Which type of integral membrane protein has its NH3+ group outside the cell?

The correct answer, a Type 1 integral membrane protein, is distinguished by its orientation and structure. These proteins have a single transmembrane domain that typically contains a hydrophobic segment allowing it to span the lipid bilayer. The NH3+ group refers to the amino terminus (N-terminus) of the protein, which is positively charged at physiological pH and is located on the extracellular side of the membrane in Type 1 proteins.

This configuration occurs because Type 1 proteins have their carboxyl terminus (C-terminus) anchored within the cytoplasm, and their N-terminus facing outward towards the extracellular environment. This arrangement is crucial for the function of these proteins, many of which serve as receptors or enzymes that interact with ligands or other signal molecules outside the cell.

In contrast, other types of integral membrane proteins do not present their NH3+ group to the outside in the same way. For example, Type 2 proteins have their N-terminus in the cytoplasm, which would place the NH3+ group internally. Meanwhile, Type 3 proteins span the membrane multiple times, and Type 4 proteins may exist in complexes or as multi-subunit structures that do not have a defined orientation like