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Enzymes III Kevin Ahern's BB 450 Lecture #11 2016

4,170 views · Published 15 October 2016 · 49:27 · Indexed 21 September 2026

Channel: Kevin Ahern's YouTube Videos - Lectures and More · 2016 · Education

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2. Kevin's lectures with The Great Courses - https://www.thegreatcoursesplus.com/biochemistry-and-molecular-biology-how-life-works?tn=Expert_tray_Course_1_0_12943 
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1. Important cofactors/coenzymes include biotin for carboxylation reactions, FAD and NAD+ for oxidation/reduction reactions, and Coenzyme A (CoA).

2. Metal ions are used by some enzymes. Some of these, such as seleneium, are needed in extremely small quanitites and are toxic when in higher concentrations. 

3. 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.

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

5. Ribozymes are catalytic RNA molecules. The most widespread  of these is the one in the ribosomes that catalyzes the formation of peptide bonds in protein synthesis.

6. Enzymatic reactions can be inhibited by reversible and irreversible processes. Reversible processes involve binding of an inhibitor and its release.

a. Competitive Inhibition - This reversible inhibition occurs when the inhibitor resembles the substrate and competes for the enzyme substrate binding site. The greater the concentration of inhibitor, the greater the inhibition. Competitive inhibition can be overcome by increasing amounts of substrate. The apparent Vmax of competitive inhibition does not vary from the Vmax of the same reaction when uninhibited. The apparent Km, however, does vary, because it requires more substrate to get the same velocity as the uninhibited reaction.

b. Non-competitive inhibition - This type of reversible inhibition occurs when an inhibitor binds an enzyme at a site unrelated to the substrate binding site. The inhibitor's binding to the enzyme is unrelated to the binding of the substrate and it does not have to have a structure like that of the substrate.The inhibitor can inhibit the enzyme without interference from the substrate. Therefore, increasing substrate concentrations cannot eliminate the effect of the inhibitor. In this case, the Vmax is lowered, but the Km is unchanged.
7. Methotrexate is a competitive inhibitor of dihydrofolate reductase.  It resembles the normal substrate (dihydrofolate) and can stop the enzyme from making tetrahydrofolate - an important intermediate in nucleotide biosynthesis.
8. An irreversible type of enzyme inhibition is demonstrated by penicillin, which covalently binds to the enzyme required for synthesis of the bacterial cell wall. Such a compound is an effective antibiotic because it inhibits an enzyme not found in humans and stops a process not found in humans. 
9. Allosterism is a phenomenon in which a small molecule interacts with a protein and affects the proteins activity. Such an enzyme is an allosteric enzyme. Allosterism can be positive (activating an enzyme) or negative (inhibiting an enzyme). The terms activation and inhibition are not perfectly accurate for action of allosterism - more acccurately these are thought of as increasing enzyme activity or decreasing enzyme activity, respectively.
10. There is similarity between the kinetics of allosterically acting enzymes and the cooperativity of binding of oxygen by hemoglobin, since both involve binding of a molecule by a protein.
11. A homotropic effector of an enzyme is a substrate that affects the binding of other substrates by an enzyme, much like binding of oxygen by hemoglobin affects binding of other hemoglobins. Such an effector will (typically) cause a V vs [S] plot to have a sigmoidal shape.
12. A heterotropic effector of an enzyme is a non-substrate molecule.
13. The sequential model of catalysis is very much the same as the cooperativity scheme shown for binding of oxygen by hemoglobin. It states that there is a cause/effect between binding of a substrate or effector by an enzyme and the conversion of the enzyme from T state to R state (or vice versa).
14. The MWC/Concerted model of catalysis states that there is no cause/effect result for flipping of R and T states and that the enzyme routinely switches as a whole between these states independently of binding of substrate/effector. The role of the substrate/effector in the MWC model is to "lock" the enzyme into one state or the other.
15. The Morpheein model of catalysis is much like the MWC model, but instead of the enzyme as a whole flipping between R and T states, the multisubunit enzyme first dissociates into individual subunits, which then flip and reassemble. Only individual subunit of the same configuration (T or R) can reassemble.

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