U01.05.001 Enzyme kinetics

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Learning Objectives

Master the principles of Enzyme Kinetics. Understand the relationship between substrate concentration and reaction velocity, interpret Michaelis-Menten and Lineweaver-Burk plots, and distinguish between different types of enzyme inhibition for the USMLE Step 1.

1. Michaelis-Menten Kinetics

Most enzymatic reactions follow a hyperbolic curve, describing how the reaction velocity ( $V$ ) changes with substrate concentration ( $[S]$ ).

Variable	Definition & Properties
$V_{max}$	The maximum velocity of the reaction. It is directly proportional to enzyme concentration.
$K_m$	The $[S]$ required to reach $1/2 V_{max}$ . It is inversely related to enzyme affinity ( $\uparrow K_m = \downarrow$ affinity).
Cooperativity	A sigmoidal curve (instead of hyperbolic) indicates positive cooperativity (e.g., Aspartate transcarbamoylase).

2. Lineweaver-Burk Plot

The Lineweaver-Burk plot is a double-reciprocal transformation ($1/V$ vs. $1/[S]$ ) that creates a straight line, making it easier to determine $V_{max}$ and $K_m$.

Plot Feature	Mathematical Value	Interpretation
Y-intercept	$1 / V_{max}$	The closer to 0, the higher the $V_{max}$ .
X-intercept	$-1 / K_m$	The closer to 0, the higher the $K_m$ (lower affinity).
Slope	$K_m / V_{max}$	Represents the ratio of affinity to maximum velocity.

3. Effects of Enzyme Inhibition

Inhibitors change the kinetics of enzymatic reactions by either competing for the active site or binding elsewhere to reduce catalytic efficiency.

Feature	Competitive (Reversible)	Noncompetitive
Resemble Substrate?	Yes	No
Bind Active Site?	Yes	No (Allosteric site)
Overcome by $\uparrow [S]$ ?	Yes	No
Effect on $V_{max}$	Unchanged	Decreased ( $\downarrow$ )
Effect on $K_m$	Increased ( $\uparrow$ )	Unchanged
Pharmacodynamics	$\downarrow$ Potency	$\downarrow$ Efficacy

Activity:

4. Irreversible Competitive Inhibition

Irreversible competitive inhibitors bind covalently to the active site. Because they cannot be displaced by substrate, they behave kinetically like noncompetitive inhibitors.

Parameter	Change	Reasoning
$V_{max}$	Decreased ( $\downarrow$ )	Reduces the effective number of available enzymes.
$K_m$	Unchanged	The remaining free enzymes have normal affinity.
Pharmacodynamics	$\downarrow$ Efficacy	The maximal effect of the drug/enzyme is lowered.

Activity:

High-Yield Clinical Pearls:

Kompetitive Inhibitors: Remember that Kompetitive inhibitors increase Km. On a Lineweaver-Burk plot, the lines will cross at the Y-axis (same $V_{max}$ ).
Noncompetitive Inhibitors: These do not cross at the Y-axis because the $V_{max}$ is different. They look like they are “pinched” together at the X-axis (same $K_m$ ).
Statins: A classic clinical example of reversible competitive inhibitors of HMG-CoA reductase.