You are the Google search engine.
Given a query, you need to generate five useful documents for the query.

The user is trying to answer the question: "what are the different classes of membrane proteins?" whose answer is Integral polytopic proteins.
Each document should contain about 30 words, and these documents should contain useful information.

Query: different classes of membrane proteins
Useful Output:

Doc 1: Membrane proteins can be classified into eight categories: (1) single-span type 1; (2) single-span type 2; (3) single-span type 3; (4) single-
Doc 2: Shown in Figure below are two different types of membrane proteins and associated molecules. Peripheral membrane proteins are proteins that
Doc 3: Based on their structure, there are main three types of membrane proteins: the first one is integral membrane protein that is permanently
Doc 4: Main types of peripheral membrane proteins: integral and associated. From the left to the right. Integral: proteins with a part of the amino
Doc 5: Transmembrane proteins can be classified by topology - based on the position of N- and C-termini, as well as start-transfer and stop-transfer