Clinical Chemistry 2: Enzyme Kinetics

Vocabulary terms and concepts regarding enzyme kinetics, Michaelis-Menten dynamics, factors affecting activity, and laboratory measurement methods.

Michaelis-Menten Theory

A theory describing the relationship between the rate of an enzymatic reaction and substrate concentration, stating that the reaction rate is initially proportional to substrate concentration until it reaches a maximum velocity ($V_{max}$).

$V_{max}$ (Maximum Velocity)

The point in an enzymatic reaction where the rate becomes constant and reaches its peak because all enzyme active sites are saturated with substrate.

$K_m$ (Michaelis Constant)

The substrate concentration at which the velocity of an enzymatic reaction is half of $V_{max}$, providing a measure of the enzyme's affinity for its substrate.

Low $K_m$

An indicator that an enzyme has a high affinity for its substrate, meaning it reaches half its maximum velocity at a low substrate concentration.

High $K_m$

An indicator that an enzyme has a low affinity for its substrate, requiring a higher substrate concentration to reach half of its maximum velocity.

First-order Reactions

Reactions where the enzyme concentration is fixed and the rate of reaction is directly proportional to the substrate concentration because the substrate concentration is low.

Zero-order Reactions

Reactions occurring when substrate concentration is high enough to saturate all available enzymes, resulting in a maximum reaction velocity ($V_{max}$) that does not increase with additional substrate.

Second-order Reactions

Reactions where the rate is proportional to the product of the concentrations of two reactants or the square of the concentration of one reactant ($S_1 + S_2 \rightarrow P$).

Aspartate Aminotransferase (AST)

An enzyme example for second-order reactions because it requires two substrates, aspartate and $\alpha\text{-ketoglutarate}$, to proceed.

Optimal Enzyme pH

The specific pH range where most enzymes operate, typically between $7.0$ and $8.0$, although specific enzymes like pepsin ($1.5$) and ALP ($10.5$) vary.

Pepsin

An enzyme noted for its low specific optimal pH range of $1.5$.

ALP (Alkaline Phosphatase)

An enzyme noted for its high specific optimal pH range of $10.5$.

Optimal Temperature

The temperature, typically set at $37\,^{\circ}C$, at which enzymes catalyze reactions at their maximum efficiency.

Denaturation

The alteration of an enzyme's protein structure which leads to loss of activity, occurring due to extreme pH changes or high temperatures between $40-50\,^{\circ}C$.

Cofactors

Nonprotein entities, such as activators or coenzymes, that must bind to specific enzymes before a reaction can occur.

Competitive Inhibitor

An inhibitor that binds to the active site of an enzyme, preventing the substrate from binding and resulting in no product formation.

Noncompetitive Inhibitor

An inhibitor that binds to an allosteric site rather than the active site, preventing the substrate from binding and resulting in no product formation.

Uncompetitive Inhibitor

An inhibitor that binds to the enzyme-substrate complex.

Allopurinol

A pharmacological inhibitor that targets xanthine oxidase to prevent the creation of uric acid.

Penicillin

An antibiotic that inhibits transpeptidase, preventing the formation of bacterial cell walls.

Coupled-Enzyme Assay

An indirect measurement method where the activity of the enzyme being tested is linked to another more easily detectable enzyme reaction.

Fixed-Time Method (End-Point)

An assay where reactants are combined, the reaction proceeds for a designated time and is then stopped, after which a measurement of the overall reaction is made.

Continuous-Monitoring Method (Kinetic Assay)

An assay where the rate of product formation is monitored via multiple absorbance measurements while the reaction is still proceeding.

International Unit (U)

The amount of enzyme that will catalyze the reaction of $1\,\mu mol$ of substrate per minute under specified conditions.

Katal (mole/s)

The SI unit of enzyme activity, representing the amount of enzyme that catalyzes the reaction of $1\,mol$ of substrate per second.

$$U/L$$

International Unit per Liter, the unit used in the laboratory to represent enzyme concentration.