Mon 5:33 PM C Exam 4 Open CHEM 400500 Summer 2018 2. ATP is one of the energy cu

Question

Mon 5:33 PM C Exam 4 Open CHEM 400500 Summer 2018 2. ATP is one of the energy currencies of the cell. ATP is a negative allosteric effector of enzyme B of the following pathway. NAD NADH+H What conclusions might you make about this enzyme based on the results that ATP is a negative allosteric effector of this enzyme? Include the sign and relative size of AGe compared to other enzymes of the pathway, the expected highest order of enzyme structure of enzyme B, the expected type of cooperativity demonstrated by enzyme B, the expected shape of the rate vs [S] curve for this enzyme and whether you feel that B is part of a catabolic or anabolic pathway. Explain in excruciating detail giving complete reasons for your answers. (4 pts)

Explanation / Answer

Conclusions about the enzyme:

Sign and size of standard free energy (Delta Go) compared to other enzymes:

As this step requires ATP and is regulated by it, it has to be the irreversible step with negative free energy change (delta Go ) resulting in the formation of product. The value would be more negative than the other enzymes involved which will drive the pathway. The other enzymatic steps are in equilibrium and have almost zero delta Go of reactions they catalyze.

Order of enzyme structure of B:

As the enzyme is allosteric in nature, it must be having more than one subunit with each subunit have active site and allosteric site. Such enzymes have tense (low affinity) and relaxed (high affinity) states. The enzyme accepts ligand in relaxed state while allosteric inhibitor (ATP in present case) prevents the enzyme to come in relaxed state and hence preventing it to bind the substrate

Expected Cooperativity by enzyme:

The enzyme is expected to show cooperativity with substrate and activators while negative with the allosteric inhibitors. This is because enzyme has more than one subunit and a change in one subunit will cause similar change in other. In present case the enzyme will show negative cooperativity for substrate when ATP binds to one of allosteric site. This prevents that subunit to come in relaxed state which due to cooperatvity will prevent another subunit to be in relaxed state hence blocking the binding sites of both the subunits.  

Expected shape of rate v/s [S] curve:

The enzyme in question is an allosteric enzyme and this type of enzymes show sigmoidal curves. This is due to cooperativity effect. The rate of reaction will be less initially when the substrate concentration is less but increase will be higher as substrate concentration increases.

Part of Catabolic/anabolic pathway

Enzyme B is a part of Catabolic pathway as the pathway involves inhibition at high ATP concentration which means if the pathway proceeds it will result in ATP formation and also NADH + H+ is produced. So, the pathway seems to be a catabolic pathway.

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