Explanation of the structure and function of amino acids and proteins in the human body, as well as where protein is found in the diet.
PROTEINS
Amino Acids
The human body produces over 100 000 different proteins. Amino acids are the monomers that are the building blocks of the thousands of different types of proteins. Amino acids that are incorporated into proteins are called proteinogenic. There are 20 different amino acids. These amino acids differ only by their functional group, and they can be arranged in multiple ways to produce proteins (Figure 2.6). Every cell in the body can build the correct proteins in the correct amount throughout our lives, and different cells can produce different proteins. For example, after we eat and blood sugar levels increase, the pancreas beta islet cells are the only cells that synthesize the protein hormone insulin (Chapters 17 and 20). Whereas every cell in the body produces enzyme proteins involved in the production of ATP, every protein is formed by very specific amino acid sequences. A typical protein contains anywhere from 100 to 1000 amino acids. The order of amino acids in a protein is critical for the proper folding of the protein, which is based on interactions between the functional groups. The protein can function only if it contains the correct amino acids and is folded into the correct conformational structure. For example, a single amino acid change in a chloride-channel protein causes improper folding, with the result that the membrane protein cannot function, and this causes the disease cystic fibrosis. However, not all changes in the amino acid sequence will cause disease.
An individual amino acid has a carboxyl group, an amino group, and a functional group. The functional group is what differs among all 20 amino acids and gives each amino acid its distinctive characteristics (Figure 2.7). The covalent bond that links the amino acids is called a peptide bond. When many amino acids are covalently bonded by peptide bonds, a polypeptide—a protein—is formed.
Did You Know?
People with curly hair have more cysteine amino acids in their hair proteins. Cysteine amino acids have a sulfur atom that can form disulfide bonds with other cysteine molecules; this causes kinks in the proteins and, therefore, curly hair.
Essential Amino Acids
The human body can synthesize 11 of the 20 amino acids from other molecules. The remaining nine, which are called essential amino acids, cannot be synthesized by the human body and need to be acquired from our diet: phenylalanine, valine, threonine, tryptophan, isoleucine, methionine, leucine, histidine, and lysine. The most common food sources that contain all amino acids are meat, fish, eggs, chicken, soy products, quinoa, hemp seeds, and dairy products. Other foods contain some, but not all, of the essential amino acids. See Table 2.1.
Did You Know?
Vegetarians can become deficient in essential amino acids if they do not eat adequate combinations of plant protein sources. Legumes, beans, nuts, seeds, and grains each contain some of the essential amino acids, which is why it is important for vegetarians to eat these food groups in combinations. Amino acid deficiencies can cause numerous symptoms, including chronic fatigue, hair loss, irritability, and reproductive problems.
Protein Structure
The sequence of amino acids in a protein is called the primary structure (Figure 2.8). For example, the formation of the protein insulin requires the correct amino acids in the correct order, and the protein has to fold into the correct shape so that it will bind specifically to insulin receptors. The body’s cells know the correct order of amino acids because of the DNA sequences called genes.
As amino acids become arranged in the correct order on the ribosomes in each cell, they take on a secondary structure; this is based on the interactions of the functional groups of the individual amino acids (Figure 2.8e). Hydrogen bonds form between polar amino acids, just like hydrogen bonds form between water molecules. Hydrophobic amino acids are attracted to other hydrophobic amino acids, and the secondary structure develops. Most of the secondary structures formed are called alpha helices or beta sheets.
The protein takes on a three-dimensional shape based on the primary amino acid sequence and the secondary structures that are formed. Eventually, the protein forms a three-dimensional tertiary structure, which constitutes the functional structure of the protein. To function properly, every distinctive protein has to have not only the correct amino acid sequence but also the correct tertiary structure, which is determined by the interactions between functional groups. Nonpolar amino acids are attracted to one another, polar or charged amino acids are attracted to one another, and sulfur groups will form disulfide bonds. Sometimes multiple tertiary structures combine to form a final functional protein product, called a quaternary structure. An example of a quaternary protein in our body is hemoglobin, where four protein subunits combine to form the final functional macromolecule.
Denaturation
When you cook an egg, the increased temperature causes the egg proteins to unfold; this explains why the egg changes from a semi-liquid to a semi-solid state. The increased temperature adds enough energy to break the weak hydrogen bonds that form the secondary and tertiary protein structures, but the covalent peptide bonds remain intact. A change in the pH can also affect the bonds that hold proteins in their conformational shape: for example, the acidic environment of the stomach. This process of breaking the interactions between functional groups in a protein is called denaturation. Denaturation is an irreversible reaction—for example, cooling a cooked egg does not change it back to its original state—and denatured proteins no longer function (Figure 2.9) because their folded structure is critical for protein function. However, denaturation does not change the structure of individual amino acids. When we eat proteins, digestive enzymes break the proteins into the amino acids that we absorb, so cooking food or digesting it in our acidic stomach does not affect the structure of the amino acids.
Did You Know?
Having an extremely high fever is fatal because cellular proteins can start to denature. Without the thousands of proteins doing their normal cellular functions, death could occur quite rapidly.
Enzymes
Enzymes are very important globular proteins that facilitate thousands of chemical reactions in our body’s cells. In humans, more than 75 000 different enzymes are produced in various types of cells and perform a myriad of essential functions. Enzymes have grooves that are specific tertiary structures that form active sites whereby an enzyme can catalyze, or speed, the rate of chemical reactions. Enzymes increase the contact between specific regions of molecules so that a particular chemical reaction can occur: for example, bringing amino acids in close proximity so that dehydration synthesis can occur (Figure 2.10).
Did You Know?
Enzymes require cofactors or coenzymes in order to function properly. Cofactors are most often minerals such as zinc, magnesium, manganese, copper, and iron. Coenzymes are vitamins such as the B vitamins. It is important to eat a variety of foods that contain vitamins and minerals, such as vegetables, whole grains, nuts, seeds, liver, eggs, and fish so that cellular enzymes can function properly.
Amino Acids That Have Other Roles
Amino acids are not always incorporated into proteins, in which case they are referred to as non-proteinogenic amino acids. Non-proteinogenic amino acids have a variety of important functions, such as the production of neurotransmitters. See Table 2.2.
