The Great Supermarket Analogy
Imagine a supermarket - and that includes the usual employees - the check-out clerks, stockers, managers, janitors. This supermarket is like a huge complicated enzyme. How can this be? Well, you already know all about the activation energy profiles of chemical reactions, and you know that catalysts (like enzymes), do something to tunnel through the activation energy mountain to facilitate the reaction. Just imagine the days before a supermarket came to this small imaginary town. All the residents had to raise their own food - and sometimes due to lack of skill, crops did not always fare very well. A tremendous amount of work had to be expended to get food on the table. That great amount of work is the activation energy of this small town.
Now, with a supermarket, not too much time and effort need go into getting food on the table. The activation energy of food provision is much lower - and leaves the townspeople more time to do other constructive things. Thus the supermarket is like an enzyme.
The reaction in which this supermarket plays a major role is this: it takes hungry people with money in their pockets and converts them into happy people with bags of groceries in their arms. Actually that is only half of the reaction! So let's be complete about it:
Now let us play a little with this notion. What happens if we change the number of people entering the store? Say we have this huge supermarket and only 10 customers per hour enter and leave. Just a small amount of money is converted to groceries. What if we increase the business so that 20 people per hour come and go? In this huge store, we find that twice as much money is converted to groceries. What about if 3,000 customers come in per hour? Or 5,000 come in per hour? Soon the check-out clerks have long lines and it doesn't matter if we increase the number entering the store. The customers have saturated the check-out lines and no additional increase of money is converted to groceries. The rate of exchange has reached a maximum. (Or as an enzymologist would say: "We've reached the maximum velocity of the reaction - 'Vmax'.")
Thus, and this is important! The velocity of the reaction is only proportional to the substrate concentration (number of entering customers) when the substrate is somewhat below saturation of the catalyst. After that, if the substrate increases, the reaction velocity plateaus. Totally understand this before continuing further!!!
Inhibition of the Reaction
Inhibition is slowing down the reaction. In the supermarket analogy, inhibition can come about in many ways - power failure, cashier gets sick, food trucks don't arrive often enough, and even if the food trucks arrive too often and the place gets overwhelmed with too much food - boxes blocking all the aisles, etc. And there are other things that can go wrong with the supermarket - just ask any manager!
Competitive Inhibition. This is where the inhibition occurs at the very same place (site) that the desired reaction takes place. Customers are separated from their money and given groceries at the cash register. Suppose that a group of mischievious persons decide to enter a supermarket with no money. They pile their shopping carts full of food, and get in line with the other customers who have money. After the cashier scans all the groceries, the fake customer declares the lack of money. All that wasted time! Consider now if fully half of the customers were fake, the cashiers would be wasting half of their time. The supermarket's efficiency would be cut in half. This is called "competitive inhibition" because fake customers compete with real customers.
Let's continue with this. Consider that initially there were 10 real customers and 10 fake ones. The rate of successful checkout would be half of that where there were 20 real customers. Now let's keep the number of fake customers constant, and increase the number of real customers. Scientists like to go to extremes, so let's increase the real customers to thousands per hour. The 10 fake customers are now so diluted by all the real ones that the checkout lines are moving at 'Vmax'.
Thus if an inhibitor of an enzymatic reactions is "competitive", it cannot change 'Vmax'. All you have to do is run the reaction several times - holding the inhibitor's concentration constant, and increasing the substrate.
Allosteric Inhibition. (Another term for this is "non-competitive inhibition" where inhibiting the enzyme is at some place other than at the active site.) So can you think of any way to slow the efficiency of the supermarket without dealing with the checkout lanes? Let's try this simple one: our supermarket had two entrance doors. One got jammed closed. At first, when there were only a few customers coming in early in the morning, it didn't make any difference. But as more and more shoppers came by, they couldn't get inside as fast as they would have had both doors been working. What happens to 'Vmax'? It would be cut in half. Thus allosterically inhibited reactions can be identified by the fact that a constant amount of inhibitor affects 'Vmax'.
Astute merchants thus know when to use more advertising and when to install more checkout lanes. More advertising is like adding more doors to our imaginary store. But if the swarm of customers is already large - advertizing for even more customers will not help sales. But adding more checkout lanes will proportionately help.
There is another class called "un-competitive inhibitors. This is rare in nature, and is when the inhibitor can only bind to the enzyme-substrate complex at the moment of reaction. Here are two "supermarket" examples.
When this occurs, it functions very much like the active site is partially destroyed. No amount of dilution by "good" customers helps: the supermarket's overall efficiency is diminished by a constant proportion no matter how many customers.