user16651: I approved an edit on this question where the metabolism tag was added. I am not 100% sure that I would agree that the essence of the question is about metabolism, but I did not disagree strongly enough to reject the edit either.
That then raised the question of what the appropriate use of the metabolism tag is. The tag description states:

The set of chemical reactions that happen in the cells of living
organisms to sustain life.

Technically any question related to a cellular process could be classified as cell metabolism, but metabolism may not be at the heart of the question. Nature's Scitable gives the following definition:

Cell Metabolism
A cell's daily operations are accomplished through the biochemical reactions that take place within the cell. Reactions are turned on and off or sped up and slowed down according to the cell's immediate needs and overall functions. At any given time, the numerous pathways involved in building up and breaking down cellular components must be monitored and balanced in a coordinated fashion. To achieve this goal, cells organize reactions into various enzyme-powered pathways.

So my question is when is it appropriate to tag a question as being a question about metabolism and when should the question not be tagged with the metabolism tag?
Would we want to reserve the tag for only those questions that dealt with cellular processes such as Glycolysis, The Citric Acid Cycle, Oxidative Phosphorylation, etc. or is any question about cell function fair game for the tag?
A secondary question would be is the question that I linked above an example of a question that would warrant the metabolism tag.
Thank you in advance.

user15695: Good question. I agree the formulation "The set of chemical reactions that happen in the cells of living organisms to sustain life" for this tag is way too broad. Pretty much all of biology is chemical reactions that happens in cells!
I would define metabolism as the collection of chemical processes whose main purpose is to synthesize biomass or extract energy for cellular work. This includes breakdown of fat, protein and carbohydrates derived from the diet / environment, and synthesis of other compounds required to build cellular structures, as well as energy metabolism. These are "physical" processes that provide matter and energy.
Many enzymatic reactions (and other chemical processes) do not provide matter or energy and should not be considered as "metabolism" in my view. For example, protein kinases that transmit signals to regulate other proteins, or synthesis and release of neurotransmitters and hormones. These processes may be catalyzed by enzymes, and they may involve small molecules that we might call "metabolites", but their purpose is to send or receive information (signals); they do not by themselves produce biomatter or energy, and therefore I would not consider them as truly metabolic processes.
This is a bit subjective of course, and there certainly are gray zones. For example, phosphatidylinositol metabolism is mainly involved in cell signalling, but many would include it in lipid metabolism. But generally I think the dividing line is whether a process actually provides matter or energy, or merely information.
That would be my definition of metabolism itself. I would agree with Vance though that questions about regulation of metabolic processes is also relevant for the "metabolism" tag. This includes effects of metabolic hormones on metabolism of various cell types, and regulatory enzymes such as pyruvate dehydrogenase kinase or fructose-2,6-biphosphatase, for example. But I would not include questions about "pure" signalling processes such as the MAPK cascade or PI3 kinase, unless it immediately relates to their effects on a metabolic process.
As for the specific question you mentioned, I would probably not tag it "metabolism", because it relates mainly to how cAMP signalling achieves specificity. It does fit endocrinology though.
Sorry for the long post :) Anyway, I would suggest revising the "metabolism" tag description to something like "Biochemical processes whose main purpose is to synthesize biomass or extract energy, and their regulation".

