An enzyme is defined as a
A. a modified protein that lowers the activation energy needed for chemical reactions to occur.
B. a protein that functions as a hormone.
C. a protein that only forms blood.
D. fibrous protein that is used to help build certain tissues in the body.

Option A.

A modified protein that lowers the activation energy needed for chemical reactions to occur.

Explanation:

An enzyme is a protein or biological catalysts that speed up the rate of chemical reactions by lowering the activation energy.

Activation energy is the amount of energy required for a reaction to proceed and over it's barrier.

Enzymes lower the activation energy by binding to one of the reactants called substrate, thereby sticking to it till the energy lowers.

The lower the activation energy, the faster the reaction .

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carbon atoms, hydrogen, oxygen, and nitrogen.