Biochemistry, 4th Edition P16. 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. | What Is the Nature of Amino Acid Sequences 113 the same amino acid residues are always found Figure . These invariant residues serve roles crucial to the biological function of this protein and thus substitutions of other amino acids at these positions cannot be tolerated. The number of amino acid differences between two cytochrome c sequences is proportional to the phylogenetic difference between the species from which they are derived. Cytochrome c in humans and in chimpanzees is identical human and another mammalian sheep cytochrome c differ at 10 residues. The human cytochrome c sequence has 14 variant residues from a reptile sequence rattlesnake 18 from a fish carp 29 from a mollusc snail 31 from an insect moth and more than 40 from yeast or higher plants cauliflower . The Phylogenetic Tree for Cytochrome c Figure displays a phylogenetic tree a diagram illustrating the evolutionary relationships among a group of organisms constructed from the sequences of cytochrome c. The tips of the branches are occupied by contemporary species whose sequences have been determined. The tree has been deduced by computer analysis of these sequences to find the minimum number of mutational changes connecting the branches. Other computer methods can be used to infer potential ancestral sequences represented by nodes or branch points in the tree. Such analysis ultimately suggests a primordial cytochrome c sequence lying at the base of the tree. Evolutionary trees constructed in this manner that is solely on the basis of amino acid differences occurring in the primary sequence of one selected protein show remarkable agreement with phylogenetic relationships derived from more classic approaches and have given rise to the field of molecular evolution. Related Proteins Share a Common Evolutionary Origin Amino acid sequence analysis reveals that proteins with related functions often show a high degree of sequence similarity. Such findings suggest a common ancestry for .
