**Core Concept**
Glutathione is a tripeptide antioxidant that plays a crucial role in protecting cells from oxidative damage. It is composed of three amino acids: glutamic acid, cysteine, and glycine. The reactive group of glutathione is responsible for its antioxidant properties.

**Why the Correct Answer is Right**
The correct answer is **Cysteine thiol (-SH) group**. The cysteine thiol group in glutathione is responsible for its antioxidant properties. It has a high reducing potential, which allows it to donate electrons and neutralize free radicals. The thiol group also participates in the formation of disulfide bonds, which help to detoxify reactive oxygen species.

**Why Each Wrong Option is Incorrect**
**Option A:** The amino group (-NH2) of glutamic acid is not directly involved in the antioxidant properties of glutathione. While glutamic acid is an essential component of glutathione, its amino group is not the reactive group.
**Option B:** The carboxyl group (-COOH) of glutamic acid is also not directly involved in the antioxidant properties of glutathione. Like the amino group, the carboxyl group is an essential component of glutathione, but it is not the reactive group.
**Option D:** The amino group (-NH2) of glycine is not directly involved in the antioxidant properties of glutathione. While glycine is an essential component of glutathione, its amino group is not the reactive group.

**Clinical Pearl / High-Yield Fact**
Glutathione is an essential antioxidant in the body, and its levels can be depleted in various diseases, including cancer, AIDS, and neurodegenerative disorders. Supplementing with glutathione or its precursors may help to improve antioxidant defenses and reduce oxidative stress.

**Correct Answer:** C. Cysteine thiol (-SH) group.