Can you talk more about this please?
In a negative feedback loop, the build up of the product of a reaction will cause the reaction that produces it to slow down. To use a random example, let's take body temperature. Your body wants to maintain its temperature within a certain range - not too hot or cold. When the temperature runs too hot, the body takes action to cool itself down (like sweating). When the temperature has lowered and is becoming too low, there is a negative feedback - the body shuts off the processes that caused your temperature to go down (e.g. stops sweating), to keep its temperature balanced. The mechanism by which negative feedback takes place can vary (in the Cliff's question, it's direct negative feedback because the product of the reaction acts as an allosteric inhibitor on the enzyme that produces it).
In question 2, product E is normally consumed in other reactions, so its levels don't build up and cause negative feedback to occur. But the question asks what would have if product E were not consumed. At first, levels of E would build up. Since we know E acts as an allosteric inhibitor on its own production, the enzyme D' would be inhibited. As a result, E would no longer be produced, since the conversion of D --> E can no longer take place. The direct consequence of this is that levels of product E will go down (it's not being made anymore), but levels of D will go up - it's no longer being converted to another substance. As a result of product D going up, you're going to see negative feedback on the reaction that produces D (this is the conversion of C to D via the enzyme C1'). This feedback isn't from allosteric inhibition, it's just normal negative feedback from excess product. D will no longer be produced as quickly, because the reverse reaction is going to be favored (to oversimplify things, there's too much D building up, so its production gets slowed down).
Question 3 is a little more straightforward. Just like in question 2, product E is normally consumed but now it isn't, so it's going to act like an allosteric inhibitor. Levels of E would build up, and this time it inhibits the enzyme C1'. Since C1' is needed to catalyze the production of D, J, E, K, and L, all those products would have a decreased production rate.
To recap negative feedback briefly, for any reaction like A--> B when you end up with too much B for whatever reason, the body says "there's too much B, slow down its production" and A builds up instead. I hope this helps, but let me know if you need further clarification in specific areas.
