This is a real-world medical entrance exam question, please give the true answer based on the question and selection. Topic name: Metabolism of carbohydrate, Subject name: Biochemistry
Coenzyme required for transketolase reaction is 
Select and explain. 
A. Ca2+, B. Mg2+, C. H+, D. PO4-
B: Mg2+
Answer: B: Mg2+ 
Explanation: Transketolase transfers the two-carbon unit comprising carbons 1 and 2 of a ketose onto the aldehyde carbon of an aldose sugar. It, therefore, effects the conversion of a ketose sugar into an aldose with two carbons less and an aldose sugar into a ketose with two carbons more. The reaction requires Mg2+ and thiamin diphosphate (vitamin B1) as the coenzyme. Measurement of erythrocyte transketolase and its activation by thiamin diphosphate provides an index of vitamin B1 nutritional status.Ref: Harper&;s Biochemistry; 30th edition; Chapter 20; The Pentose Phosphate Pathway & Other Pathways of Hexose Metabolism