Metabolic Pathways

Metabolic Pathways

There are a number of steps in each of the processes of cellular respiration, anabolic reactions, and catabolic reactions. A specific sequence of enzymatic actions controls these reactions; therefore, the enzymes are organized in the exact same sequence as the reactions they control. Each sequence of enzyme- controlled reactions is called a metabolic pathway (FIGURE 4-8).

An enzyme-controlled reaction usually increases its rate if the number of substrate molecules or enzyme molecules increases; however, the rate is often determined by an enzyme that regulates one of the reaction’s steps. Regulatory enzyme molecules are limited, and when the substrate concentration exceeds a certain level, the enzyme supply can become saturated. When this occurs, increasing ­substrate molecules will no longer have any effect on the reaction rate. Because of this, just one enzyme can control the entire pathway.

A rate-limiting enzyme is usually the first enzyme in a series. Being first is critical because if a rate-­ limiting enzyme were located somewhere else in the ­chemical pathway, an intermediate chemical could accumulate at that point. Fats and proteins as well as glucose can be broken down to release energy needed to synthesize ATP. In all three cases, aerobic respira-tion is still the result of these breakdown processes. The most common point of entry is into the citric acid cycle as acetyl CoA (FIGURE 4-9).

1. What is a metabolic pathway?

2. Why is being the first enzyme in a series critical?