DIFFERENT FORMS OF FEED BACK CONTROL

I had mentioned in my earlier posts the difference between Feedback inhibition and Feedback repression. But both can be considered under a single name i.e, Feedback Control.

Feedback Control in Unbranched Pathways

Unbranched pathways give only one end product.

In unbranched pathways control is achieved in two ways:

• The end product inhibiting the first enzyme of the pathway.
• The end product or its derivatives repressing all the enzymes involved in the pathway

Feedback Control in Branched Pathways

In branched pathways there will be more than one end product. So control cant be as simple as in unbranched pathways. Each of the end product has its own physiological significance and its own levels of optimum concentration. If one of the end product exerts control on an enzyme common to two or more end products then the organism may suffer from the lack of those end products whose synthesis has been blocked. So different control mechanisms have evolved that avoid these sort of problems. They include:

• Concerted/ Multivalent Feedback control
• Co-operative Feedback Control
• Cumulative Feedback Control
• Sequential Feedback Control
• Control by Isoenzymes

Concerted/Multivalent Feedback control:
When all the end products are present in excess, they togather exert control over the first enzyme of the pathway. When either of them is not present in excess control can't be exerted.

Co-operative Feedback control:
This is similar to concerted feed back control. The difference is that in Co-operative feed back control when when of the endproducts is in excess, it can exert a weak control independent of the other endproducts. When both the endproducts occur in excess they exert a synergistic control.

Cumulative Feedback control:
Each of the end products exert a certain proportion of control over the first enzyme of the pathway independently. They donot exert a synergistic control as in case of the co-operative feedback control.

In both Co-operative feedback control and Cumlative feedback control, each of the end products exert control immediately after the branch point so that the common intermediate for the end products is diverted away from the pathway of the product in excess.


Sequential Feedback control:
Each endproduct controls the enzyme immediately after the branchpoint leading to the end product. Due to this the intermediates build up that inturn exerts control on previous enzymes.

Control by Isoenzymes:
Isoenzymes are enzymes which catalyse the same reaction but different in their control characteristics.
The critical reaction (usually the first reaction) is catalysed by more than one isoenzymes. In this situation different end products inhibit different isoenzymes. So, when one of the end product is in excess it exerts control on one of the isoenzymes leaving the other isoenzyme active so that the production of the other endproduct is not stopped.