Proteins are the functional structures of life. Nearly all of the roughly 24,000 genes in each of your cells are blueprints for different proteins. Twenty amino acids are the building blocks from which all of these proteins are made. Amino acids join together chemically by forming peptide bonds. Peptides, polypeptides and proteins are terms used to describe amino acid strings of various lengths.
Small Proteins: Peptides
All amino acids have a cluster of atoms called an amino group at one end and another cluster called a carboxyl group at the other end. A variable cluster of atoms called the R group is attached to the center of the amino acid and differentiates the 20 amino acids. Amino acids join when the carboxyl group of one amino acid reacts with the amino group of another to form a peptide bond. Any string of two or more amino acids is called a peptide. Dipeptides, tripeptides and tetrapeptides consist of two, three and four amino acids, respectively. An oligopeptide is the general term for peptides that contain approximately 12 to 20 amino acids. Although there is not a firm cutoff, the term peptide typically refers to strings of less than roughly 30 amino acids.
Important human peptides include gastrin, which stimulates the release of stomach acid; enkephalins and endorphins, brain neurotransmitters that block pain; and oxytocin, the so-called "love hormone," which stimulates uterine contractions during labor, milk letdown to facilitate breastfeeding and pleasant feelings associated with close interpersonal relationships.
Medium Proteins: Polypeptides
Polypeptide is a term used by biochemists to describe medium-size proteins with an amino acid chain of roughly 30 amino acids or more. Protein is a nonspecific term that includes amino acid chains of any length; the term polypeptide refers to proteins of a particular size. The difference between these terms is similar to the difference between the words pants and jeans. You could refer to a pair of jeans as pants, and you would be correct. Using the word jeans instead of pants, however, communicates more information because you've specified a particular type of pants.
The pancreatic hormone insulin is a polypeptide, which helps your body use and store sugar. The hormone glucagon, which is also produced in your pancreas, stimulates the breakdown of stored sugar between meals.
Although peptides and polypeptides are proteins, biochemists and doctors usually use the word protein to refer to large protein molecules. Large proteins may consist of a single, long amino acid chain or several amino acid chains joined together. For example, antibodies produced by your immune system to fight infections consist of four amino acid chains. The protein hemoglobin in your red blood cells also contains four amino acid chains.
The number of amino acids in large proteins varies broadly, usually from roughly 100 to a few thousand. Some proteins, however, are much larger. The muscle protein titin consists of nearly 27,000 amino acids.
Importance of Diet
Your body requires a daily supply of amino acids from dietary proteins to maintain production of new proteins. The recommended daily allowance of protein is 56g for men and 46g for women; pregnancy and breastfeeding increase your daily protein needs. Dairy products, fish, lean meat, eggs, beans, nuts and seeds are good sources of dietary protein.