user16299: Great question and thanks for posting!
My perspective is that of a physiologist who studies metabolism in general. I tagged the question specifically with the metabolism tag and didn't think twice about doing so. From my perspective, I consider "metabolism" to include not only the strict biochemical reactions (e.g. Glycolysis, Krebs Cycle, Glycogenolysis, Gluconeogenesis, Cori Cycle, etc), but also the regulation of those metabolic enzymes and pathways. This regulation includes not only the classical allosteric effects and feedback/product inhibition that is taught in basic biochemistry courses, but also includes the hormonal and organ-to-organ communication that is emphasized in graduate level biochemistry and integrative physiology courses.
If you study metabolism in a vacuum (i.e. just the strict biochemistry reactions) you easily miss the complex regulation, subcellular sequestration, and organ-to-organ communication that is vital to having a thorough understanding of intermediary metabolism. For example, the organ-to-organ flux of citrulline between the intestinal-kidney-liver axis is a vital part of whole body citrullin/arginine/glutamine metabolism and nitrogen balance. No biochemist would argue that those processes are somehow separate from "metabolism" - they are an important part of the reactions that control whole-body nitrogen balance. 
As for the question that sparked this discussion, it was asking about a fundamental concept of metabolic regulation - that is, how hormones like insulin, glucose and epinephrine signal through second messengers with specificity. 
I thought the question was excellent! This is one of those questions that is fundamental to graduate study in physiology. It is those second messenger processes that can have very different effects depending on the tissue type and receptor expression. The concept of second messengers actually grew out of studies from biochemists like Earl Sutherland and others at Vanderbilt University, Washington University in St. Louis, Case Western Reserve (and others). These scientists were trying to understand the hormonal regulation of metabolic enzymes in tissues like liver, skeletal muscle, adipose tissue, etc. 
Also, note that the definition above is for cell metabolism, however, I would argue that metabolism is much more than the metabolism of an individual cell - which is what I have tried to emphasize in my answer above.

16651: <p>I approved an edit on <a href="https://biology.stackexchange.com/questions/41084/if-so-many-different-hormones-molecules-work-by-activating-adenyl-cyclase-how-d?noredirect=1#comment69647_41084">this question</a> where the <a href="https://biology.stackexchange.com/questions/tagged/metabolism" class="post-tag" title="show questions tagged &#39;metabolism&#39;" rel="tag">metabolism</a> tag was added. I am not 100% sure that I would agree that the essence of the question is about metabolism, but I did not disagree strongly enough to reject the edit either.</p>
<p>That then raised the question of what the appropriate use of the <a href="https://biology.stackexchange.com/questions/tagged/metabolism" class="post-tag" title="show questions tagged &#39;metabolism&#39;" rel="tag">metabolism</a> tag is. The tag description states:</p>
<blockquote>
<p>The set of chemical reactions that happen in the cells of living
organisms to sustain life.</p>
</blockquote>
<p>Technically any question related to a cellular process could be classified as cell metabolism, but metabolism may not be at the heart of the question. Nature's Scitable gives the following definition:</p>
<blockquote>
<h1>Cell Metabolism</h1>
<p>A cell's daily operations are accomplished through the biochemical reactions that take place within the cell. Reactions are turned on and off or sped up and slowed down according to the cell's immediate needs and overall functions. At any given time, the numerous pathways involved in building up and breaking down cellular components must be monitored and balanced in a coordinated fashion. To achieve this goal, cells organize reactions into various enzyme-powered pathways.</p>
</blockquote>
<p>So my question is when is it appropriate to tag a question as being a question about metabolism and when should the question not be tagged with the metabolism tag?</p>
<p>Would we want to reserve the tag for only those questions that dealt with cellular processes such as Glycolysis, The Citric Acid Cycle, Oxidative Phosphorylation, etc. or is any question about cell function fair game for the tag?</p>
<p>A secondary question would be is the question that I linked above an example of a question that would warrant the <a href="https://biology.stackexchange.com/questions/tagged/metabolism" class="post-tag" title="show questions tagged &#39;metabolism&#39;" rel="tag">metabolism</a> tag.</p>
<p>Thank you in advance.</p>
 15695: <p>Good question. I agree the formulation "The set of chemical reactions that happen in the cells of living organisms to sustain life" for this tag is way too broad. Pretty much all of biology is chemical reactions that happens in cells!</p>

<p>I would define metabolism as the collection of chemical processes whose main purpose is to synthesize biomass or extract energy for cellular work. This includes breakdown of fat, protein and carbohydrates derived from the diet / environment, and synthesis of other compounds required to build cellular structures, as well as energy metabolism. These are "physical" processes that provide matter and energy.</p>

