Question

In the fed state, glucose is directed towards certain metabolic pathways in the liver
(many of which are regulated at least in part by insulin). Indicate the potential metabolic fates
of glucose in the fed condition and indicate how glucose enters the pathway (e.g., for entry into
the Pentose Phosphate Pathway, glucose must be converted to Glucose-6-Phosphate by
Glucokinase).

Answer 1

Each class of biomolecule has alternative fates depending on the metabolic state of the body.

• Glucose: The intracellular form of glucose is glucose-6- phosphate.

• Only liver cells have the enzyme glucose-6-phosphatase that dephosphorylates G-6-P and releases glucose into the blood for use by other tissues

• G-6-P can be oxidized for energy in the form of ATP and NADH • G-6-P can be converted to acetyl CoA and then fat.

• Excess G-6-P is stored away as glycogen.

• G-6-P can be shunted into the pentose phosphate pathway to generate NADPH and ribose-5-phosphate.

Pentose phosphate pathway

When the ratio of NADP⁺ : NADPH increases, the body realizes it needs to produce more NADPH (a reducing agent for several reactions like fatty acid synthesis and glutathione reduction in erythrocytes). This will cause the G6P to be dehydrogenated by glucose 6-phosphate dehydrogenase. This irreversible reaction is the initial step of the pentose phosphate pathway, which generates the useful cofactor NADPH as well as ribulose-5-phosphate, a carbon source for the synthesis of other molecules. Also, if the body needsnucleotide precursors of DNA for growth and synthesis, G6P will also be dehydrogenated and enter the pentose phosphate pathway.