Developing nanozymes with well-defined yet synthetically simple coordination environments remains essential from a rational design perspective. Here, we present a minimalistic glycine-copper (GCu) nanozyme, where the smallest amino acid acts as the only ligand to stabilize mixed-valence Cu(I)/Cu(II) centers. Structural analysis confirms the formation of a glycine-coordinated copper complex in a distinct mixed-valence state, supporting efficient phenol oxidase-like catalysis. The GCu nanozyme shows catechol oxidase activity toward epinephrine (EP), catalyzing its oxidation to adrenochrome, which strongly absorbs at 486 nm. Kinetic studies across different buffer systems reveal that GCu exhibits the highest catalytic turnover (Kcat = 0.353 min−1) in Tris buffer (pH 7.0). Compared with the phenylalanine-copper (PheCu) nanozyme, which contains bulkier ligand side chains, GCu exhibited 3-fold higher catalytic turnover. This indicates the effectiveness of smaller ligands in improving substrate accessibility. The color change of the oxidized product correlates directly with EP concentration, enabling colorimetric detection of EP at levels as low as 0.21 μg/mL. Analysis of EP-spiked urine samples showed recovery rates of 90−98%, confirming the potential of this colorimetric sensor for diagnostic applications. In addition to catechol oxidase activity, GCu also oxidizes various phenolic pollutants, indicating broader substrate versatility and dual oxidase activity. Overall, this work demonstrates glycine as a minimal coordination ligand for stabilizing a mixed-valence copper nanozyme, thereby providing a simpler structure for colorimetric sensing and the oxidation of phenolic pollutants.