tailieunhanh - Biochemistry, 4th Edition P15

Biochemistry, 4th Edition P15. Continuing Garrett and Grisham's innovative conceptual and organizing framework, "Essential Questions," BIOCHEMISTRY guides students through course concepts in a way that reveals the beauty and usefulness of biochemistry in the everyday world. Streamlined for increased clarity and readability, this edition also includes new photos and illustrations that show the subject matter consistently throughout the text. New end-of-chapter problems, MCAT practice questions, and the unparalleled text/media integration with the power of CengageNOW round out this exceptional package, giving you the tools you need to both master course concepts and develop critical problem-solving skills you can draw upon. | How Is the Primary Structure of a Protein Determined 103 in the protein. The efficiency with larger proteins is less a typical 2000-amino acid protein provides only 10 to 20 cycles of reaction. B. C-Terminal Analysis For the identification of the C-terminal residue of polypeptides an enzymatic approach is commonly used. Carboxypeptidases are enzymes that cleave amino acid residues from the C-termini of polypeptides in a successive fashion. Four carboxypeptidases are in general use A B C and Y Carboxypeptidase A from bovine pancreas works well in hydrolyzing the C-terminal peptide bond of all residues except proline arginine and lysine. The analogous enzyme from hog pancreas carboxypeptidase B is effective only when Arg or Lys are the C-terminal residues. Carboxypeptidase C from citrus leaves and carboxypeptidase Y from yeast act on any C-terminal residue. Because the nature of the amino acid residue at the end often determines the rate at which it is cleaved and because these enzymes remove residues successively care must be taken in interpreting results. Carboxypeptidase Y cleavage has been adapted to an automated protocol analogous to that used in Edman sequenators. Steps 4 and 5. Fragmentation of the Polypeptide Chain The aim at this step is to produce fragments useful for sequence analysis. The cleavage methods employed are usually enzymatic but proteins can also be fragmented by specific or nonspecific chemical means such as partial acid hydrolysis . Proteolytic enzymes offer an advantage in that many hydrolyze only specific peptide bonds and this specificity immediately gives information about the peptide products. As a first approximation fragments produced upon cleavage should be small enough to yield their sequences through end-group analysis and Edman degradation yet not so small that an overabundance of products must be resolved before analysis. A. Trypsin The digestive enzyme trypsin is the most commonly used reagent for specific proteolysis. .