**Core Concept**
Glutathione is a tripeptide molecule composed of cysteine, glycine, and glutamic acid. It plays a crucial role in cellular defense mechanisms by scavenging free radicals and reducing oxidative stress. Glutathione is also involved in various biochemical reactions, including conjugation reactions and serving as a cofactor for enzymes.

**Why the Correct Answer is Right**
The correct answer, option D, is false because glutathione does not convert hemoglobin to methemoglobin. Instead, methemoglobin is formed by the oxidation of hemoglobin, typically due to exposure to certain chemicals or drugs, such as nitrites or aniline. Glutathione's primary function is to reduce oxidative stress and protect cells from damage caused by free radicals. It does not directly participate in the conversion of hemoglobin to methemoglobin.

**Why Each Wrong Option is Incorrect**
**Option A:** Glutathione indeed decreases free radicals and reduces oxidative stress, making this statement true. It achieves this through its antioxidant properties, which help to neutralize free radicals and prevent cellular damage.
**Option B:** Glutathione helps in conjugation reactions, where it serves as a cofactor for enzymes involved in the conjugation of detoxified substances. This statement is true, as glutathione plays a crucial role in these reactions.
**Option C:** Glutathione acts as a cofactor for various enzymes, including glutathione S-transferase and glutathione peroxidase. These enzymes are involved in the conjugation and reduction of reactive oxygen species, respectively. This statement is true.

**Clinical Pearl / High-Yield Fact**
A key function of glutathione is its ability to recycle vitamins C and E, which are essential antioxidants in the body. This recycling process helps to maintain the antioxidant capacity of these vitamins and prevent oxidative stress.

**✓ Correct Answer:** D. Converts hemoglobin to methemoglobin