Iron

Second post for the last BIOC3004 topic! In this post, we will be looking at iron as it is the most abundant metal in our body. That's not to say it's the only important one: zinc is found in the active sites of many enzymes (zinc fingers!) and manganese can have a protective effect against oxidation. We can't focus on all of them, though, which is why we're just sticking with iron for now.

Understand the two states of iron and how they affect function in iron bearing proteins.

The two states of iron are Fe²⁺ (ferrous) and Fe³⁺(ferric). Fe²⁺ is water-soluble, whereas Fe³⁺ is not. These two forms are inter-convertible, and there are several different reactions that can cause this. One such reaction is the Fenton reaction:

H₂O₂ + Fe²⁺ --> OH* + Fe³⁺ + OH^-

Note: the * denotes a free radical. I don't know if there's a way to type in the dot thing, but I do know for sure that I'm too lazy and tired to find out right now.

As you can probably guess, if this reaction was allowed to go on unchecked, you'd eventually end up producing a shit-ton of hydroxyl radicals, which isn't good. Good thing that Fe²⁺ is often stored or associated with something, keeping it out of mischief. Around 5-30% of the iron in our bodies is stored within a big protein called ferritin. A lot of iron is also chelated (associated with a ligand), such as heme, ascorbate, or citrate. As for Fe³⁺, it can be shuttled around by transferrin or serotransferrin and enter cells via transferrin receptors.

Basic functions of haemoglobin and myoglobin as oxygen carriers

Haemoglobin and myoglobin both contain a molecule called heme (or Fe-protoporphyrin IX), so let's look at that first. Heme is a tetrapyrrole, which means that it is a fairly large molecule consisting of four pyrrole rings as well as some side groups. There are four types of heme (A, B, C and O), which are classified according to these side groups.

There are many steps involved in the creation of heme, and thankfully we don't need to remember every step. The most important steps to know are the first step and the last step. The first step involves two simple molecules: glycine and succinyl-CoA, which combine to form 5-aminolevulic acid (5-ALA). One special thing about 5-ALA is that in some organisms it can go on to form B12, but not in us as we lack the enzyme. The last step involves the insertion of Fe into protoporphyrin IX. The special thing about this step is that it can be blocked by lead, which is why lead poisoning is bad.

As a side note, chlorophyll is somewhat similar in structure to heme. In fact, it is also synthesised via protoporphyrin IX, but instead of having a Fe in the middle, it has Mg in the middle. Vitamin B12 apparently has a similar porphyrin ring structure, but we'll find out more about that later.

Anyway, back to haemoglobin and myoglobin! Haemoglobin is made up of four subunits: 2 α and 2 β. As each subunit binds a heme group, and each heme group can bind one oxygen molecule, one molecule of haemoglobin can bind 4 oxygen molecules. Myoglobin, which is an oxygen-binding protein found in muscle, can only bind one heme and thus only one oxygen molecule. It makes up for this by binding oxygen more tightly than haemoglobin, however.

Cooperativity and role of myoglobin as an oxygen store

Haemoglobin displays cooperativity- the phenomenon in which when one oxygen binds, the next one binds more readily. Not really sure what else I can say here, other than that there are some other factors that affect oxygen binding. For example, lower pH, presence of DPG (a.k.a. BPG) and raised temperature can all release oxygen from haemoglobin. Also, foetal haemoglobin binds oxygen more tightly than maternal haemoglobin, ensuring that the foetus gets all of the oxygen that it needs.

Myoglobin, as I said before, binds oxygen more tightly than haemoglobin, making it suitable for storing oxygen. It acts as a "reserve oxygen store," as oxygen is only released from myoglobin when oxygen levels are low.

Examples of iron in heme and non-heme protein systems

Cytochromes

You're probably already familiar with a few cytochromes: cytochrome C, cyt b and cyt c in Complex III, cyt a in complex IV and cytochrome P450, to name a few. There are also others: cytochrome C oxidase converts reduced cyt C to oxidised cyt C, and cytochrome b5 reductase converts methemoglobin (which I'll describe later) back into working haemoglobin. Just like haemoglobin and myoglobin, cytochromes bind heme.

Lactoferrin (lactotransferrin)

Lactoferrin is a non-heme protein that contains iron. It is one of the components of the innate immune system as it has antimicrobial activity. Lactoferrin is found in many secretory fluids, including breastmilk, so it can provide antibacterial activity to babies. It can also bind with nucleic acids (preferably double-stranded), though I'm not sure how important that is to know.

Iron-Sulfur proteins

Iron-sulfur proteins come in three main arrangements: 2Fe-2S, 4Fe-4S and P-clusters. One such protein is a mitochondrial protein called frataxin, which is deficient in Friedreich's ataxia. Friedreich's ataxia is caused by an autosomal recessive mutation, and has symptoms such as poor coordination, scoliosis, heart disease and diabetes.

Know the daily requirements for iron and disorders related to over and under supply

Roughly 0.9mg of iron is lost per day, and more during menstruation. Iron is lost when heme is converted to biliverdin and Fe²⁺ and then into bilirubin, which is excreted (mainly through the GI tract but some through the skin and urine as well). Therefore, we need to consume some iron to make up for what we are losing. Iron is found in foods such as meat and fortified cereals. Babies need around 7mg/day, children 4mg/day, teenagers 4-6mg/day, men 6mg/day and women 8mg/day.

Probably the most well-known disorder involving iron is anaemia (low iron). This can be due to iron deficiency or a parasitic infection (worms like to steal your heme). This has symptoms such as feeling tired and weak, difficulty in maintaining body temperature, decreased immune function, breathlessness and headaches.

Haemochromatosis is the opposite problem, where you have too much iron. This is pretty rare but can be due to genetic causes. In haemochromatosis, iron builds up and forms deposits in the heart, joints and liver, which can lead to cirrhosis. Haemochromatosis can be treated by clinical phlebotomy (removing blood).

High levels of nitrate can also cause issues. Nitrate is converted to nitrite by gut microflora, and nitrite can cause the creation of methemoglobin. Methemoglobin is a form of haemoglobin that does not bind oxygen, so if you have too much methemoglobin, you're in for some serious trouble. This effect of nitrites can actually be used to our advantage, though: amyl nitrite can be used to treat cyanide poisoning. Amyl nitrite causes methemoglobin to form, which cyanide binds to in order to form nontoxic cyanomethemoglobin.