Скачать или смотреть Ahern's Biochemistry #10 - Enzymes 2

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Highlights Enzymes II

1. Velocity of an enzymatic reaction is measured as the concentration of product formed per time. It can be estimated in a V vs. [S] plot. Maximum velocity (Vmax) occurs in a reaction when the enzyme is saturated with substrate. Vmax depends on the amount of enzyme used to measure it. It is NOT a constant for an enzyme.

2. We define Km as the substrate concentration that gives Vmax/2. Whereas the Vmax varies, depending on the amount of enzyme that one uses, the Km is a constant for a given enzyme for its substrate.

3. The higher the Km of an enzyme, the LOWER its affinity for its substrate. This is because a high Km means that it takes a LOT of substrate before the enzyme gets to Vmax/2. Km is frequently referred to as the affinity of the enzyme for a substrate, though that is not 100% correct. Nevertheless, we say that a high Km is consistent with a low affinity of enzyme for substrate and conversely, that a low Km is consistent with a high affinity of enzyme for substrate.

4. If one lets a reaction go for a long time, it will reach equilibrium. At equilibrium, the relative concentration of products and reactants do not change. Initial velocities of reactions are therefore measured so as to avoid allow the product to accumulate and favor the reverse reaction.

5. Lineweaver-Burk plots are alternative plots of V vs S data obtained by taking the inverse of each and plotting it, thus making a 1/V vs 1/[S] plot (also called a double reciprocal plot).

6. On a Lineweaver-Burk plot, the Y intercept is 1/Vmax and the X intercept is -1/Km.

7. The catalytic actions of enzymes appear to be related to their ability to be at least slightly flexible. Originally, Fischer proposed a model of catalysis called the Lock and Key model. It described enzymes as inflexible and the substrate as like a key fitting into a lock. While substrates do, in fact, fit into enzymes somewhat like a key, the enzyme is NOT inflexible.

8. Koshland's model of enzyme action, called the Induced Fit model says that not only does the enzyme change the substrate (via catalysis), but the substrate also changes the enzyme shape upon binding. This transient change of enzyme shape is important for catalysis because it may bring together molecular groups (such as a phosphate and a sugar) that may not be close together in the enzyme prior to the change in enzyme shape. Remember that enzymes also work by orienting substrates together in the proper way to maximize their likelihood of bouncing together in a way that leads to making a bond.

9. Chemical changes brought about by catalysis facilitate a last change in enzyme shape to allow for the release of the products. When this happens, the enzyme returns to its original shape and remains unchanged by acting to catalyze the reaction.

10. In contrast to Vmax, Kcat is a constant for an enzyme. It is also known as the turnover number and corresponds to the number of molecules of product made per molecule of enzyme per second. 1000/second means 1000 molecules of product per molecule of enzyme per second.

11. "Perfect" enzymes are enzymes that have evolved to the point where any additional mutation will reduce their ability to catalyze reactions. They are not common. Perfect enzymes have a very high ratio of Kcat/Km and are such that the only thing that inhibits their ability to function more efficiently is the rate of diffusion of substrate in water.