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Proteins may be classified as to their three-dimensional structure (also known a protein fold). The two most widely used classification schemes are: [2] CATH database [3] Structural Classification of Proteins database (SCOP) [4] Both classification schemes are based on a hierarchy of fold types.
In an early study of 170,000 proteins, about two-thirds were assigned at least one domain, ... Proteins can be informally divided into three main classes, ...
All-β proteins are a class of structural domains in which the secondary structure is composed entirely of β-sheets, with the possible exception of a few isolated α-helices on the periphery. Common examples include the SH3 domain , the beta-propeller domain , the immunoglobulin fold and B3 DNA binding domain .
There are two basic types of transmembrane proteins: [4] alpha-helical and beta barrels. Alpha-helical proteins are present in the inner membranes of bacterial cells or the plasma membrane of eukaryotic cells, and sometimes in the bacterial outer membrane. [5] This is the major category of transmembrane proteins.
Alpha and beta proteins (a+b) [53931] (376): Mainly antiparallel beta sheets (segregated alpha and beta regions) Multi-domain proteins (alpha and beta) [56572] (66): Folds consisting of two or more domains belonging to different classes; membrane and cell surface proteins and peptides [56835] (58): Does not include proteins in the immune system
The two main types of proteins involved in such transport are broadly categorized as either channels or carriers (a.k.a. transporters, or permeases). Examples of channel/carrier proteins include the GLUT 1 uniporter , sodium channels , and potassium channels .
These proteins may have different transmembrane topology. [4] [5] These proteins have one of two structural architectures: Helix bundle proteins, which are present in all types of biological membranes; Beta barrel proteins, which are found only in outer membranes of Gram-negative bacteria, and outer membranes of mitochondria and chloroplasts. [6]
Protein tertiary structure can be divided into four main classes based on the secondary structural content of the domain. [25] All-α domains have a domain core built exclusively from α-helices. This class is dominated by small folds, many of which form a simple bundle with helices running up and down.