Why is it often the case that an enzyme is favorable only towards one direction of a reaction and not both directions?

Question

In class when we're studying enzymes like amylase or protease it only works well when you're using it to break down compounds like polysaccharides. I'm just curious but why is it not possible for enzymes to work efficiently in a reverse reaction? I mean a formula like $\\ce{amino\\ acid + amino\\ acid + energy<=>H2O + protein}$ does imply that our reaction can go both ways , right? But why is it that the enzyme favors the catabolic reaction?

Answer 1

Very interesting question indeed, and very critical to understand enzyme dynamics. Let me start from the end.

Firstly, enzymes are needed not only for catabolic reactions but also for anabolic reactions. For example, synthesis of glycogen from glucose, proteins from amino acids, and literally all biological anabolic reactions need enzymes; viz., to synthesize an alpha-1-4 glycosidic linkage (in glycogen) needs the enzyme glycogen synthase, synthesis of peptide linkage (in protein) needs the enzyme peptidyltransferase, and so on.

Secondly, enzymes can never change direction of a reaction. They only helps to reach the equilibrium faster, millions, billions, trillions ... times faster.

Now, why is that in one reaction glycogen is broken down, and in another it is synthesized? One fact must be noted is that such reactions are not really an example of change of direction of the same reaction. The reactants are actually different in such two sets of reaction. For example in case of alpha-amylase acting upon glycogen, it needs only glycogen and water, but in synthesis of the same bond it needs UDP-glucose and a pre-synthesized piece of glycogen (glycogen primer). Plain molecules of glucose or glycogen are not able to join as synthesis (or anabolic) reactions typically needs and cataboic reactions gives off energy. Here UDP-glucose is an energized form of glucose, and 2 moles of ATPs are spent to energize one mole of glucose. The energy from UDP-glucose pushes the reaction towards synthesis, and the enzyme glycogen synthase binds specifically to theses molecules and pose them optimally to react and reach equilibrium faster.

Salivary amylase in our mouth cannot favor synthesis of glycogen, because firstly, there is no mechanism to provide UDP-glucose in our mouth; secondly, even it were supplied, alpha-amylase has no capacity to bind UDP-glucose.

I hope it answers the question in the simplest biochemical terms. Good luck.