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?

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1. Identify and list the substrates and enzymes involved in the metabolic pathway. For this scenario, you have substrates numbered 1 to 5 (S1, S2, S3, S4, S5) and enzymes labeled A to E (E_A, E_B, E_C, E_D, E_E). Each enzyme catalyzes the conversion of one substrate to the next, leading up to the product nirvana.
2. Draw the pathway starting from substrate 1 (S1) being converted by enzyme A (E_A) to substrate 2 (S2), and continue this pattern until substrate 5 (S5) is converted by enzyme E (E_E) into the product nirvana.
3. Analyze the mutant strain's growth conditions. Since the mutant can survive with the addition of substrate 5 but not with substrates 1, 2, 3, or 4, focus on the role of the enzymes related to these substrates.
4. Determine which enzyme might be dysfunctional in the mutant strain. Since adding substrate 5 allows for survival, the enzyme that converts substrate 5 into the product (enzyme E) is likely functional. The issue must occur before substrate 5 in the pathway.
5. Conclude that the enzyme likely affected in the mutant is enzyme D (E_D), which is responsible for converting substrate 4 (S4) to substrate 5 (S5). This is because the addition of S5 rescues the growth, suggesting that the pathway blockage occurs at the step involving E_D.