What is a common characteristic of proteins with two or four subunits?

Proteins with two or four subunits exhibit quaternary structure, which is a crucial concept in understanding protein assembly and function. Quaternary structure refers to the organization of multiple polypeptide chains (subunits) into a single functional protein complex. Proteins can have varying numbers of subunits, and those that assemble into groups of two or four are very common in biological systems.

The presence of multiple subunits allows for more complex regulation and interaction within the protein complex, often enhancing its functional diversity. For example, each subunit can contribute to the protein's overall function, stability, and ability to bind to other molecules, including substrates, inhibitors, or activators.

Many crucial biological functions and processes are mediated by proteins with quaternary structures, making this characteristic fundamental to their biology. Understanding this feature helps explain how proteins can acquire novel properties that are not present in single-chain proteins.