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Like the complex carbohydrates, proteins (or polypeptides) are large molecules made up of smaller subunits which have been bonded together. In this case, amino acids joined by peptide bonds.

Proteins are crucial for cells. Some are major structural components of cells, others serve as enzymes. Without enzymes to speed up the chemical reactions inside our cells, the reactions which keep us alive would simply stop.

One interesting feature of proteins is that their 3-dimensional shape plays a huge role in how (and if) they function. Many enzymes change shape while they work. However, when they are through reacting they resume their original shape. When the shape of a protein is permanently changed, it generally ceases to function as it should, and we say that it has been denatured.

Denaturing Proteins

Much of the 3-D shape of a protein relies upon hydrogen bonds, which are relatively weak and easily broken. Within your cells, the temperature and pH are generally stable (around 37' C and a pH of 7). Your proteins are adapted to work best under these conditions. If the temperature gets too high, or the pH deviates too far from neutral, these proteins may denature.

For example, when you fry an egg, the
heat denatures the egg proteins, and they change from being liquid to being solid. The chemical composition of the proteins hasn't changed, but some of their hydrogen bonds have been shifted around, resulting in the change in texture.

In lab, we mixed acid with milk and saw the following results:

The tube on the left is pure milk. The tube on the right has had acid added to it. Note that the milk has curdled in the tube with the acid. The proteins in the milk were denatured by the acid, which made them go from being water soluble to being insoluble in water. This change caused the now solid proteins to precipitate out of the milk, forming the white curds which can be seen clinging to the glass of the tube.

Cheese-making takes advantage of the acid naturally produced by certain bacteria which grow in milk. They produce lactic acid as a byproduct of their growth, and the acid causes the milk to curdle. The solid chunks of protein can then be skimmed out and pressed into blocks. The type of cheese made depends on how long the curds are allowed to age and how much moisture is pressed out of them, among other things.

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