Isoelectric point protein structure

Food protein Protein component % of total protein Molecular weight Isoelectric point Molecular structure... 

Most of the E. coli ribosomal proteins are rather basic with high isoelectric points (Kaltschmidt, 1971) and a high content of basic amino acids (Tables I and II). The complete primary structures of all . coli ribosomal proteins have been determined by Wittmann-Liebold and coworkers (see Table III and Appendix). 

Module 1, Determination of Chemical and Structural Information on the Sample. The task of Module 1 is to provide non-chromato-graphic data for analytes prior to specification of the chromatographic method. Data bases have been developed for pK values of organic molecules, isoelectric points of proteins, and fluorescence spectral properties of organic molecules. 

Histones are very basic proteins with an isoelectric point between 10.31 and 11.27 for human complement. They are present in virtually all eukaryotes (with the exception of dinoflagellates [14]) where they are associated with most of the nuclear DNA. The DNA is wrapped around an octamer formed by the four core histones H2A, H2B, H3 and H4 to build a nucleosome. This particle is the fundamental repeating unit of chromatin [15]. A string of nudeosomes can fold into a higher order structure, the exact molecular nature of which is still not fully understood but clearly has a strong influence on gene expression. 

Electrostatic charges due to ionized acidic or basic amino acids influence protein solubility. At extremes of pH, many poorly soluble proteins are dissolved and their molecular structures unfolded due to surplus of similar repelling charges. Gluten proteins have few charged groups and so are poorly soluble in neutral solution (15). Dispersions of other proteins must be adjusted to their isoelectric point or have salt added to optimize cohesion and adhesion. 

When they are first isolated, proteins are usually characterized by Mr, isoelectric point, and other easily measured properties. Among these is the amino acid composition112 which can be determined by completely hydrolyzing the protein to the free amino acids. Later, it is important to establish the primary structure or amino acid sequence.51 This has been accomplished traditionally by cutting the peptide chain into smaller pieces that can be characterized easily. However, most protein sequences are now deduced initially from the corresponding DNA sequences, but further chemical characterization is often needed. 

Many enzymes exist within a cell as two or more isoenzymes, enzymes that catalyze the same chemical reaction and have similar substrate specificities. They are not isomers but are distinctly different proteins which are usually encoded by different genes.22 23 An example is provided by aspartate aminotransferase (Fig. 2-6) which occurs in eukaryotes as a pair of cytosolic and mitochondrial isoenzymes with different amino acid sequences and different isoelectric points. Although these isoenzymes share less than 50% sequence identity, their internal structures are nearly identical.24-27 The two isoenzymes, which also share structural homology with that of E. coli,28 may have evolved separately in the cytosol and mitochondria, respectively, from an ancient common precursor. Tire differences between them are concentrated on the external surface and may be important to as yet unknown interactions with other protein molecules. 

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