4. Enzyme Inhibition & Lineweaver–Burk

An irreversible inhibitor commonly:

Forms a persistent, often covalent interaction with the enzyme

The manual contrasts reversible inhibitors with irreversible inhibitors that may covalently bind.

A competitive inhibitor binds:

The active site of free enzyme and competes with substrate

Competitive inhibitors compete with substrate for the enzyme's active site.

What happens to apparent Km in competitive inhibition?

Increases

More substrate is needed to achieve half-maximal rate in the presence of a competitive inhibitor.

What happens to Vmax in ideal competitive inhibition?

Remains unchanged

Sufficiently high substrate can outcompete a competitive inhibitor, preserving Vmax.

How can competitive inhibition be overcome in the model described?

Increase substrate concentration

High substrate levels favor substrate occupancy over inhibitor occupancy.

An uncompetitive inhibitor binds:

Only the ES complex

Uncompetitive inhibition requires substrate-bound enzyme.

In uncompetitive inhibition, what happens to both apparent Km and Vmax?

Both decrease

Uncompetitive inhibition lowers both apparent Km and Vmax.

Can increasing substrate overcome uncompetitive inhibition?

No

The inhibitor binds ES, so adding more substrate does not displace it in the simple model.

A mixed inhibitor can bind:

Both free enzyme and ES complex

Mixed inhibition includes binding to E and ES with potentially different affinities.

Pure noncompetitive inhibition is the special case where the inhibitor:

Binds E and ES with equal affinity

Equal affinity for E and ES is the defining pure noncompetitive case.

In pure noncompetitive inhibition, what happens to Vmax?

Decreases

The inhibitor reduces the amount of catalytically effective enzyme.

In pure noncompetitive inhibition, what happens to Km?

Remains unchanged

Equal inhibitor affinity for E and ES changes Vmax without changing Km.

In mixed inhibition, Km can:

Increase or decrease depending on relative inhibitor affinities

Different affinities for E and ES shift the apparent Km.

What is plotted on the x-axis of a Lineweaver–Burk plot?

1/[S]

The double-reciprocal plot uses 1/[S] on x and 1/v0 on y.

What is plotted on the y-axis of a Lineweaver–Burk plot?

1/v0

Both axes are reciprocals: 1/v0 versus 1/[S].

What is the y-intercept of a Lineweaver–Burk plot?

1/Vmax

The y-intercept equals 1/Vmax.

What is the x-intercept of a Lineweaver–Burk plot?

−1/Km

The x-intercept equals −1/Km.

What is the slope of a Lineweaver–Burk plot?

Km/Vmax

The reciprocal form is 1/v0 = (Km/Vmax)(1/[S]) + 1/Vmax.

In the manual's summary, competitive-inhibition double-reciprocal lines:

Meet on the y-axis

Competitive inhibition leaves 1/Vmax unchanged, so the y-intercept is shared.

In the manual's summary, uncompetitive-inhibition double-reciprocal lines:

Are parallel

Uncompetitive inhibition scales Km and Vmax together, producing parallel lines.