Problem 15 - Biochemistry: Concepts and Connections

Question

The muscle isozyme of lactate dehydrogenase is inhibited by lactate. Steady-state kinetic analysis yielded the following data, with lactate either absent or present at a fixed concentration. (a) Pyruvate is the substrate whose concentration is varied in one plot, NADH in the other. Identify each. Use an arrow and the appropriate letter (b, c, d, or e) to identify each of the following. (b) Reciprocal of \(V_{\max }\) for the uninhibited enzyme. (c) The line representing data obtained in the presence of lactate acting as a competitive inhibitor with respect to the variable substrate. (d) The line representing data obtained in the presence of lactate acting as a noncompetitive inhibitor with respect to the variable substrate. (e) Reciprocal of \(K_{\mathrm{M}}\) in the presence of lactate acting as a competitive inhibitor. (f) If \(K_{\mathrm{M}}\) for NADH is \(2 \times 10^{-5} \mathrm{M}\), then which of the following is the most appropriate NADH concentration to use when determining \(K_{\mathrm{M}}\) for pyruvate: \(10^{-7} \mathrm{M}, 10^{-6} \mathrm{M}, 10^{-5} \mathrm{M}, 10^{-4} \mathrm{M}\), or \(10^{-3} \mathrm{M}\) ?

Step-by-Step Solution

Answer

NADH at 10⁻⁴ M is most appropriate. Use arrow to indicate: (b) Vmax without inhibition, (c) competitive inhibition line, (d) noncompetitive inhibition line, (e) new Km.

Step 1: Understand the Problem

We need to identify how lactate affects the enzyme kinetics of lactate dehydrogenase in the presence of different substrates like pyruvate and NADH.

Step 2: Identify the Types of Inhibition

Lactate can act as a competitive or noncompetitive inhibitor. A competitive inhibitor increases the apparent Km (shift in slope), while a noncompetitive inhibitor decreases Vmax (affecting y-intercept).

Step 3: Assign Variables to Substrates

Given two variables: Pyruvate and NADH. We need to decide which plot corresponds to each of the substrates. Pyruvate concentration is varied for competitive inhibition as it is the direct substrate.

Step 4: Identify Reciprocal of Vmax without Inhibition

Without any inhibitors present, the line's y-intercept in a Lineweaver-Burk plot corresponds to the reciprocal of Vmax. Identify the uninhibited enzyme line from the data.

Step 5: Identify Competitive Inhibition Line

In a Lineweaver-Burk plot, a competitive inhibitor changes the slope but not the y-intercept. Identify the line affected by the presence of lactate that undergoes such changes.

Step 6: Identify Noncompetitive Inhibition Line

For noncompetitive inhibition, both the slope and y-intercept can change. Identify the line that shows a change in y-intercept with lactate.

Step 7: Determine Reciprocal of Km with Competitive Inhibition

For competitive inhibition, the line's x-intercept shifts. Determine the 'new' x-intercept and the corresponding reciprocal of Km.

Step 8: Choose Appropriate NADH Concentration

To measure Km for pyruvate accurately, NADH should be present at a concentration much higher than its Km (2 x 10⁻⁵ M). Choose a concentration notably higher.

Key Concepts

Lactate Dehydrogenase

Lactate dehydrogenase is a crucial enzyme found in many tissues. Its primary role is to catalyze the conversion of lactate to pyruvate and back, using the coenzyme NAD⁺ or NADH. This reversible reaction is an essential step in metabolic pathways such as glycolysis and gluconeogenesis.

This enzyme has different isozymes, meaning there are variations of it in different tissues. In muscles, the isozyme of lactate dehydrogenase is slightly different compared to other tissues, which may affect how it is regulated or inhibited. Learning about how these isozymes react in different environments is key in understanding their function.

For instance, when lactate accumulates, it can act as an inhibitor to lactate dehydrogenase. Understanding how this occurs helps us in fields like exercise physiology, where the build-up of lactate is related to muscle fatigue.

Competitive Inhibition

Competitive inhibition occurs when a substance mimics a substrate and competes for the enzyme's active site. In the context of lactate dehydrogenase, lactate can act as a competitive inhibitor concerning pyruvate.

What happens in this case is that lactate competes with pyruvate for the same active site on lactate dehydrogenase. As a result, more pyruvate molecules are needed to achieve the same level of enzyme activity, effectively increasing the apparent \( K_{M} \) and changing the slope in a Lineweaver-Burk plot. However, the maximum velocity \( V_{ ext{max}} \) remains the same because if enough substrate molecules are present, they can outnumber and outcompete the inhibitor at the active sites.

Noncompetitive Inhibition

In noncompetitive inhibition, the inhibitor binds to a site on the enzyme other than the active site. This binding changes the enzyme's structure, affecting its functionality. With lactate dehydrogenase, lactate acts as a noncompetitive inhibitor when viewed in relation to NADH.

The binding of the inhibitor reduces the enzyme's activity regardless of how much substrate is present. This results in a decreased \( V_{ ext{max}} \) since the enzyme's efficiency is compromised. In a Lineweaver-Burk plot, both the slope and the y-intercept change, indicating that the enzyme cannot reach its previous maximum rate, even if there is ample substrate.

Lineweaver-Burk Plot

The Lineweaver-Burk plot is a graphical representation designed to analyze enzyme kinetics. It's essentially a double reciprocal plot, plotting \( 1/ ext{Velocity} \) against \( 1/ ext{substrate concentration} \). This linear plot helps determine important kinetic constants like \( K_{M} \) and \( V_{ ext{max}} \).

In this plot, the y-intercept represents \( 1/V_{ ext{max}} \), while the x-intercept reflects the negative reciprocal of \( K_{M} \). The slope illustrates \( K_{M}/V_{ ext{max}} \). Lineweaver-Burk plots are especially useful for visualizing how different types of inhibition affect enzyme activity. For instance:

• Competitive inhibition increases the slope due to increased apparent \( K_{M} \), but does not change the y-intercept.
• Noncompetitive inhibition alters both the slope and y-intercept, reflecting changes in both \( K_{M} \) and \( V_{ ext{max}} \).

By examining these plots, scientists can infer the type of inhibition occurring and better understand enzyme-substrate interactions.