A metabolic pathway is a sequence of enzyme-catalyzed reactions in which the product of one step becomes the substrate of the next. Each enzyme catalyzes exactly one step, converting a specific substrate into a specific product. The intermediates between steps are metabolites, and they are not consumed in the abstract; they are passed along the chain.
Flux is the rate at which material moves through the pathway, measured as the amount of substrate converted to product per unit time. When a pathway is at steady state, the concentration of each intermediate remains constant over time because the rate of its formation equals the rate of its consumption. Steady state does not mean the pathway is inactive; it means the throughput is balanced so that no intermediate accumulates or depletes.
If one step in the chain is removed, the ordered flow is interrupted at that point. The steps before the block continue to produce their products, while the steps after it receive no substrate. This is why the position of a missing enzyme within the pathway determines the pattern of consequences.