/ 1


[E] = 1.0 unit

r i

Concentration of substrate [S]

Concentration of substrate [S]

▲ FIGURE 3-18 Mechanism of phosphorylation by protein kinase A. (Top) Initially, ATP and the target peptide bind to the active site (see Figure 3-17a). Electrons of the phosphate group are delocalized by interactions with lysine side chains and Mg2+. Colored circles represent the residues in the kinase core critical to substrate binding and phosphoryl transfer. Note that these residues are not adjacent to one another in the amino acid sequence. (Middle) A new bond then forms between the serine or threonine side-chain oxygen atom and y phosphate, yielding a pentavalent intermediate. (Bottom) The phosphoester bond between the p and y phosphates is broken, yielding the products ADP and a peptide with a phosphorylated serine or threonine side chain. The catalytic mechanism of other protein kinases is similar.

High-affinity substrate (S)

Concentration of substrate ([S] or [S'])

▲ EXPERIMENTAL FIGURE 3-19 The Km and Vmax for an enzyme-catalyzed reaction are determined from plots of the initial velocity versus substrate concentration. The shape of these hypothetical kinetic curves is characteristic of a simple enzyme-catalyzed reaction in which one substrate (S) is converted into product (P). The initial velocity is measured immediately after addition of enzyme to substrate before the substrate concentration changes appreciably. (a) Plots of the initial velocity at two different concentrations of enzyme [E] as a function of substrate concentration [S]. The [S] that yields a halfmaximal reaction rate is the Michaelis constant Km, a measure of the affinity of E for S. Doubling the enzyme concentration causes a proportional increase in the reaction rate, and so the maximal velocity Vmax is doubled; the Km, however, is unaltered. (b) Plots of the initial velocity versus substrate concentration with a substrate S for which the enzyme has a high affinity and with a substrate S' for which the enzyme has a low affinity. Note that the Vmax is the same with both substrates but that Km is higher for S', the low-affinity substrate.

moving molecules between widely dispersed enzymes (Figure 3-20a). To overcome this impediment, cells have evolved mechanisms for bringing enzymes in a common pathway into close proximity.

In the simplest such mechanism, polypeptides with different catalytic activities cluster closely together as subunits of a multimeric enzyme or assemble on a common "scaffold" (Figure 3-20b). This arrangement allows the products of one reaction to be channeled directly to the next enzyme in the pathway. The first approach is illustrated by pyruvate

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