Amino acid # 1: CH3-CH (NH2)-CO Alanine
Amino acid # 2: CH2 (N2) -COOH Glycine
Water is split out to form a -NH- link. Note the blinking atoms in the formulae. The new molecule still has a -NH2 group and a -COOH group, so the processes can continue. A protein may have several hundred amino acids linked this way. This color sketch may be helpful:
Click if you don't know the
notation for carboxyl groups
The red amino acid on Line 1 is alanine. The blue amino
acid is glycine. When they split out water to form a
dipeptide, either could be the donor for either a hydrogen or
a hydroxyl. There are 4 possibliities for the product:
glycyl glycine
alanyl alanine
glycyl alanine
and the one that is shown on Line 2, alanyl glycine.
If a chemist cooked up a mixture of alanine and glycine, the 4 possible products would form. Nature uses specific enzymes to make the one desired product.
Note that the dipeptide still has an amino group and a carboxyl group that can react further. The orange amino acid is serine; it has a functional hydroxyl group. If it splits out water with the dipeptide to form a tripeptide, one of the possible products is shown, alanyl glycyl serine.
Note the the tripeptide has a carboxyl group and an amino group that can react further. Note also the hydroxyl group. Proteins that are made up of amino acids with extra functional groups will have these in their structure. The nature and orientation of these groups determines the properties of the protein. For example, the non-polar portions of the protein will attract each other to form regions that do not associate well with water but associate very well with fats and oils. Some proteins are found in membranes that a rich in lipids, so the (non)polarity of the protein determines how it functions there. In other regions of a cell, the more polar regions may have groups that participate in acid/base equilibria and thus affect profoundly the charged nature of the protein.