Multisubstrate Reactions

Thus far, we have considered enzyme-catalyzed reactions involving one or two substrates. How are the kinetics described in those cases in which more than two substrates participate in the reaction? An example might be the glycolytic enzyme glyceraldehyde-3-phosphate dehydrogenase (Chapter 19):

NAD⁺ + glyceraldehyde-3-P + P_i ® NADH + H⁺ + 1,3-bisphosphoglycerate

Many other multisubstrate examples abound in metabolism. In effect, these situations are managed by realizing that the interaction of the enzyme with its many substrates can be treated as a series of uni- or bisubstrate steps in a multi-step reaction pathway. Thus, the complex mechanism of a multisubstrate reaction is resolved into a sequence of steps, each of which obeys the single- and double-displacement patterns just discussed.

14.6 · RNA and Antibody Molecules as Enzymes: Ribozymes and Abzymes

Catalytic RNA Molecules: Ribozymes

It was long assumed that all enzymes are proteins. However, in recent years, more and more instances of biological catalysis by RNA molecules have been discovered. These catalytic RNAs, or ribozymes, satisfy several enzymatic criteria: They are substrate-specific, they enhance the reaction rate, and they emerge from the reaction unchanged. For example, RNase P, an enzyme responsible for the formation of mature tRNA molecules from tRNA precursors, requires an RNA component as well as a protein subunit for its activity in the cell. In vitro, the protein alone is incapable of catalyzing the maturation reaction, but the RNA component by itself can carry out the reaction under appropriate conditions. In another case, in the ciliated protozoan Tetrahymena, formation of mature ribosomal RNA from a pre-rRNA precursor involves the removal of an internal RNA segment and the joining of the two ends in a process known as splicing out. The excision of this intervening internal sequence of RNA and ligation of the ends is, remarkably, catalyzed by the intervening sequence of RNA itself, in the presence of Mg²⁺ and a free molecule of guanosine nucleo-side or nucleotide (Figure 14.23). In vivo, the intervening sequence RNA probably acts only in splicing itself out; in vitro, however, it can act many times, turning over like a true enzyme.

Figure 14.23 •RNA splicing in Tetrahymena rRNA maturation: (a) the guanosine-mediated reaction involved in the autocatalytic excision of the Tetrahymena rRNA intron, and (b) the overall splicing process. The cyclized intron is formed via nucleophilic attack of the 3'-OH on the phosphodiester bond that is 15 nucleotides from the 5'-GA end of the spliced-out intron. Cyclization frees a linear 15-mer with a 5'-GA end.

Date: 2016-01-03; view: 796