Enzyme Kinetics Basics

Why Enzyme Kinetics Matters for Peptides

Many peptide drugs target enzymes — either as inhibitors, substrates, or activators. Understanding enzyme kinetics is essential for designing peptides that interact with enzymes in predictable, therapeutically useful ways.

The Michaelis-Menten Equation

The foundation of enzyme kinetics is the Michaelis-Menten equation:

v = (Vmax * [S]) / (Km + [S])

Where:

v = reaction velocity (rate of product formation)
Vmax = maximum reaction velocity when the enzyme is fully saturated
[S] = substrate concentration
Km = Michaelis constant (substrate concentration at half Vmax)

At low substrate concentrations (when [S] << Km), the reaction rate increases almost linearly with [S]. At high concentrations (when [S] >> Km), the rate plateaus at Vmax because all enzyme active sites are occupied.

Km and Vmax: What They Tell You

Km (Michaelis constant) reflects the enzyme’s affinity for its substrate:

Low Km = high affinity (the enzyme binds the substrate tightly and reaches half-maximal velocity at low concentration)
High Km = low affinity (more substrate is needed to achieve the same rate)

Vmax depends on two factors:

The total enzyme concentration ([E]T)
The turnover number (kcat): how many substrate molecules each enzyme molecule converts per second

The ratio kcat / Km is called the catalytic efficiency and is used to compare how well different enzymes catalyze their reactions.

Enzyme Inhibition

Understanding how inhibitors work is critical for drug design.

Competitive Inhibition

A competitive inhibitor binds to the enzyme’s active site, competing directly with the substrate. Key effects:

Km increases (apparent affinity decreases — more substrate is needed)
Vmax stays the same (enough substrate can outcompete the inhibitor)
Example: Methotrexate competes with folate for the active site of dihydrofolate reductase

Non-Competitive Inhibition

A non-competitive inhibitor binds to a site other than the active site (an allosteric site). It changes the enzyme’s conformation so it can no longer catalyze the reaction efficiently. Key effects:

Km stays the same (substrate binding is unaffected)
Vmax decreases (some enzyme molecules are rendered inactive)
Example: Heavy metal ions (lead, mercury) binding to cysteine residues away from the active site

Mixed Inhibition

In mixed inhibition, the inhibitor can bind to both the free enzyme and the enzyme-substrate complex, but with different affinities. Both Km and Vmax are affected.

The Lineweaver-Burk Plot

The Lineweaver-Burk plot (double reciprocal plot) linearizes the Michaelis-Menten equation by plotting 1/v against 1/[S]:

1/v = (Km/Vmax)(1/[S]) + 1/Vmax

This transformation makes it easier to visually distinguish inhibition types:

Inhibition Type	Km Effect	Vmax Effect	Lineweaver-Burk Pattern
Competitive	Increases	Unchanged	Lines intersect on y-axis
Non-competitive	Unchanged	Decreases	Lines intersect on x-axis
Mixed	Changes	Decreases	Lines intersect in quadrant

Connection to Peptide Drug Design

Enzyme kinetics directly informs peptide drug development:

Km tells you how potent a peptide substrate analog might be
Ki (inhibitor constant) measures how tightly a peptide inhibitor binds
kcat/Km identifies the best substrates for designing competitive inhibitors
IC50 values from inhibition assays guide lead optimization

When designing peptide-based enzyme inhibitors, medicinal chemists aim for low Ki values (tight binding) and high selectivity for the target enzyme over related family members to minimize side effects.

Summary

Concept	Meaning
Km	Substrate concentration at half Vmax; reflects affinity
Vmax	Maximum velocity at enzyme saturation
kcat	Turnover number (substrate molecules per enzyme per second)
kcat/Km	Catalytic efficiency
Competitive inhibition	Inhibitor binds active site; Km up, Vmax same
Non-competitive inhibition	Inhibitor binds allosteric site; Km same, Vmax down
Lineweaver-Burk	Double-reciprocal plot for visualizing kinetics