Protein is mainly composed of chemical elements such as carbon, hydrogen, oxygen and nitrogen. It is an important biological macromolecule. All proteins are multimers formed by the connection of 20 different amino acids. After Forming proteins, these amino acids are also called as a residue.
The boundaries between proteins and peptides are not very clear. Some people believe that the number of residues required for a functionally acting domain is called a polypeptide or peptide if the number of residues is less than 40. To function biologically, proteins need to be properly folded into a specific configuration, mainly through a large number of non-covalent interactions (such as hydrogen bonds, ionic bonds, van der Waals forces and hydrophobic interactions); in addition, in some proteins (especially in the case of secreted proteins), disulfide bonds also play a key role. In order to understand the mechanism of action of proteins at the molecular level, it is often necessary to determine the three-dimensional structure of a protein. Structural biology has been developed by studying protein structure, using techniques including X-ray crystallography, nuclear magnetic resonance, etc. to resolve protein structures.
A certain number of residues are necessary to exert a certain biochemical function; 40-50 residues are usually the lower limit of the size of a functional domain. Protein size can range from such a lower limit up to thousands of residues. The current estimated average length of proteins differs between different species, typically about 200-380 residues, while eukaryotes have an average protein length of about 55% longer than prokaryotes. Larger protein aggregates can be formed by many protein subunits; for example, by the polymerization of thousands of actin molecules to form protein fibers.
In 1959, Perutz and Kendrew analyzed the structure of hemoglobin and myoglobin, solved the three-dimensional structure, and won the 1962 Nobel Prize in Chemistry.
Pauling discovered the basic structure of the protein. Based on the X-ray diffraction data, Crick and Watson proposed a model of the three-dimensional structure of DNA. Received the 1962 Nobel Prize in Physiology or Medicine. After the 1950s, Hauptmann and Karle established a purely mathematical theory for the direct determination of crystal structures using X-ray analysis, which has epoch-making significance in crystal research, especially in the study of macromolecular biological substances such as hormones, antibiotics, and proteins. And the molecular structure of new drugs played an important role. They were awarded the 1985 Nobel Prize in Chemistry.
Protein molecules are covalent polypeptide chains formed by the condensation of amino acids end-to-end, but natural protein molecules are not loose random polypeptide chains. Each natural protein has its own unique spatial structure or three-dimensional structure, which is often referred to as the conformation of the protein, ie the structure of the protein.
The molecular structure of a protein can be divided into four levels to describe its different aspects:
Primary structure: A linear amino acid sequence that makes up a protein polypeptide chain.
Secondary structure: a stable structure formed by hydrogen bonds between C=O and N-H groups between different amino acids, mainly α-helix and β-sheet.
Tertiary structure: The three-dimensional structure of a protein molecule formed by the arrangement of multiple secondary structural elements in three dimensions.
Quaternary structure: used to describe a protein complex molecule that is functionally formed by interactions between different polypeptide chains (subunits).
In addition to these structural levels, proteins can be transformed in multiple similar structures to perform their biological functions. For functional structural changes, these tertiary or quaternary structures are usually described in a chemical conformation, and the corresponding structural transformation is referred to as a conformational change.
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