Proteins and Enzymes

ID #1617

In class you talked about how biosynthetic reactions that link together smaller molecules into larger ones require energy, so they are called endogonic. Similarly, catabolic reactions break down larger molecules into smaller ones, release energy, and are called exogonic. However, in Chemistry classes I have always been taught that breaking a bond requires energy because it takes work to pull the two bonded atoms apart. Likewise, bond formation occurs because the two atoms can lower their net energy by bonding. So how does the breaking apart of large molecule in a catabolic reaction release energy? And how does bond formation in a biosynthetic reaction require energy?

You're absolutely correct that even the breaking of a bond requires energy -- what your chemistry teacher told you wasn't wrong, but neither was I. That small amount of energy needed to pull the bonded atoms apart is precicely what we mean by the activation energy. An enzyme creates an environment where this energy can be provided simply by the internal kinetic energy of the molecules as they collide with each other. On the other side, with the endergonic reactions, again it's true that the formation of a covalent bond occurs when the participating atoms achieve their most stable configuration together instead of separate. But in a living cell, the atoms that form this bond are already bonded to something else, creating a significant energy barrier of breaking the "old" bonds, "introducing" the new partners, and making it energetically favorable to form the "new" bonds. This is why in biology, endergonic reactions have such a significantly positive delta G.

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