Which one of the following about enzymes is true?
**Core Concept**
Enzymes are biological molecules, typically proteins, that significantly speed up the rate of virtually all of the chemical reactions that take place within cells. They are highly specific, with each enzyme catalyzing only one or a few related chemical reactions.
**Why the Correct Answer is Right**
Enzymes are highly specific due to their unique shape, which allows them to bind to specific substrates. This binding causes a conformational change in the enzyme, positioning the substrate for optimal reaction conditions. Enzymes can be inhibited by various factors, including competitive inhibitors, which bind to the active site of the enzyme and reduce its activity. The correct answer highlights the importance of substrate specificity in enzyme function.
**Why Each Wrong Option is Incorrect**
**Option A:** Incorrect, as enzymes are not solely composed of protein; some enzymes, such as ribozymes, are composed of RNA.
**Option B:** Incorrect, as enzymes are not solely responsible for the regulation of metabolic pathways; other factors, such as allosteric regulation and feedback inhibition, also play crucial roles.
**Option C:** Incorrect, as enzymes do not directly control the rate of chemical reactions; rather, they catalyze reactions, allowing them to occur at a faster rate.
**Clinical Pearl / High-Yield Fact**
Enzyme specificity is crucial for maintaining proper cellular function, and alterations in enzyme activity can lead to various diseases, including inherited disorders such as phenylketonuria (PKU) and glycogen storage diseases.
**Correct Answer: C. Enzymes catalyze chemical reactions, but they do not directly control the rate of these reactions.**