Question

34. Consider the following hypothetical metabolic pathway: a) Under intracellular conditions, the activity of enzyme X is 1 Calculate the effect on metabolic flux rate (Ipmol(10 cells x sec)D of B C of the following treatments. Calculate as % change (increase or decrease). 00pmou(10 cells x sec). I U Sre'l。re % change in flux Treatment inhibitor that reduces activity of X by 10% Activator that increases activity of X by 10% b) Now consider a substrate cycle operating with enzymes X and Y in the hypothetical pathway shown below. Under intracellular conditions, the activity of enzyme X is 100pmou(10 cells x sec) and that of enzyme Y is 80pmol(10 cells x sec). What are the direction and rate (Ipmol/(10 cells x sec)D of metabolic flux between B and C? A B Direction rate c) Calculate the effect on direction and metabolic flux of the following treatments. Calculate as % change (increase or decrease). Direction of flux % change in flux Treatment B to C or C to B inhibitor that reduces activity of X by 10% Activator that increases activity of X by 10% Doubling the activity of enzyme Y

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Answer 1

The metabolic flux or flux is the rate of turnover of molecules in a metabolic pathway. The metabolic flux depends on activity of the enzyme which is involved in the reaction. Consider the number of moles of B converted to C is 100. Thus, when the activity of enzyme changes by 10 % the conversion rate also changes and affects the metabolic flux by 10 %. Hence, when inhibitor reduces the activity of enzyme X' by 10 %, the change in flux is 10 % (decrease). Similarly, with the increase in enzyme activity by 10 % by an activator, the change in flux is 10 % (increase).

In the given information, the rate of activity of enzyme X is 100 pmol/ 106 cells/ sec and the rate of activity of enzyme Y is 80 pmol 106 cells/ sec. The metabolic flux rate is calculates as follows: Metabolic flux rate = rate of X-rate of Y 100 80 20 pmol 10 cells sed Hence, the metabolic flux rate in the direction of B-C is 20 pmolu 106 cells/ sec

The metabolic flux rate in the direction of B C is 20 pmol/ 106 cells/ sec. The decrease in enzyme activity of X by 10 % affects the metabolic flux rate in the direction of B→C by 10 pmol/ 106 cells/ sec. 90-80 = 10 pmol/10°cells/sec The 10 pmol 106 cells/sec are exactly half of the normal flux rate. Hence, there is 50 % (decrease) change in flux. The positive value indicates the direction of flux is from B→C. . The increase in enzyme activity of X by 10% affects the metabolic flux rate in the direction of B-C by 30 pmol/ 106 cells/ sec. 100 80 30 pmol/10 cells/sec The above value is 10 pmol/ 106 cells/ sec more than the normal value. Hence, there is 50 % (increase) change in flux. The positive value indicates the direction of filux is from B-C Doubling the enzyme activity of Y, affects the metabolic flux rate in the direction of B-C. 100-160--60 pmol/10' cells/sec The negative value indicates the direction of flux is from C→B. Normally, the rate of enzyme Y in conversion of C-B is 20 pmolu 106 cells/ sec less than rate of enzyme X. By doubling the enzyme activity of Y has increased its activity by 60 pmol/ 106 cells/ sec than X. Hence, there is 400% (decrease) change in flux.

The substrate cycle is a cycle of phosphorylation and dephosphorylation catalyzed by two enzymes. The net effect of this cycle is hydrolysis of adenosine tri phosphate (ATP) and generation of heat. It allows amplification of a small change in an enzyme's activity. For example, the 10 % change in enzyme activity gives a 50 % change in flux.