What is the net change in molecular weight when a reaction involving TPMT occurs?

• A. +15 g/mol
• B. 0 g/mol
• C. -15 g/mol
• D. -16 g/mol

Answer: (B)

When evaluating the net change in molecular weight when a reaction involving TPMT (thiopurine S-methyltransferase) occurs, it is important to consider the function of the enzyme and the overall chemical process involved. TPMT primarily catalyzes the S-methylation of thiopurine compounds, which are several medications used primarily in the treatment of certain autoimmune diseases and cancers.

During this enzymatic reaction, the process involves the transfer of a methyl group (CH₃) from S-adenosylmethionine (SAM) to a thiol group in the substrate. The key aspect of this reaction is that while there is a conversion of substrates and products, the total number of atoms and their respective masses remains constant in a closed system in terms of molecular weight. This is due to the principle of conservation of mass, which states that matter cannot be created or destroyed in a chemical reaction.

Thus, the net change in molecular weight through the reaction can be considered to be zero, as the products formed will still account for the same total weight as the substrates consumed. The small changes in specific isotopes or atomic masses could occur, but overall, in terms of molecular weight as calculated, there is no increase or decrease.

As