Which metal substitutes for iron in some enzyme reactions in S. mutans?

• A. Zinc
• B. Copper
• C. Magnesium
• D. Manganese

Answer: (D)

Metals act as cofactors that enable enzymes to carry out their chemical reactions, and some enzymes can use different metals without losing function. In Streptococcus mutans, manganese can step in for iron in certain enzymes because Mn2+ shares similar size and charge and can coordinate in the enzyme’s active site, allowing the catalytic chemistry to proceed. This substitution is particularly helpful when iron is limited or when the oxidative environment makes iron less available. A well-known example is manganese-dependent superoxide dismutase, which helps protect the cell from reactive oxygen species when iron-containing versions aren’t available. Because of this capacity for iron-free activity in some enzymes, manganese becomes the most likely substitute for iron in these reactions.