This is post #4 of this series on the Brain (First Post and Last Post). As I mention every time, these posts are from a series of lectures by Prof. Jan Schnupp, and I want to make sure he is properly quoted and credited. Many parts are his lectures verbatim. However, for any errors, mistakes or inaccuracies in anything I write in here, I take all the credit. I probably misunderstood him or made it up. His course material is available here.
So what are proteins? Proteins are huge. They are again molecules. So basically we have atoms that we link together to make molecules. But proteins are very, very large molecules.
They are very important in the functioning of the human body, and your brain is no exception. They're so-called macromolecules. They may contain thousands of atoms linked together. The thing about proteins actually is that there are smaller molecules, which are the so-called amino acids, which you link together to make larger molecules, which are the proteins.
There could be in principle infinitely many of different amino acids, but nature uses 20 amino acids to make all the proteins in your body.
Your body needs 20 different kinds of amino acids to function correctly. These 20 amino acids combine in different ways to make proteins in your body.
Your body makes hundreds of amino acids, but it can’t make nine of the amino acids you need. These are called essential amino acids. You must get them from the food you eat. The nine essential amino acids are:
Histidine: Histidine helps make a brain chemical (neurotransmitter) called histamine. Histamine plays an important role in your body’s immune function, digestion, sleep and sexual function.
Isoleucine: Isoleucine is involved with your body’s muscle metabolism and immune function. It also helps your body make hemoglobin and regulate energy.
Leucine: Leucine helps your body make protein and growth hormones. It also helps grow and repair muscle tissue, heal wounds and regulate blood sugar levels.
Lysine: Lysine is involved in the production of hormones and energy. It’s also important for calcium and immune function.
Methionine: Methionine helps with your body’s tissue growth, metabolism and detoxification. Methionine also helps with the absorption of essential minerals, including zinc and selenium.
Phenylalanine: Phenylalanine is needed for the production of your brain’s chemical messengers, including dopamine, epinephrine and norepinephrine. It’s also important for the production of other amino acids.
Threonine: Threonine plays an important role in collagen and elastin. These proteins provide structure to your skin and connective tissue. They also help with forming blood clots, which help prevent bleeding. Threonine plays an important role in fat metabolism and your immune function, too.
Tryptophan: Tryptophan helps maintain your body’s correct nitrogen balance. It also helps make a brain chemical (neurotransmitter) called serotonin. Serotonin regulates your mood, appetite and sleep.
Valine: Valine is involved in muscle growth, tissue regeneration and making energy.
(list of amino-acids from the site https://my.clevelandclinic.org/health/articles/22243-amino-acids)
Tryptophan is an interesting one because you use it to make a neurotransmitter called serotonin, which is very important for mood disorders.
If you have people who are depressed, often doctors say that maybe something is wrong with the serotonin levels.
Tryptophan is what you make serotonin out of. So you want to make sure you get enough tryptophan in your diet. Broccoli is also a good source of tryptophan.
These all, by the way, are hydrophobic amino acids. They like to mix with fat. They don't like to mix with water. There are also hydrophilic amino acids: serine, threonine, cysteine, tyrosine, asparagine, and glutamine.
And then there are charged ones, charged meaning that they themselves form ions. They will give off electrons or attract electrons, extra ones. These are: aspartate, glutamate, lysine, arginine and histidine.
Anybody ever heard of monosodium glutamate? It's a much maligned substance.
It is a substance that everybody says it is bad for you. Well, actually, it is the most important chemical neurotransmitter in your brain. Your body needs it, and your body needs it in copious quantities. But it needs it in the right place at the right time.
You do not want to get a super huge amount because glutamate goes straight into your brain and too much would be a sure-fire recipe for an epileptic fit.
But because that would be such a bad idea, to flood your brain with glutamate, your brain actually has a very effective barrier to keep glutamate out of your brain.
That is the so-called blood-brain barrier. Glutamate is not nearly as bad as people think it is. All the anchovies and many lovely tasty cheeses and so on, they are actually tasty because they have glutamate in them.
So, there are these amino acids, which are themselves relatively small molecules, and you string them together like pearls on a string to make proteins.
So imagine you've got 20 different types of beads, and you make long chains out of these beads. And you can basically pick them in any order you like, but the order in which your body wants to put them together is written down in your genes. Your genome, the DNA in your cells, basically tells your body which order it has to stick these amino acids together in order to make the proteins in your body.
And that is the only thing that your genes tell your body. It is just a recipe for making the different proteins in your body. Because once you have stuck these things together in the right way, they will do all the rest. Pretty much everything.
So how do they do this? How do they stick together in the first place? The thing is, amino acids are called amino acids because they have got an amino group and a so-called carboxy group, which makes it slightly acidic. And the thing is that you can basically link these together through a so-called peptide bond to end up with two chains where you can still have an amino group at one end and a carboxy group at the other. And then you can stick another one on, and another one on, and another one on.
And you can make very long chains.
And a typical protein will have many hundreds to many thousands of these things stuck together. And because, of course, some of these will then be hydrophobic, water repellent, others will be hydrophilic and like to mix with water, others will be positively charged, others will be negatively charged, this long chain will then fold spontaneously into different sorts of shapes. It can form into so-called alpha helices, which themselves can form together into so-called beta pleated sheets. And you'll get a very complicated shape.
This shape will be attracting ions because of its electric properties at one end and will repel them at another end and so on. And because of that, it can do a variety of things. It can, for example, form a little pore that will sit quite happily in our phospholipid bilayer.
Because it likes to mix with water somewhere and it doesn't want to mix with water somewhere else and it has the shape of a pore, stuff can go through it.
A pore is like a little channel, like a hollow little tube. It's like a tiny little tube. So we've got a tiny little tube that we can embed into our membrane, and that now means that we've got a place where, for example, the water can't go through on one side and ions can't go through another place, but they can go through this little tube to the little pore, this little channel, but only if the channel lets it. And this channel may change its mind about whether it lets a particular ion through or not.
And of course, if this now lets ions through only in certain times and not in others, then it will allow electrical currents, which are carried by the charges in these ions, to flow only under certain conditions. So we basically made ourselves a little switch, a little electrical switch that's made out of protein that sits in that membrane. So proteins really do a number of different things.
These transmembrane channels are these little pores that sit in the fatty membrane.
Other important things are so-called enzymes.
All enzymes are proteins. They are proteins that are wrapped up in a particular way that encourages other molecules to come together so that they can react together and form different bonds or break up different bonds and so on. Then we have the channels that we just talked about.
You can get so-called structural proteins, which just form long chains that are like scaffolding. And then you get so-called receptors, which can sense the presence or the absence of other substances.
So you can imagine that if you've got a protein that's folded into a particular shape because it's got positive and negative charges sitting on it, if there's another molecule that has a particular charge configuration that slots into it, it's shaped a bit like a lock, and you've got another molecule that fits into it like a key, it might change the shape of a protein and thereby make it do something else. And this would then be a receptor. And you can have receptors like this that are sitting on top of nerve cells and find other things.
And we'll talk more about that next time when we will discuss Membrane Voltage and Electrical Signaling.