Start Material for Exam 2
Highlights Enzyme Control I
1. Allosterism is one way of controlling enzyme activity. ATCase is an excellent example of an allosterically controlled enzyme. It catalyzes the first step in a multi-step process. The end product of this pathway/process is CTP.
2. CTP binds to the regulatory subunits for ATCase and inactivates the enzyme allosterically. By contrast, ATP binds to the same subunits and causes the enzyme to be activated. Activation of an enzyme results in conversion of it from the T to the R state. Inactivation of an enzyme causes the enzyme to convert from the R to the T state. Aspartate is a substrate for the enzyme that also activates it and converts the enzyme into the R state. Note that aspartate binds to the enzyme’s catalytic subunits, not the regulatory subunits that ATP and CTP bind to.
3. We have two mechanisms for explaining how enzymes are activated or inactivated allosterically. They are called the sequential and the concerted models.
4. The sequential model says that binding of a molecule by an enzyme causes the change in the enzyme from R to T or from T to R. There is a cause-effect relationship between the allosteric effector (molecule affecting the enzyme) and the state of the enzyme (T or R).
5. The concerted model says that there is no cause-effect relation between the effector and the state of the enzyme. Instead, it states that the enzyme “flips” from R to T or T to R by itself and then binding of an effector LOCKS the enzyme into that state.
6. Another powerful mechanism for controlling some enzymes is covalent modification. Covalent modification involves making or breaking covalent bonds of an enzyme. One common covalent modification is phosphorylation (putting on a phosphate) or dephosporylation (taking off a phosphate). Only the hydroxylated aminos acids (serine, threonine, or tyrosine) are typically phosphorylated.
7. Enzymes that put phosphates onto things are called kinases. Enzymes that take phosphates off are known as phosphatases.
8. A very common type of covalent modification is phosphorylation, as exhibited by glycogen phosphorylase. Phosporylation converts glycogen phosphorylase b (less active form) to glycogen phosphorylase a (more active form). The kinase that puts the phosphates on is controlled by epinephrine (adrenalin).