Amino acids build proteins

What happens when you fry an egg? What happens when milk curdles? Both can be explained by how amino acids shape proteins. These twenty different amino acids are the building blocks of all proteins. When they form a protein, they attach to each other along their backbone structure... ...

with their side chains sticking out. All the cells in our bodies have the ability to build new proteins, as long as they get a constant supply of amino acids. Some of the amino acids we can produce in our bodies ourselves, using atoms from sugar and fat, or from other proteins. But nine of the twenty amino acids can't be built in our bodies. They need to be supplied from food.

These are called the essential amino acids. we don't get any of the essential amino acids. If we don't eat protein at all, Then the new proteins that require them can't be built. These new proteins are made in our cells by linking hundreds - or even thousands of them together in long strings. The strings of amino acids twist and turn in different ways.

In some proteins, they form long spirals that are wrapped around each other. In other proteins, the string of amino acids is folded together in a ball shape. Why is that? What makes the proteins fold themselves at all? Let's make a short string of amino acids to watch one such folding mechanism.

Although there are about twenty different amino acids to choose from, let's just use three: Glycine aspartic acid and lysine. The side chain of the aspartic acid has a negative end... And that of the lysine has a positive end. Even if we make the string of amino acids straight like this... Look what happens.

The negative and the positive areas of the side chains attract each other. So the string gets bent into a loop. If nothing interferes, this is how this particular string of amino acids will fold itself. But the attraction between the positive and negative side chains is not a very strong bond. Suppose this sequence of amino acids is from some egg white that you've just put in a frying pan.

As you heat it, the bond breaks. The amino acids are still attached to each other by the backbone structure, but the side chains lose contact with each other. That means that some of the twists and bends that folded the protein into its shape start to unravel. This is a process called denaturation. When a protein becomes denatured its molecules unfold out of their naturally occurring structures.

As protein molecules change shape, they start to get tangled up with each other. Instead of floating around like balls in a liquid, the protein molecules get entwined. That is the reason why egg solidifies with heat. These bonds can also be broken by changing the acidity of the protein. Milk gets sour when bacteria in the milk produce lactic acid.

Acid can break the weak bonds between the positive and negative side chains, just as heat can. As the bonds break, the bends in the protein start to unfold. As the protein changes its shape, the protein molecules get stuck to each other, forming clumps in the milk. We say the milk has curdled. So whether you are frying an egg, or forgetting to put the milk in the fridge, you're doing the same thing: making strings of amino acids unfold themselves and get tangled with each other!

Now, the amino acids in the egg probably taste better than the ones in the curdled milk... but that's another experiment...