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Chemguide: Support for CIE A level Chemistry Learning outcome 11.1(n) This statement looks at some of the problems associated with heavy metals in the environment. Before you go on, you should find and read the statement in your copy of the syllabus. Heavy metals include things like mercury, lead and silver. The ions of these metals have the ability to change the structure of proteins, including enzymes. This particular statement deals with heavy metals in the environment, and so it makes sense to concentrate on mercury and lead. Although silver ions do interfere with proteins, they aren't a major environmental problem. Sources of the mercury and lead in the environment Mercury The US Environmental Protection Agency (EPA) reported in 2005 that 50% of all US human-based mercury emissions came from burning coal. Coal contains all sorts of other minerals apart from carbon, and these include mercury compounds. Mercury is also used in small-scale gold mining. Associated Press reported in 2009 that small-scale gold mining was second only to burning fossil fuels as a source of mercury pollution. A major source of mercury pollution used to be the production of sodium hydroxide solution using the Castner-Kellner Process. This involved the electrolysis of sodium chloride solution using a mercury cathode. This is being phased out because of the serious pollution that it has caused in the past. Lead In the past, lead was used for water pipes, and so lead ions ended up in the water you drank. Lead was added to petrol, and lead compounds were present in car exhausts. This has now largely been stopped. Lead and its compounds were also used in things as various as toys or paints. Again, this use has been largely discontinued. Concentration in food chains Mercury poisoning that occurred in Minamata in Japan in the mid-1900s is a good example of this. The pollution was in the form of methylmercury, which was taken up by shellfish and fish, which were then eaten by people in the fishing communities nearby. The biochemical effects of heavy metal ions You will already have come across these in earlier statements. This just brings it all together, and adds some equations that you might need for exam purposes. Heavy metal ions can react with side groups in the amino acid residues which make up the protein chains. If those side-groups are important to the way the protein folds into its tertiary structure, then that can have an effect on the shape of the active site of an enzyme molecule. The metal ions can be non-competitive inhibitors. In June 2009, CIE asked a fairly typical question about the toxicity of mercury. They asked for a possible source, and then for an explanation of two reasons why mercury is toxic. Their mark schemes showed that they wanted you to describe (with equations) the effect of mercury ions on two different types of amino acid side groups - those containing sulphur and those containing -COOH groups. So let's look at the rest of this topic based on that question. You would NOT need to write everything below. You would need to choose ONE of the equations involving sulphur-containing groups, and ONE of the equations involving -COOH groups to talk about. As you will see, it is always easier to talk about mercury(I) ions than mercury(II) ions. The effect of mercury ions on side groups containing sulphur There are two possible effects you can talk about - and either was acceptable in the question I mentioned. Reaction with -SH groups in cysteine
This is easier to see with mercury(I) ions. The reaction is:
Silver ions would behave in exactly the same way - just replace Hg by Ag. | |
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Note: CIE use Hg+ for the mercury(I) ion. In fact, the ion should be Hg22+. It is, of course, easier to use the simplified version, and if that's what CIE wants, that is what you have to give them. | |
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Mercury(II) ions are a bit more of a problem. One of the equations which CIE suggested as a possibility in their mark scheme was the following. This forms one bond to the sulphur, and leaves a single positive charge on the mercury.
If the -SH group was important either in the active site of an enzyme, or in determining the tertiary structure of the protein, then these reactions would interfere with that. Reaction with -S-S- bridges
Again, this is easier to see with mercury(I) ions. The reaction is:
The -S-S- bridge is broken (and, incidentally, the mercury has been oxidised from the +1 to +2 oxidation state). Sulphur-sulphur bridges are important in the tertiary structure of a protein, and changes to that could well affect the shape of an active site. I am not certain exactly how this happens with mercury(II) ions. Interestingly, CIE didn't actually offer a version of this equation in the mark scheme for the question. If CIE just ask a question about mercury in this context, always use the Hg(I) ion when you are discussing this. The effect of mercury ions on side groups containing -COOH A salt is formed between the -COOH and the metal ion. With mercury(I) ions, you get:
Exactly the same thing happens with silver ions, and CIE asked a question about that on another occasion. -COOH groups in side chains are involved in either hydrogen bonding or in ionic attractions in the tertiary structure of proteins. Ionic bonds come from a transfer of a hydrogen ion to a nearby -NH2 in the side group on another amino acid residue, giving an attraction between -COO- and -NH3+. Neither hydrogen bonding nor the formation of ionic bonds can happen if the hydrogen in the -COOH group has been replaced by a metal atom. What happens if you have mercury(II) ions? I don't really know the answer to that! I can't find it anywhere reliable. CIE suggest this equation in the mark scheme for the question:
If you have a free choice in an exam, always stick to mercury(I) ions in your answers.
© Jim Clark 2011 (modified August 2013) |
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