An enzyme is a:
a. biological catalyst that increases the concentration of the products.
b. biological catalyst that increases the concentration of the reactants.
c. chemical that is used in a reaction, after which it can no longer be used by the cell.
d. biological catalyst that works by bringing its substrates closer to their transition states.

Verified step by step guidance

1. Understand the definition of an enzyme: An enzyme is a biological catalyst, meaning it speeds up chemical reactions in the body without being consumed in the process.
2. Review the role of a catalyst: Catalysts work by lowering the activation energy required for a reaction to occur, making it easier for reactants to reach their transition state and form products.
3. Analyze the options provided: Option (a) suggests enzymes increase product concentration, which is incorrect because enzymes do not alter the equilibrium of a reaction. Option (b) suggests enzymes increase reactant concentration, which is also incorrect for the same reason. Option (c) states enzymes are used up in reactions, which contradicts their role as reusable catalysts.
4. Focus on option (d): This option states that enzymes work by bringing substrates closer to their transition states, which aligns with the mechanism of action of enzymes. Enzymes bind to substrates at their active sites, stabilizing the transition state and facilitating the reaction.
5. Conclude that the correct answer is the option that accurately describes the function of enzymes as biological catalysts that assist substrates in reaching their transition states without being consumed in the reaction.

Key Concepts

Enzymes as Biological Catalysts

Enzymes are proteins that act as biological catalysts, meaning they accelerate chemical reactions without being consumed in the process. They lower the activation energy required for reactions, allowing them to occur more quickly and efficiently. This catalytic function is crucial for various biological processes, including metabolism and DNA replication.

Substrate and Active Site Interaction

The active site of an enzyme is a specific region where substrates bind. This interaction is highly specific, often described by the 'lock and key' model, where the enzyme's active site (the 'lock') is shaped to fit a particular substrate (the 'key'). This proximity facilitates the transition state, leading to the formation of products more efficiently.

Transition State Theory

Transition state theory posits that during a chemical reaction, reactants must reach a high-energy state known as the transition state before converting into products. Enzymes stabilize this transition state, effectively lowering the energy barrier and increasing the likelihood of the reaction proceeding. This concept is fundamental to understanding how enzymes enhance reaction rates.