生命科學(一) Ch6-4 An Introduction to Metabolism

Summary

This lecture covers the role of enzymes (biological catalysts) in accelerating biochemical reactions by lowering the activation energy (EA). It explains how enzymes interact with substrates, inducing a shape change (induced fit) that facilitates reaction progress. The discussion includes enzyme specificity, optimal conditions like temperature and pH, and the effects of cofactors and inhibitors on enzyme activity. Examples of competitive and non-competitive inhibition are provided, as well as how enzymes can evolve based on environmental changes. The lecture emphasizes enzymes' role in catalyzing and optimizing metabolic reactions in living organisms.

Takeaways

• 🔬 Enzymes (catalysts) are proteins that accelerate chemical reactions without altering the reaction's spontaneity.
• ⚡ Activation energy (EA) is the energy required to start a reaction by breaking and forming chemical bonds.
• 🍬 Sucrase is an enzyme that catalyzes the breakdown of sucrose into glucose and fructose, speeding up the otherwise slow reaction.
• 🧩 Enzymes lower activation energy by undergoing induced fit, where their shape changes upon substrate binding, enhancing their catalytic ability.
• 🔄 Enzymes do not alter the delta G of a reaction, only reduce the activation energy to increase reaction speed.
• 🔥 Enzyme activity is affected by environmental factors like temperature and pH, with each enzyme having an optimal range for these conditions.
• 🧪 Cofactors and coenzymes are substances that assist enzymes in their catalytic activity; they can be inorganic (e.g., metal ions) or organic (e.g., vitamins).
• 🚫 Enzyme inhibitors can be competitive, binding to the active site, or noncompetitive, binding elsewhere and altering enzyme shape, both of which hinder enzyme function.
• 🔄 Enzymes can be reused multiple times as they do not change during the reactions they catalyze.
• 🌱 Enzymes evolve to adapt to changing environments, altering their structure and activity to suit the conditions, as seen in E. coli adapting to metabolize different sugars.

Q & A

What role do enzymes (catalysts) play in chemical reactions?

Enzymes act as catalysts in chemical reactions, accelerating the reaction by lowering the activation energy (EA) without altering the reaction's spontaneity or the final products.

How do enzymes accelerate reactions like the breakdown of sucrose into glucose and fructose?

Enzymes like sucrase bind to substrates (sucrose in this case), lowering the activation energy needed for the hydrolysis reaction, allowing the breakdown of sucrose into glucose and fructose to occur more quickly.

What is the significance of activation energy (EA) in a chemical reaction?

Activation energy (EA) is the initial energy required to weaken and break chemical bonds, allowing the reaction to proceed. Even if a reaction is spontaneous, a high EA can slow its progress.

Why doesn't the presence of an enzyme change the delta G of a reaction?

Enzymes only reduce the activation energy required for the reaction but do not alter the free energy difference between the reactants and products (delta G), which determines whether the reaction is spontaneous.

What happens to the enzyme after the reaction it catalyzes is complete?

After catalyzing a reaction, the enzyme remains unchanged and can be reused for multiple reaction cycles, continuing to accelerate the same type of reaction.

What is 'induced fit,' and how does it relate to enzyme-substrate interaction?

'Induced fit' refers to the change in the enzyme's shape when it binds to the substrate, enhancing the enzyme's ability to lower the activation energy and facilitate the reaction.

How do temperature and pH affect enzyme activity?

Each enzyme has an optimal temperature and pH at which it works most efficiently. Deviating from these conditions can reduce enzyme activity. For example, human enzymes work best at body temperature (around 37°C) and specific pH levels depending on their environment, like pH 2 in the stomach.

What are cofactors and coenzymes, and how do they assist enzymes?

Cofactors are non-protein substances that assist enzymes in catalysis. They can be inorganic (metal ions) or organic molecules. Organic cofactors are called coenzymes, such as vitamins, which help enzymes function efficiently.

What is the difference between competitive and noncompetitive enzyme inhibition?

In competitive inhibition, the inhibitor binds to the enzyme's active site, preventing the substrate from binding. In noncompetitive inhibition, the inhibitor binds to another part of the enzyme, causing a conformational change that reduces the enzyme's ability to catalyze the reaction.

Can enzymes evolve, and if so, how does that happen?

Yes, enzymes can evolve. Over time, changes in environmental conditions can lead to mutations in the enzyme's structure (amino acid sequence), potentially enhancing the enzyme's efficiency or altering its substrate specificity to adapt to new conditions.