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At the top level are all alpha proteins (domains consisting of alpha helices), all beta proteins (domains consisting of beta sheets), and mixed alpha helix/beta sheet proteins. While most proteins adopt a single stable fold, a few proteins can rapidly interconvert between one or more folds. These are referred to as metamorphic proteins. [5]
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 .
The Structural Classification of Proteins (SCOP) database is a largely manual classification of protein structural domains based on similarities of their structures and amino acid sequences. A motivation for this classification is to determine the evolutionary relationship between proteins.
A protein superfamily is the largest grouping of proteins for which common ancestry can be inferred (see homology). Usually this common ancestry is inferred from structural alignment [ 1 ] and mechanistic similarity, even if no sequence similarity is evident. [ 2 ]
Modified amino acids are sometimes observed in proteins; this is usually the result of enzymatic modification after translation (protein synthesis). For example, phosphorylation of serine by kinases and dephosphorylation by phosphatases is an important control mechanism in the cell cycle. Only two amino acids other than the standard twenty are ...
Protein structures range in size from tens to several thousand amino acids. [2] By physical size, proteins are classified as nanoparticles, between 1–100 nm. Very large protein complexes can be formed from protein subunits. For example, many thousands of actin molecules assemble into a microfilament.
A successful search yields the class, folds, superfamilies, families, and individual proteins matching the query. Domain Assignments The database has domain assignments, alignments, and architectures for completely sequence eukaryotic and prokaryotic organisms, plus sequence collections.
The CATH Protein Structure Classification database is a free, publicly available online resource that provides information on the evolutionary relationships of protein domains. It was created in the mid-1990s by Professor Christine Orengo and colleagues including Janet Thornton and David Jones , [ 2 ] and continues to be developed by the Orengo ...