A. Large Km: A high Km indicates a low affinity between the enzyme and substrate, requiring a high concentration of substrate to half-saturate the enzyme.

2. Relationship between velocity and enzyme concentration: The reaction rate is directly proportional to the enzyme concentration when the substrate concentration ([S]) is not limiting. For example, if the enzyme concentration is halved, the initial rates of the reaction (vo) and Vmax are both reduced to half of the original values.

3. Reaction order: When [S] is much less than Km, the reaction velocity is approximately proportional to the substrate concentration. This is known as first-order kinetics with respect to substrate. When [S] is much greater than Km, the velocity remains constant and equal to Vmax. In this case, the enzyme is saturated with substrate, and the reaction is said to have zero-order kinetics with respect to substrate concentration.

D. Lineweaver-Burk plot: Plotting 1/vo against 1/[S] creates a straight line, unlike plotting vo against [S]. This plot, known as the Lineweaver-Burk plot or double-reciprocal plot, is useful for determining Km and Vmax, as well as understanding the mechanism of action of enzyme inhibitors. The intercept on the x-axis represents -1/Km, and the intercept on the y-axis represents 1/Vmax. The slope of the line is equal to Km/Vmax.

VII. Enzyme Inhibition: Inhibitors are substances that decrease the velocity of enzyme-catalyzed reactions. They can be reversible or irreversible. Irreversible inhibitors form covalent bonds with enzymes, such as lead binding to the sulfhydryl side chain of cysteine in proteins. Reversible inhibitors, on the other hand, bind to enzymes through noncovalent bonds and can be dissociated by dilution of the enzyme-inhibitor complex.

A. Competitive Inhibition: This type of inhibition occurs when the inhibitor binds reversibly to the same site as the substrate, competing for binding. 

1. Effect on Vmax: The effect of a competitive inhibitor can be overcome by increasing the concentration of substrate. At a sufficiently high substrate concentration, the reaction velocity reaches the maximum observed in the absence of the inhibitor (Vmax).

2. Effect on Km: A competitive inhibitor increases the apparent Km for a given substrate. This means that more substrate is required to achieve half of the maximum reaction velocity (Vmax) in the presence of the inhibitor.

3. Effect on the Lineweaver-Burk plot: Competitive inhibition is characterized by a Lineweaver-Burk plot where the inhibited and uninhibited reactions intersect on the y-axis at 1/Vmax. The value of Vmax remains unchanged.
b. Large Km: A numerically large (high) Km reflects a low affinity of enzyme for substrate because a high concentration of substrate is needed to half-saturate the enzyme.
2.
Velocity relationship to enzyme concentration: The rate of the reaction is directly proportional to the enzyme concentration because [S] is not limiting. For example, if the enzyme concentration is halved, the initial rates of the reaction (vo) and that of Vmax are reduced to half that of the original.
3.
Reaction order: When [S] is much less (<<) than Km, the velocity of the reaction is approximately proportional to the substrate concentration (Fig. 5.10). The rate of reaction is then said to be first order with respect to substrate. When [S] is much greater (>>) than Km, the velocity is constant and equal to Vmax. The rate of reaction is then independent of substrate concentration (the enzyme is saturated with substrate) and is said to be zero order with respect to substrate concentration (see Fig. 5.10).
D. Lineweaver-Burk plot
When vo is plotted against [S], it is not always possible to determine when Vmax has been achieved because of the gradual upward slope of the hyperbolic curve at high substrate concentrations. However, if 1/vo is plotted versus 1/[S], a straight line is obtained (Fig. 5.11). This plot, the
Lineweaver-Burk plot (also called a double-reciprocal plot) can be used to calculate Km and Vmax as well as to determine the mechanism of action of enzyme inhibitors.
The equation describing the Lineweaver-Burk plot is: where the intercept on the x axis is equal to − 1/Km, and the intercept on the y axis is equal to 1/Vmax. [Note: The slope = Km/Vmax.]
VII. ENZYME INHIBITION
Any substance that can decrease the velocity of an enzyme-catalyzed reaction is called an inhibitor. Inhibitors can be reversible or irreversible. Irreversible inhibitors bind to enzymes through covalent bonds. Lead, for example, forms covalent bonds with the sulfhydryl side chain of cysteine in proteins. Ferrochelatase, an enzyme involved in heme synthesis (see p. 279), is irreversibly inhibited by lead. [Note: An important group of irreversible inhibitors are the mechanism-based inhibitors that are converted by the enzyme itself to a form that covalently links to the enzyme, thereby inhibiting it. They also are referred to as “suicide” inhibitors.] Reversible inhibitors bind to enzymes through noncovalent bonds and, thus, dilution of the enzyme–inhibitor complex results in dissociation of the reversibly bound inhibitor and recovery of enzyme activity. The two most commonly encountered types of reversible inhibition are competitive and noncompetitive.
A. Competitive inhibition
This type of inhibition occurs when the inhibitor binds reversibly to the same site that the substrate would normally occupy and, therefore, competes with the substrate for that site.
1.
Effect on Vmax: The effect of a competitive inhibitor is reversed by increasing the concentration of substrate. At a sufficiently high [S], the 2.
Effect on Km: A competitive inhibitor increases the apparent Km for a given substrate. This means that, in the presence of a competitive inhibitor, more substrate is needed to achieve one half Vmax.
3.
Effect on the Lineweaver-Burk plot: Competitive inhibition shows a characteristic Lineweaver-Burk plot in which the plots of the inhibited and uninhibited reactions intersect on the y axis at 1/Vmax (Vmax is reaction velocity reaches the Vmax observed in the absence of inhibitor, that is, Vmax is unchanged (Fig. 5.12).