The herbicide glyphosate (Roundup®) kills plants by inhibiting an enzyme needed for synthesis of phenylalanine. Deprived of phenylalanine, the plant cannot make the proteins it needs, and it gradually weakens and dies. Although a small amount of glyphosate is deadly to a plant, its human toxicity is quite low. Suggest why this powerful herbicide has little effect on humans.

Verified step by step guidance

1. Step 1: Understand the mechanism of glyphosate's action. Glyphosate inhibits an enzyme called EPSP synthase, which is involved in the shikimate pathway. This pathway is essential for the synthesis of aromatic amino acids like phenylalanine, tyrosine, and tryptophan in plants.
2. Step 2: Recognize that humans and animals do not possess the shikimate pathway. Instead, humans obtain aromatic amino acids through their diet, as these are essential amino acids that cannot be synthesized by the human body.
3. Step 3: Analyze the chemical structure of glyphosate (provided in the image). Glyphosate contains functional groups such as a phosphonate group and an amine group, which are specifically designed to interact with the active site of EPSP synthase in plants. These interactions are highly specific to the plant enzyme and do not affect human enzymes.
4. Step 4: Consider the biochemical differences between plants and humans. The specificity of glyphosate for the plant enzyme ensures that it does not interfere with human metabolic pathways, making its toxicity to humans very low.
5. Step 5: Conclude that glyphosate's low human toxicity is due to the absence of the shikimate pathway in humans and the specificity of glyphosate for the plant enzyme EPSP synthase, which humans do not possess.

Key Concepts

Amino Acid Synthesis

Phenylalanine is an essential amino acid that plants synthesize through specific enzymatic pathways. Glyphosate inhibits the enzyme EPSP synthase, which is crucial for the shikimic acid pathway, leading to a deficiency in phenylalanine. This disruption prevents the formation of proteins necessary for plant growth and survival.

Selective Toxicity

Glyphosate exhibits selective toxicity, meaning it targets specific biological pathways in plants that are not present in humans. Humans do not possess the shikimic acid pathway, which makes glyphosate ineffective against human cells, thus explaining its low toxicity in humans despite its potency as a herbicide.

Chemical Structure and Function

The chemical structure of glyphosate, as shown in the image, includes a phosphonate group that plays a key role in its herbicidal activity. This structure allows glyphosate to mimic a substrate in the shikimic acid pathway, effectively blocking the enzyme's function in plants while remaining non-reactive in human metabolic processes.