There are three basic forms of super secondary structure found at present: α - helix combination (α - α);β - fold combination (β β) and α - helix β - fold combination (β α β), of which β α - β combination is the most common.They can be directly used as the "building blocks" of the tertiary structure or the constituent units of the domain, called super secondary structure, which is not a level of protein structure.
Super secondary structure
Combined unit
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α α combination form
Several super secondary structures
Several secondary structures can appear in special geometric combinationsprotein structureThese combinedStructural unitIt is called super secondary structure or pattern.The super secondary structure can be associated with some special biological functions, or can only be used as an assembly block of the structure.The α - ring - α pattern contains twoAlpha helixAnd connected by a ring area, it is a super secondary structure with special functions.In knownprotein structureTwo such patterns were observed in theDNACombination pattern, and calcium combination pattern, also known as EF hand, each has its owngeometryAnd requiredAmino acid residueSequence.
EF hand appears in the protein Parvalbumin, troponin, calmodulin and other structures from muscle, and they passBound calciumTo regulate changes in cell function.EF hand provides a maintenance calciumLigandsThe scaffold ofprotein structureOne of the first functional patterns recognized in.
β β combination form
Hairpinβ or β - ring - β pattern is a super secondary structure composed of two anti parallel β - chains connected by a ring, which frequently appears in protein structure.The two adjacent β chains in the β - turn are easy to form this hairpin β pattern.The length of rings between two β chains is unequal, generally 2-5Residue。Unlike the alpha ring alpha pattern, the hairpin beta pattern has no special function.
Greek pattern
Four adjacent antiparallel beta chains are usually arranged likeancient GreekDecorative patterns, therefore called Greek patterns, are common in protein structures but have no special functionsDirect connection。
β α β combination form
The hairpin pattern is usually used to connect two anti parallel β - chains.How to connect two parallel beta chains?If the sequence of residues of two adjacent parallel β - chains is continuous, their connecting parts must be at both ends of β - folding.The polypeptide chain must be rotated twice depending on the ring region to make these two chains parallel, which is in turn: β - ring 1-Alpha helix-Ring 2 - β chain, such pattern is called β - α - β pattern.This pattern has a parallel β - foldingprotein structureBoth exist in.In such a structureCarboxyl terminalThe ring 1 connected to the amino end of the alpha helix often contains a functional binding site orActive siteHowever, another ring 2 connected with the amino end of the β - chain and the terminal end of the α - helix has not been found to be related to the active site.This β - α - β pattern can be considered as a loose spiral circle. In the known protein structure, basically every β - α - β pattern is a right-handed α - helix, so it is called a "right-handed" pattern. There is no satisfactory explanation for this.
Supersecondary structure refers to the secondary structures that are adjacent to each other in sequence within the polypeptide chain are often close in space folding and interact with each other to form regular secondary structure aggregates.There are three basic forms of super secondary structure found at present:Alpha helixCombination (α α);Beta foldingCombination (β β β) and α spiral β folding combination (β α β), of which β α β combination is the most common.They can be directly used asTertiary structure"Building block" orDomainThe constituent unit of isproteinA level between the secondary structure and the tertiary structure in the conformation, so it is called super secondary structure.
Domain(domain) is also a layer between secondary structure and tertiary structure in protein conformation.In larger protein moleculesPeptide chainThe adjacent super secondary structure on the top is closely linked, forming two or more structures that can be clearly distinguished fromProtein subunitStructural differences.Generally, each domain consists of 100-200Amino acid residueThey have unique spatial conformation and assume different biological functions.asimmunoglobulin（IgG）It consists of 12 domains, two of which arelight chainTwo on eachheavy chainThere are 4 on each;complementJunction siteAndAntigen binding siteThey are in different domains.Some domains in a protein molecule are the same, some are different;The domains in the peptide chain between different protein molecules can also be the same.aslactate dehydrogenase, 3-glyceraldehyde phosphatedehydrogenase、Malate dehydrogenaseAll belong toNAD+ForcoenzymeThey are composed of two different domains, but the conformation of the domain they bind to NAD+is basically the same.
Terminology
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Super secondary structure: secondary structure unitA-HelixandBeta foldingThey gather together to form a regular structure of higher level but lower than the third level structure
