The union of several polypeptide chains to form a complete protein is regarded at what level of protein structure?

Explanation

To determine the level of protein structure that involves the union of several polypeptide chains to form a complete protein, we can follow these steps:

1. **Understand the Levels of Protein Structure**: Proteins have four distinct levels of structure: primary, secondary, tertiary, and quaternary. Each level has unique characteristics and functions.

2. **Define Each Level**:
- **Primary Structure**: This is the sequence of amino acids in a polypeptide chain.
- **Secondary Structure**: This refers to the local folding of the polypeptide chain into structures such as alpha-helices and beta-pleated sheets, stabilized by hydrogen bonds.
- **Tertiary Structure**: This level involves the overall three-dimensional shape of a single polypeptide chain, formed by various interactions among the side chains (R groups) of the amino acids.
...