Question

Ann O'Rexia suffers from anorexia nervosa. In addition to a low body weight, decreased muscle mass, glycogen, and fat stores, she has iron deficiency anemia. She constantly feels weak and tired. When she walks, she feels pain in her calf muscles. On this visit to her nutritionist, they discuss the vitamin content of her diet and its role in energy metabolism.  She is deficient in thiamine, riboflavin, biotin and niacin (the vitamin precursor of NAD+). In the context of the metabolic pathways discussed in Chapter 14, specifically how would these affect carbohydrate metabolism.

Answer 1

Ans) thiamine or vitamin B1 is essential in carbohydrate metabolism as it associated with energy releasing rea tions in carbohydrate metabolism

* the enzyme pyruvate dehydrogenase catalyses the conversion of Puruvate --------> acetyl Co A.

So this reaction requires TPP or thiamine pyrophosphate enzyme.

* alpha keto glutarate enzyme in kreb's cycle requires TPP.

* transketolase enzyme is required in HMP shunt pathway. It is also dependant on TPP.

So, the carbohydrate metabolism will be impaired, pyruvate accumulates in the tissue as it is not converted to acetyl Co A. Normally, pyruvate cannot cross BBB but in case of thiamine deficiency pyruvate accumulates in brain and results in polyneuritis.

Riboflavin.

Riboflavin has two coenzyme forms.

FAD and FMN. Flavin adenine nucleotide and flavin mononucleotide.

The flavin coenzyme participate in many redox reactions like -

In carbohydrate metabolism -

* pyruvate dehydrogenase (pyruvate--------> acetyl Co A)

*Alpha ketoglutarate dehydrogenase. ( alpha KG-------> succinyl Co A.

* succinate dehydrogenase ( succinate -------> fumarate)

Niacin.

Coenzymes are NAD ⁺ and NADP⁺​​​​​​.

Required in the following reactions in carbohydrate metabolism

* glyceraldehyde 3 phosphate dehydrogenase.

* lactate dehydrogenase

* pyruvate dehydrogenase complex

* alpha ketoglutarate dehydrogenase.

Lack of niacin will halt these mitochondrial cellular reactions and accumulate intermediates of carbohydrate metabolism like pyruvate and alpha ketoglutarate. Also cells fail to release energy required for the processes

Biotin

*gluconeogenesis and krebs cycle

Conversion of pyruvate to oxaloacetate by biotin dependant pyruvate carboxylase is necessary for synthesis glucose non carbohydrate sources and also for continuation of citric acid cycle.