tailieunhanh - Biochemistry, 4th Edition P88

Biochemistry, 4th Edition P88. 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 Thymine Nucleotides Synthesized 833 Energy status of cell is robust ATP is high. Make DNA Q ATP occupies activity site A ribonucleotide reductase ON Q ATP in specificity site S favors CDP or UDP in catalytic site C - dCDP dUDP f C DA--------- ----- dUMP------ dTMP------ ----- dTTP dUDP Q dTTP occupies specificity site S favoring GDP or ADP in catalytic site C GDP----- dGDP------ dGTP @ dGTP occupies specificity site S favoring ADP in catalytic site C- dADP t @ dATP replaces ATP in activity site A ribonucleotide reductase OFF FIGURE Regulation of deoxynucleotide biosyn-thesis the rationale for the various affinities displayed by the two nucleotide-binding regulatory sites on ribonucleotide reductase. enzyme must be turned on and off in response to the need for dNTPs. Second the relative amounts of each NDP substrate transformed into dNDP must be controlled so that the right balance of dATP dGTP dCTP dTTP is produced. The two different effector-binding sites on ribonucleotide reductase discrete from the substrate-binding catalytic site are designed to serve these purposes. As noted previously these two regulatory sites are designated the overall activity site and the substrate specificity site. Only ATP and dATP are able to bind at the overall activity site. ATP is an allosteric activator and dATP is an allosteric inhibitor and they compete for the same site. If ATP is bound the enzyme is active whereas if its deoxy counterpart dATP occupies this site the enzyme is inactive. That is ATP is a positive effector and dATP is a negative effector with respect to enzyme activity and they compete for the same site. The second regulatory site the substrate specificity site can bind either ATP dTTP dGTP or dATP and the substrate specificity of the enzyme is determined by which of these nucleotides occupies this site. If ATP is in the substrate specificity site ribonucleotide reductase preferentially binds pyrimidine nucleotides UDP or CDP at its active site