What is the main difference between protein domains and protein subunits? Group of answer choices1. Protein domains do not have secondary structure, whereas protein subunits always have secondary structures 2. Each subunit represents a separate polypeptide chain, while one polypeptide chain may contain several domains 3. Domains are parts of membrane proteins, while subunits are parts of water-soluble proteins 4. Domains are composed mostly of the alpha-helices, while subunits contain both alpha-helices and beta-strands 5. The tertiary structure of domains is stabilized by hydrogen bonds, while the structure of subunits is stabilized by disulfide bonds