Problem 79 - Chemistry: The Central Science

Question

(a) Explain the importance of enzymes in biological systems. (b) What chemical transformations are catalyzed (i) by the enzyme catalase, (ii) by nitrogenase?

Step-by-Step Solution

Answer

Enzymes are vital to biological systems as they speed up chemical reactions, regulate metabolism, aid in energy production, assist in DNA replication and repair, synthesize proteins, and participate in defense mechanisms. (i) Catalase catalyzes the breakdown of hydrogen peroxide, a toxic byproduct, into water and oxygen: \[2H_2O_2 (aq) → 2H_2O (l) + O_2 (g)\]. (ii) Nitrogenase, found in diazotrophs, converts atmospheric nitrogen into ammonia for use in essential biological molecules: \[N_2 (g) + 8H^+ + 8e^- + 16ATP → 2NH_3 + H_2 (g) + 16ADP + 16P_i\].

Step 1: Understanding Enzymes

Enzymes are biological catalysts that speed up the chemical reactions in living organisms without being consumed or changed themselves. They facilitate these reactions by lowering the activation energy required, helping maintain the organism's metabolic processes.

Step 2: Importance of Enzymes in Biological Systems

Enzymes play critical roles in biological systems, such as: 1. Metabolism regulation: Enzymes enable the organism's metabolic processes to occur at a faster rate, thus allowing the organism to maintain its normal functions. 2. Energy production: Enzymes are involved in the breakdown of nutrients to generate energy needed for cellular functions. 3. Replication and repair of DNA: Enzymes are involved in DNA replication, allowing cells to divide and reproduce. They are also responsible for repairing damaged DNA. 4. Protein synthesis: Enzymes are crucial components of the ribosome, which plays a major role in the synthesis of proteins. 5. Defense mechanisms: Enzymes help protect the organism from harmful substances and participate in immune system responses.

Step 3: The Enzyme Catalase

(i) The enzyme catalase facilitates the breakdown of hydrogen peroxide (H2O2), a toxic byproduct produced as a result of metabolic processes within cells. Catalase effectively prevents the accumulation of hydrogen peroxide by converting it into water (H2O) and molecular oxygen (O2). The chemical transformation catalyzed by catalase can be represented as follows: \[2H_2O_2 (aq) → 2H_2O (l) + O_2 (g)\]

Step 4: The Enzyme Nitrogenase

(ii) Nitrogenase is an enzyme complex that plays a critical role in the biological nitrogen fixation process, particularly in certain bacteria known as diazotrophs. Nitrogenase catalyzes the conversion of atmospheric nitrogen gas (N2) into ammonia (NH3), which can be used by plants and other organisms to form amino acids, nucleic acids, and other essential biological molecules. The chemical transformation catalyzed by nitrogenase involves the following reaction: \[N_2 (g) + 8H^+ + 8e^- + 16ATP → 2NH_3 + H_2 (g) + 16ADP + 16P_i\] In summary, enzymes play crucial roles in various biological systems, facilitating chemical reactions essential for life, such as the breakdown of toxic substances by catalase and the fixation of nitrogen by nitrogenase.

Key Concepts

Metabolism

Metabolism encompasses all the chemical reactions that occur in living organisms to sustain life. It can be broadly divided into two categories: catabolism and anabolism. Catabolic reactions involve breaking down complex molecules into simpler ones, releasing energy. Anabolic reactions, on the other hand, use energy to build complex molecules from simpler ones.
Enzymes are crucial in metabolism, acting as biological catalysts. They lower the activation energy required for reactions, making them occur quickly and efficiently. Without enzymes, metabolic reactions would be too slow to sustain life.
Additionally, enzymes ensure that metabolic pathways are regulated and coordinated. They play a critical role in maintaining the balance of metabolism, ensuring that energy production, nutrient processing, and waste elimination occur smoothly. This balance is vital for normal functioning of cells and organisms as a whole.

Catalase

The enzyme catalase plays a vital role in protecting cells from oxidative damage. Hydrogen peroxide, a byproduct of metabolic processes, is toxic to cells. Its accumulation can cause cellular damage. Catalase facilitates the rapid decomposition of hydrogen peroxide into water and oxygen.
The reaction catalyzed by catalase can be expressed as:

• \[2H_2O_2 (aq) → 2H_2O (l) + O_2 (g)\]

This reaction prevents the build-up of hydrogen peroxide, thereby safeguarding the cell. Catalase is remarkably efficient; a single molecule can convert millions of hydrogen peroxide molecules per second. It is crucial in the detoxification process and highlights the importance of enzymes in preserving cellular health and function.

Nitrogenase

Nitrogenase is a key enzyme in the nitrogen cycle, crucial for life on Earth. This enzyme is mainly found in certain bacteria known as diazotrophs, which reside in soil or within the root nodules of legumes.
Nitrogenase catalyzes the conversion of inert atmospheric nitrogen gas (N_2) into ammonia (NH_3), a form that plants and other organisms can assimilate to form vital biomolecules like amino acids and nucleic acids. The reaction is as follows:

• \[N_2 (g) + 8H^+ + 8e^- + 16ATP → 2NH_3 + H_2 (g) + 16ADP + 16P_i\]

This reaction is energy-intensive, requiring ATP, and occurs under ambient conditions unlike most industrial processes. Nitrogenase is essential because it provides a pathway for nitrogen to enter the biosphere, supporting the growth and productivity of various ecosystems.

Biological Catalysts

Biological catalysts, also known as enzymes, are proteins that speed up chemical reactions in living organisms. Their primary function is to increase reaction rates without being consumed in the process. Enzymes achieve this by lowering the activation energy, which is the energy barrier that must be overcome for a reaction to proceed.
Unlike chemical catalysts, enzymes are highly specific, usually catalyzing only one type of reaction for a particular substrate. This specificity is due to the unique three-dimensional shape of the enzyme's active site, where the reaction takes place. The active site's shape complements the substrate, ensuring precise interactions.
Enzymes are involved in numerous processes:

• Digestion of food to provide nutrients and energy
• DNA replication for cell division
• Speeding up metabolic pathways
• Regulating various biochemical pathways by acting as control points

The efficiency and precision of enzymes underscore their vital role in maintaining life's complexity and functionality. They provide a scaffold for reactions, ensuring life processes occur swiftly and with proper regulation.