Why it's important to know the location of an intermediate is upstream or downstream? the concept says " "if a strain with a mutant enzyme that blocks a particular step in a linear metabolic pathway can grow when an intermediate is added to the growth.."
1 Answer
It is important to know the position of the intermediate so that the organism can grow (as stated in the question).
This particular concept applies mainly to microbial fermentation. Say a linear metabolic pathway is as follows:
A ---a---> B ---b---> C ---c---> D ---d---> E
A is the substrate
E is the required product necessary for microbial growth
B, C and D are intermediates
a, b, c and d are enzymes catalysing the reactions shown by the respective arrows
Since this is a linear metabolic pathway, there is no way to get E from A except by going through B, C and D
So, if a microorganism has fully functioning a, b, c and d enzymes, it can synthesise E from A and hence, grow. However, if a microorganism has, say, a mutant non-functional enzyme c. Substrate A will only be converted to intermediate C, product E will not be synthesised and the growth of the organism will be arrested.
Nevertheless, if we still want the mutant microorganism to grow, we can provide intermediate D (which is downstream to the mutant enzyme) which will be converted to E via enzyme d. But if we provide intermediate B (which is upstream to mutant enzyme c) it will only be converted to intermediate C.
Hence, knowing the location of the intermediate tells us whether supplying that particular intermediate to the microorganism allows it to grow of not.
