tailieunhanh - Biochemistry, 4th Edition P108

Biochemistry, 4th Edition P108. 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 Are Receptor Signals Transduced 1033 More than 170 calcium-modulated proteins are known. All possess a characteristic peptide domain consisting of a short -helix a loop of 12 amino acids and a second -helix Figure . Robert Kretsinger at the University of Virginia initially discovered this pattern in parvalbumin a protein first identified in the carp fish and later in neurons possessing a high firing rate and a high oxidative metabolism. Kretsinger lettered the six helices of parvalbumin A through F. He noticed that the E and F helices joined by a loop resembled the thumb and forefinger of a right hand and named this structure the EF hand a name in common use today to identify the helix-loop-helix motif in calcium-binding proteins. In the EF hand Ca2 is coordinated by six carboxyl oxygens contributed by a glutamate and three aspartates by a carbonyl oxygen from a peptide bond and by the oxygen of a coordinated water molecule. The EF hand was subsequently identified in calmodulin troponin C and calbindin-9K. Most of the known EF-hand proteins possess two or more as many as eight EF-hand domains usually arranged so that two EF-hand domains may directly contact each other. Calmodulin Target Proteins Possess a Basic Amphiphilic Helix The conformations of EF-hand proteins change dramatically upon binding of Ca2 ions. This change promotes binding of the EF-hand protein with its target pro-tein s . For example calmodulin CaM a 148-residue protein found in many cell types modulates the activities of a large number of target proteins including Ca2 -ATPases protein kinases phosphodiesterases and NAD kinase. CaM binds to these and to many other proteins with extremely high affinities KD values typically in the high picomolar to low nanomolar range . All CaM target proteins possess a feasic amphiphilic alpha helix a Baa helix to which CaM binds specifically and with high affinity. Viewed end-on in the so-called helical wheel representation Figure a Baa .