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Regulation of Enzymatic Activity – Metabolism and Regulation

by Kevin Ahern, PhD

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    00:00 fatty acids, or even to make sugars depending upon the cell.

    00:01 Now as I said cells must control reactions, but they don't have the tools available to them to control reactions with respect to energy because that's one of those principles of the universe that the cell can't alter. So instead, I want to talk a little bit about mechanisms that cells have to control pathways, but before I do that I want to give some examples about the importance of doing that. In cells, enzymes catalyze the reactions that are occurring.

    00:24 And enzymes are incredibly powerful tools. These enzymes can speed up reactions by enormous amounts compared to the same reactions being catalyzed without the enzyme. Some enzymes for example, can catalyze a reaction that is faster but on the order of 180 quadrillion fold faster than the same reaction that's uncatalyzed. Now that 180 quadrillion, a quadrillion is 10 to the 15th times faster, is a mind-boggling amount of speed that an enzyme is giving to a reaction. Now to think of a real world analogy that we have for this, I'd like to think of going to the grocery store. We can walk to the grocery store, we can take a bicycle to the grocery store, or we can take an Indianapolis racecar to the grocery store. We will get there a lot faster if we take an Indianapolis racecar, but the likelihood that we're going to have an accident or run into something if we drive at full speed in that racecar to the grocery store increases as the speed increases. So controlling things with respect to speed is very, very important and that's really what cells are trying to do here.

    01:27 So cells have available to them, four mechanisms for controlling enzymatic reactions. The first of these is called allosterism. Allosterism occurs for some enzymes, not all enzymes.

    01:40 But these enzymes that are controlled allosterically will bind small molecules and the small molecule will affect the enzyme. It will actually help the enzyme in some cases to be faster, that is, to activate the enzyme, or in some cases to inhibit or slow down the enzyme. In some cases it may completely physically turn it off. A second mechanism that cells have for them is the covalent modification of enzymes. Now in covalently modifying enzymes, it might seem like what is happening is you just simply stopping the enzyme from functioning, but the reality is actually the opposite of that. If you go to the store and you buy something it has been sealed for your protection, the only way that you can get to that and use it is if you open the package, and so some enzymes are synthesized in a form called zymogens, an inactive form of the enzyme. And to open the package, peptide bonds have to be broken in strategic places in order for the enzyme to become active. Other covalent modifications that are performed to enzymes include the addition of phosphate, called phosphorylation, or the removal of that same phosphate called dephosphorylation. Now depending upon the enzyme, phosphorylation may activate or inactivate the enzyme and dephosphorylation will have the opposite effect.

    02:59 Cells also have available to them the ability to control whether or not an enzyme is made.

    03:04 We've seen in other modules, the importance of gene regulation, that is controlling the transcription and translation of specific proteins and if cells do that with enzymes, they are able to physically start or stop. So a good example for us, is that of the synthesis of glucose. In our bodies, only specialized cells in our bodies, will make glucose. They occur mostly in our liver and kidney. The other cells of our body will not make glucose.

    03:31 And the reason that they don't make glucose, is because those other cells of our body control the synthesis of a key enzyme that's necessary for making glucose. By not making the enzyme, glucose can't be made. The last tool available to cells to control metabolic pathways is that of organelle sequestration. This is a tool in eukaryotic cells which have of course, organelles like mitochondria. Mitochondria are membrane enclosed and so to get things in and out of there is not as easy as if the reactants are floating around in the soup, the soup of the cytoplasm of course. In the mitochondrion, there are reactions occur that are the citric acid cycle, the breakdown, the oxidation of molecules, whereas the reactions of glycolysis are sequestered in the cytoplasm. These two pathways don't easily cross and the molecules to be used in each pathway must cross the lipid bilayer of the mitochondrion in order to be used.


    About the Lecture

    The lecture Regulation of Enzymatic Activity – Metabolism and Regulation by Kevin Ahern, PhD is from the course Biochemistry: Basics.


    Included Quiz Questions

    1. Small molecules are essential for allosterism.
    2. Allosterism involves phosphorylation/dephosphorylation.
    3. Covalent modification does not involve the cleavage of peptide bonds.
    4. Sequestration is the primary means of enzymatic regulation.
    5. DNA splicing is the only way to regulate enzymes.
    1. Feedback mechanism — controlling a central metabolic pathway by inhibiting the synthesis of the last enzyme of the pathway via end product molecules
    2. Allosterism — binding of small molecules at specific sites to modulate enzyme activities
    3. Irreversible covalent modifications — maturation of zymogens
    4. Reversible covalent modifications — addition or removal of the phosphoryl group to control enzymatic activities
    5. Enzyme synthesis control — controlling the transcription of mRNA of specific gene in response to the cellular surroundings

    Author of lecture Regulation of Enzymatic Activity – Metabolism and Regulation

     Kevin Ahern, PhD

    Kevin Ahern, PhD


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