Draw a hypothetical metabolic pathway in Neurospora crassa composed of five substrates, five enzymes, and a product called nirvana. Number the substrates 1–5, and label the enzymes A–E, in order. (For instance, enzyme A catalyzes the reaction between substrates 1 and 2.)
(b) Suppose a mutant strain can survive if substrate 5 is added to the growth medium, but it cannot grow if substrates 1, 2, 3, or 4 are added. Which enzyme in the pathway is affected in this mutant?

Verified step by step guidance

1. Begin by understanding the concept of a metabolic pathway. A metabolic pathway is a series of chemical reactions occurring within a cell, where each step is facilitated by a specific enzyme. In this problem, you need to create a pathway with five substrates and five enzymes leading to a final product called nirvana.
2. Draw the pathway: Start with substrate 1, which is converted to substrate 2 by enzyme A. Then, substrate 2 is converted to substrate 3 by enzyme B. Continue this pattern, where substrate 3 is converted to substrate 4 by enzyme C, substrate 4 is converted to substrate 5 by enzyme D, and finally, substrate 5 is converted to the product nirvana by enzyme E.
3. Label each part of the pathway: Assign numbers to the substrates (1 to 5) and letters to the enzymes (A to E). Ensure that enzyme A catalyzes the reaction between substrates 1 and 2, enzyme B between substrates 2 and 3, and so on, until enzyme E catalyzes the conversion of substrate 5 to nirvana.
4. Analyze the mutant strain scenario: The mutant strain can survive only if substrate 5 is added, indicating that the conversion from substrate 4 to substrate 5 is not occurring naturally. This suggests that enzyme D, which catalyzes the conversion of substrate 4 to substrate 5, is affected in the mutant strain.
5. Conclude the analysis: Since the mutant strain cannot grow with substrates 1, 2, 3, or 4, but can grow with substrate 5, enzyme D is likely non-functional or absent in the mutant strain, preventing the conversion of substrate 4 to substrate 5.

Key Concepts

Metabolic Pathway

A metabolic pathway is a series of chemical reactions occurring within a cell, where each step is facilitated by a specific enzyme. These pathways convert substrates through intermediate steps into a final product, playing a crucial role in maintaining cellular functions and energy production.

Enzyme Function

Enzymes are biological catalysts that speed up chemical reactions in metabolic pathways without being consumed. Each enzyme is specific to a particular reaction, facilitating the conversion of substrates into products. In the context of the question, enzymes A-E catalyze sequential reactions between substrates 1-5 to produce the final product, nirvana.

Mutant Strain Analysis

Analyzing mutant strains involves understanding how genetic mutations affect metabolic pathways. In the given scenario, the mutant strain's inability to grow with substrates 1-4 suggests a disruption in the pathway before substrate 5. This indicates that the enzyme responsible for converting substrate 4 to 5 is likely defective, preventing the completion of the pathway.