Why is the snot greenish?
Runny noses frequently produce yellowish-greenish mucus. The color comes from white blood cells called neutrophils that produce a haeme enzyme myeloperoxidaze. The latter makes HOCl from hydrogen peroxide and chloride that kills bacteria. The green color of this haeme pigment is extremely unusual. There are many yellow and red haeme proteins, but green haeme is rare; it occurs (apart from the animal defenses) only in some bacteria, eg http://www.ncbi.nlm.nih.gov/pubmed/16817906
It's likely that animal myeloperoxidases are of the bacterial origin.
http://www.ncbi.nlm.nih.gov/pubmed/11080366
HOCl is chlorine bleach: combining Cl2 and water makes HOCl, too. The concentration of bleach in the mucus is very low; how does it kill bacteria?
This is still unsettled. HOCl modifies amine and thiol side chains in proteins, reacts with unsaturated lipids and (very slowly) with nucleobases. However, these are rather slow and inefficient reactions, whereas bleach kills a bacterial cell almost on contact, clearly by inactiviting some important biochemistry in the outer membranes. There are several theories of this action, but the question still remains open. The old answer (inhibition of oxidation) was shown to be incorrect. Another popular idea (ATP depletion) also seems to be incorrect; the idea was that there is an easily modifiable membrane protein that is involved in ATP synthesis, regulation, or removal. Another theory was that this mystery protein is involved in DNA replication.
http://en.wikipedia.org/wiki/Hypochlorous_acid#Mode_of_disinfectant_action
The latest, much hyped theory posits that HOCl acts by aggregation of proteins:
...we demonstrate that low molar ratios of HOCl to protein cause oxidative protein unfolding in vitro and target thermolabile proteins for irreversible aggregation in vivo. As a defense mechanism, bacteria use the redox-regulated chaperone Hsp33, which responds to bleach treatment with the reversible oxidative unfolding of its C-terminal redox switch domain. HOCl-mediated unfolding turns inactive Hsp33 into a highly active chaperone holdase, which protects essential Escherichia coli proteins against HOCl-induced aggregation and increases bacterial HOCl resistance. http://www.cell.com/abstract/S0092-8674(08)01181-1
There is little doubt that HOCl activates this chaperon holdase, but it is a long stretch to claim that HOCl acts mainly through inducing aggregation of membrane proteins. I am pretty sure this latest theory will join its predecessors.
What seems certain is that a long time ago, ancient bacteria discovered that making bleach somehow kills their competition and they developed green haeme peroxidases to make HOCl; these peroxidases were subsequently passed on to animals that also use this bleach as an antimicrobial reagent. The real mystery is that it still works so remarkably well, as this defence mechanism must be of great antiquity. Pathogens avoid the bleach by forcing our own cells to produce more mucus that dilutes it; this mucus also helps the bacteria to spread out in aerosols and infect more animals. However, dilution comes first; that's why we make more snot than sneeze it out.
Every time I look at the greenish stuff running from my nose, like I did this morning, I have the same uneasy thought. Since most of our drinking water is chlorinated, we are engaged in a massive experiment in growing HOCl-resistant bacteria. If side groups cause such great vulnerability to HOCl in certain membrane enzymes, these groups can be eliminated, or better protected, or chaperones can be engaged to refold these proteins, like this Hsp33 holdase, etc. We are breeding superbugs that will be able to evade our own innate immune system that is also based on the use of the bleach! Simple logic tells that using chlorine (as opposed to, say, ozone or UV/peroxide) as disinfectant can do enormous harm in the future, as its action too closely mimics our own cellular defenses. Perhaps chloramine (that makes water greenish) is a safer alternative overall.
Why is the goo greenish?
Normal haeme vs. green haeme.
It's likely that animal myeloperoxidases are of the bacterial origin.
http://www.ncbi.nlm.nih.gov/pubmed/11080366
HOCl is chlorine bleach: combining Cl2 and water makes HOCl, too. The concentration of bleach in the mucus is very low; how does it kill bacteria?
This is still unsettled. HOCl modifies amine and thiol side chains in proteins, reacts with unsaturated lipids and (very slowly) with nucleobases. However, these are rather slow and inefficient reactions, whereas bleach kills a bacterial cell almost on contact, clearly by inactiviting some important biochemistry in the outer membranes. There are several theories of this action, but the question still remains open. The old answer (inhibition of oxidation) was shown to be incorrect. Another popular idea (ATP depletion) also seems to be incorrect; the idea was that there is an easily modifiable membrane protein that is involved in ATP synthesis, regulation, or removal. Another theory was that this mystery protein is involved in DNA replication.
http://en.wikipedia.org/wiki/Hypochlorous_acid#Mode_of_disinfectant_action
The latest, much hyped theory posits that HOCl acts by aggregation of proteins:
...we demonstrate that low molar ratios of HOCl to protein cause oxidative protein unfolding in vitro and target thermolabile proteins for irreversible aggregation in vivo. As a defense mechanism, bacteria use the redox-regulated chaperone Hsp33, which responds to bleach treatment with the reversible oxidative unfolding of its C-terminal redox switch domain. HOCl-mediated unfolding turns inactive Hsp33 into a highly active chaperone holdase, which protects essential Escherichia coli proteins against HOCl-induced aggregation and increases bacterial HOCl resistance. http://www.cell.com/abstract/S0092-8674(08)01181-1
There is little doubt that HOCl activates this chaperon holdase, but it is a long stretch to claim that HOCl acts mainly through inducing aggregation of membrane proteins. I am pretty sure this latest theory will join its predecessors.
What seems certain is that a long time ago, ancient bacteria discovered that making bleach somehow kills their competition and they developed green haeme peroxidases to make HOCl; these peroxidases were subsequently passed on to animals that also use this bleach as an antimicrobial reagent. The real mystery is that it still works so remarkably well, as this defence mechanism must be of great antiquity. Pathogens avoid the bleach by forcing our own cells to produce more mucus that dilutes it; this mucus also helps the bacteria to spread out in aerosols and infect more animals. However, dilution comes first; that's why we make more snot than sneeze it out.
Every time I look at the greenish stuff running from my nose, like I did this morning, I have the same uneasy thought. Since most of our drinking water is chlorinated, we are engaged in a massive experiment in growing HOCl-resistant bacteria. If side groups cause such great vulnerability to HOCl in certain membrane enzymes, these groups can be eliminated, or better protected, or chaperones can be engaged to refold these proteins, like this Hsp33 holdase, etc. We are breeding superbugs that will be able to evade our own innate immune system that is also based on the use of the bleach! Simple logic tells that using chlorine (as opposed to, say, ozone or UV/peroxide) as disinfectant can do enormous harm in the future, as its action too closely mimics our own cellular defenses. Perhaps chloramine (that makes water greenish) is a safer alternative overall.
Why is the goo greenish?
Normal haeme vs. green haeme.