosphate is then dephosphorylated and isomerized, again dephosphorylated to give a molecule of glucose (6C). The second carbon of pyruvate remains at the second position in GAP. But after condensation of DHAP and GAP, the resultant molecule Fructose-1,6-bisphosphate may have the radioactively labelled carbon either at second or fifth position, as 1, 2, 3 C of DHAP are equivalent to 6,5,4 C of GAP. Simultaneously, the glucose formed will be also labelled at either second or fifth Carbon.

Glucose is also utilized in the Hexose monophosphate shunt or pathway (HMP) to form ribose-5-phosphate, one of the precursors for DNA synthesis. Here, one carbon atom (the first carbon of glucose) is removed to give rise to five carboned ribose sugar. Hence, the alpha or the second carbon becomes the first carbon in case of ribose. So, in case of ribose, the first carbon will be labelled with C14, if it is the fifth glucose carbon, then the fourth carbon of ribose will be C14 labelled.