Quaternary structure, protein

Principal micelle characteristics. The structure of the casein micelles has attracted the attention of scientists for a considerable time. Knowledge of micelle structure is important because the stability and behaviour of the micelles are central to many dairy processing operations, e.g. cheese manufacture, stability of sterilized, sweetened-condensed and reconstituted milks and frozen products. Without knowledge of the structure and properties of the casein micelle, attempts to solve many technological problems faced by the dairy industry will be empirical and not generally applicable. From the academic viewpoint, the casein micelle presents an interesting and complex problem in protein quaternary structure. 

Protein Quaternary Structures Range from Simple Dimers to Large Complexes... 

Proteins may be fibrous or globular. The structure and polarity of the particular amino acid R groups and their sequence affect the solubility properties and tertiary structure of proteins. Quaternary structure refers to the aggregation of similar protein subunits. 

MALDI has also been successfully used to study a number of intact non-covalent complexes such as protein quaternary structures [132-135]. However, MALDI has not yet contributed widely to the study of these complexes because it requires the crystallization of the sample with the matrix. Furthermore, the energy deposited to desorb the ions is not clearly known. In consequence, MALDI generally induces dissociation of the non-covalent interactions and leads to the formation of non-specific aggregates. Nevertheless, weak non-covalent interactions can survive during the MALDI process to allow the direct detection of intact complexes if specific methods, which have been developed to preserve these interactions, are followed [136],... 

Henrick, K. and Thornton, J.M. (1998) PQS a protein quaternary structure file server. Trends Biochem. Sci. 23,358-361. 

There are four recognized levels of protein structure primary, secondary, tertiary, and quaternary. The primary structure refers to the amino acid sequence of a protein. The primary structure is important to the protein s unique three-dimensional structure, its mechanism of action, and its relationship to other proteins with similar physiological roles. The amino acids in a protein are linked together by a specific type of covalent bond, called a peptide bond, that exists between adjacent amino acids in the polypeptide chain. Another important aspect of the primary structure is the sequence or order of amino acids in the polypeptide chain. The sequence of amino acids in a protein is specified by the nucleotide sequence of the segment of DNA containing the gene that codes for that protein. Each protein has a characteristic number and sequence of amino acid residues. The primary structure of a protein determines how the protein folds into a unique three-dimensional structure (further described by the secondary, tertiary, and quaternary structures), which in turn determines the biological function of the protein, see also Peptide Bond Proteins Quaternary Structure Secondary Structure Tertiary Structure. 

Different polypeptide chains may interact to form more complex multi-polypeptide proteins, wherein each individual polypeptide is known as a subunit. Subunit interactions and interrelationships are illustrated for the tetrameric protein haemoglobin (Figure 1.32). In the case of globular proteins, polypeptide chain association allows functions of individual polypeptide elements to be coordinated or indeed supplemented to give the whole molecule the opportunity to perform multiple biological functions. In the case of hbrous proteins, quaternary structure formation enhances overall molecular strength. 

Primary structure is the order of the amino acids. Secondary structure is characterized by a repetitive organization of the peptide backbone. Tertiary structure refers to the complete three-dimensional structure of the protein. Quaternary structure describes a protein that has multiple polypeptide chains. 

Hemoglobin is a classic example of protein quaternary structure. The protein has 4 subunits, two a-chains and two p-chains, and it exhibits positive cooperativity. Binding of oxygen to one subunit makes it easier for oxygen to bind to other subunits. 

Biological processes involving protein-protein recognition are of special interest since they are the basis of many relevant events in the cellular machinery. Enzyme-substrate binding, protein membrane signaling, immunologic response, and protein quaternary structural changes caused by cofactors or allosteric modulators are only a few of the examples that show the importance of protein-protein interactions at the molecular level. 

Quaternary structure refers to the arrangement of chains in proteins. Quaternary structure is maintained by interactions between amino acids on the individual chains. 

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