<p>Many enzymatic reactions (and other chemical processes) do not provide matter or energy and should not be considered as "metabolism" in my view. For example, protein kinases that transmit signals to regulate other proteins, or synthesis and release of neurotransmitters and hormones. These processes may be catalyzed by enzymes, and they may involve small molecules that we might call "metabolites", but their purpose is to send or receive information (signals); they do not by themselves produce biomatter or energy, and therefore I would not consider them as truly metabolic processes.</p>

<p>This is a bit subjective of course, and there certainly are gray zones. For example, phosphatidylinositol metabolism is mainly involved in cell signalling, but many would include it in lipid metabolism. But generally I think the dividing line is whether a process actually provides matter or energy, or merely information.</p>

<p>That would be my definition of metabolism itself. I would agree with Vance though that questions about regulation of metabolic processes is also relevant for the "metabolism" tag. This includes effects of metabolic hormones on metabolism of various cell types, and regulatory enzymes such as pyruvate dehydrogenase kinase or fructose-2,6-biphosphatase, for example. But I would not include questions about "pure" signalling processes such as the MAPK cascade or PI3 kinase, unless it immediately relates to their effects on a metabolic process.</p>

<p>As for the <a href="https://biology.stackexchange.com/questions/41084/if-so-many-different-hormones-molecules-work-by-activating-adenyl-cyclase-how-d">specific question</a> you mentioned, I would probably not tag it "metabolism", because it relates mainly to how cAMP signalling achieves specificity. It does fit endocrinology though.</p>

<p>Sorry for the long post :) Anyway, I would suggest revising the "metabolism" tag description to something like "Biochemical processes whose main purpose is to synthesize biomass or extract energy, and their regulation".</p>
 16299: <p>Great question and thanks for posting!</p>

<p>My perspective is that of a physiologist who studies metabolism in general. I tagged the <a href="https://biology.stackexchange.com/questions/41084/if-so-many-different-hormones-molecules-work-by-activating-adenyl-cyclase-how-d">question</a> specifically with the <em>metabolism</em> tag and didn't think twice about doing so. From my perspective, I consider "metabolism" to include not only the strict biochemical reactions (e.g. <em>Glycolysis, Krebs Cycle, Glycogenolysis, Gluconeogenesis, Cori Cycle, etc</em>), but also the <strong>regulation of those metabolic enzymes and pathways</strong>. This regulation includes not only the classical allosteric effects and feedback/product inhibition that is taught in basic biochemistry courses, but also includes the hormonal and organ-to-organ communication that is emphasized in graduate level biochemistry and integrative physiology courses.</p>

<p>If you study metabolism in a vacuum (i.e. <em>just the strict biochemistry reactions</em>) you easily miss the complex regulation, subcellular sequestration, and organ-to-organ communication that is vital to having a thorough understanding of intermediary metabolism. For example, the organ-to-organ flux of citrulline between the <em>intestinal-kidney-liver</em> axis is a vital part of whole body citrullin/arginine/glutamine metabolism and nitrogen balance. No biochemist would argue that those processes are somehow separate from "metabolism" - they are an important part of the reactions that control whole-body nitrogen balance. </p>

<p>As for the <a href="https://biology.stackexchange.com/questions/41084/if-so-many-different-hormones-molecules-work-by-activating-adenyl-cyclase-how-d">question</a> that sparked this discussion, it was asking about a fundamental concept of metabolic regulation - that is, how hormones like insulin, glucose and epinephrine signal through second messengers with specificity. </p>

<p><strong>I thought the question was excellent!</strong> This is one of those questions that is fundamental to graduate study in physiology. It is those second messenger processes that can have very different effects depending on the tissue type and receptor expression. The concept of second messengers actually grew out of studies from <strong>biochemists</strong> like Earl Sutherland and others at Vanderbilt University, Washington University in St. Louis, Case Western Reserve (and others). These scientists were trying to understand the hormonal regulation of metabolic enzymes in tissues like liver, skeletal muscle, adipose tissue, etc. </p>

<p>Also, note that the definition above is for <strong>cell metabolism</strong>, however, I would argue that <strong>metabolism is much more than the metabolism of an individual cell</strong> - which is what I have tried to emphasize in my answer above.</p>
