Many signaling pathways can seem quite complex at first glance, but they often f

Question

Many signaling pathways can seem quite complex at first glance, but they often follow a simple logic. One can often make sense of complex signaling pathways by first labeling activators as +1 and inhibitors as -1. Then, when you multiply all of the components of a pathway together, the sign of the answer indicates whether the end point of the pathway is inhibited or activated. This is because inhibitors of inhibitors are activators. Mathematically, this works because multiplying two negative numbers yields a positive number (-1 x -1 = 1).

Consider the hypothetical pathway depicted below, in which the molecule Rec is a receptor that is bound by the ligand Lig. Upon binding, Rec activates Enz1. When activated, Enz1 inhibits Enz2. At steady state (in the absence of signal), Enz2 is active, and is an inhibitor of the transcription factor TF1. TF1 is active unless inhibited by Enz2. You may be wondering how an enzyme could inhibit another molecule (such as a transcription factor or another enzyme). As you will learn, certain enzymes can covalently modify other proteins in ways that affect protein activity.

When Rec is bound to Lig, what is the activation state of each of the molecules in this pathway? Is gene expression active or inactive? Assume that inhibited molecules are inactive, and that a component of the pathway is inactive unless otherwise stated. Drag and drop the correct term next to each molecule. Each answer may be used once, more than once, or not at all. Drop target image

Lig-? Rec- . Enzi- H Enz2- TF1-? Gene expression Copyright The President and Fellows of Harvard Colege You may be wondering how an enzyme could inhibit another molecule (such as a transcription factor or another enzyme). As you will learn, certain enzymes can covalently modify other proteins in ways that affect protein activity. When Rec is bound to Lig, what is the activation state of each of the molecules in this pathway? Is gene expression active or inactive? Assume that inhibited molecules are inactive, and that a component of the pathway is inactive unless otherwise stated. Drag and drop the correct term next to each molecule. Each answer may be used once, more than once, or not at all. Rec Enz1 Enz2 TF1 Gene expression active inactive

Explanation / Answer

Rec - active (ligand binding to rec makes Rec active)

Enz1 - active (when Rec is active, it activates Enz1)

Enz2 - Inactive (When Enz1 is activated, it inhibits Enz2)

TF1 - Active (When Enz2 is inactive, TF1 is active)

Gene expression - Active (When TF1 gets activated, it promotes gene expression)

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