Proteins and Amino Acids: Structure, Function
Proteins
Concept: Proteins are defined as polymers formed by the union, through peptide bonds, of smaller units called amino acids. Proteins are macromolecules and are the most abundant compounds in living matter. They are specific to organisms as they express genetic information.
Functions: Proteins perform numerous vital functions, including:
- Transport
- Storage
- Movement
- Hormonal signaling
- Immunological defense
- Enzymatic catalysis
- Homeostatic regulation
- Structural support
Amino Acids
Amino acids are the structural units of proteins. All amino acids found in proteins, except for proline, have two characteristic functional groups: the carboxyl group or acid group (-COOH) and the amino group (-NH2).
The carbon chain of amino acids is numbered beginning with the carboxyl group. The carbon bearing this function is numbered 1, and the amino group is always at carbon 2 (alpha carbon). Therefore, amino acids have carbons 1 (carboxyl group) and 2 (amino group) in common, diverging in the remaining part (R) of the molecule. In the general formula, R represents the remainder of the molecule. R can be anything from a simple H, as in the amino acid glycine, to a more or less complex carbon chain which may contain other amino or carboxyl groups and other functional groups (alcohol, thiol, etc.).
The proteins of living beings are made from only about 20 different amino acids, so there are only 20 different residues commonly found in proteins. Most amino acids can be synthesized by the organism, but others, known as essential amino acids, cannot be synthesized and must be obtained through the diet. Essential amino acids are different for each species. In humans, for example, there are ten essential amino acids:
- Thr (Threonine)
- Lys (Lysine)
- Arg (Arginine)
- His (Histidine)
- Val (Valine)
- Leu (Leucine)
- Ile (Isoleucine)
- Met (Methionine)
- Phe (Phenylalanine)
- Trp (Tryptophan)
Classification of Amino Acids
Amino acids can be classified based on the properties of their R group:
Apolar Amino Acids
Amino acids whose remainder R is nonpolar. That is, it lacks significant electric charges in R and often contains long hydrocarbon chains. These amino acids, if abundant in a protein, tend to make it insoluble in water.
Polar Acidic Amino Acids
In this group are those amino acids that have more than one carboxyl group. In proteins, whether at basic or neutral pH, these groups are negatively charged.
Polar Basic Amino Acids
These are amino acids that have one or more additional amino groups. In proteins, these amino groups, if the pH is acidic or neutral, are positively charged.
The Peptide Bond
When the carboxyl group of one amino acid reacts with the amino group of another, the two amino acids are linked by a peptide bond. This is a condensation reaction that forms an amide bond and releases a water molecule. The substance that results from this union is a dipeptide.