Proteins are organic macromolecules. They provide us with energy (4 Calories/gram), but their main function in the body is to build structure such as muscle, tissue and bone. They are also the main components of enzymes, hormones, antibodies, storage and transportation organelles. Examples of high protein foods are meat, poultry, egg, dairy, and legumes.
The smallest unit of protein is an amino acid. The body uses 20 amino acids to make all the protein it needs. Nine of these amino acids are essential. That means, they cannot be made by the body and therefore must be eaten. The nine essential amino acids are phenylalanine, valine, tryptophan, threonine, isoleucine, methionine, histidine, leucine and lysine (PVT-TIM-HLL). Some amino acids are said to be conditionally essential. For example tyrosine is usually not essential, but it can be essential if the body cannot metabolize phenylalanine, used as a raw material for making tyrosine. This is what happens in the case of people with phenylketonuria disease.
The structure of an amino acid consists of an amino group, a carboxyl group, and a side (R) chain. The side chain varies, and determines the functional properties of the amino acid. For example they may be polar, non-polar, positively or negatively charged. Each of these characteristics will impart a different behavior of the protein.
Proteins are made in the body from instructions encoded in genes. In a process called transcription and translation, the DNA unwinds itself at a particular gene that encodes for a given protein. This allows the DNA to be copied to produce a strand of genetic material called messenger RNA (mRNA) which holds the code to make a specific protein. The mRNA is then processed and shipped to the cytoplasm of the cell where it attaches to a ribosome. The ribosome acts as a factory assembly point for amino acids that are subsequently stringed together according to the genetic code. Proteins may be configured into one of four different types of structures i.e., a primary structure consisting of a peptide chain of several amino acids connected at peptide bonds; a secondary structure caused by side-bonding of peptide double-strands to form flat beta sheets and spring-like alpha helices; tertiary structures caused by the overlapping of alpha and beta sheets into 3-dimensional structures; and quaternary structures caused by interaction of more than one secondary proteins.
Proteins may lose their structural integrity if the hydrogen bonds holding them together are broken. For example, adding heat, high shear, acids, alcohol, and salts to proteins can disrupt hydrogen bonds leading to a change in shape. We call this change denaturation. An example of denaturation is the hardening of egg when it is cooked, and the curdling of milk when we add acid. By understanding the basic structure of proteins we can understand their functional properties and how to apply them in food processing.
Can We Get Enough Protein Without Meat?
Certain amino acids may be missing or in small concentration in certain foods. We call these limiting amino acids. Foods that do not contain all the essential amino acids are referred to as incomplete. The most complete protein foods include egg, milk, fish, meat and poultry. By combining foods that have limiting amino acids with those that are complete, or combining two or more incomplete proteins, we can ensure we are getting all the essential amino acids. For example when you combine beans and rice you are able to get all the essential amino acids. That is because beans are high in lysine but limited in methionine and cysteine while grains such as rice are low in lysine but high in methionine and cysteine.
Since proteins vary in amino acid composition, all proteins do not have the same quality. The quality of protein can be expressed by its chemical score and its protein digestibility corrected amino acid score (PDCAAS). The chemical score is the comparison of a limiting amino acid in a food sample to the amount of the same amino acid in a reference sample. For example if the amino acid; say lysine is 45 mg in a sample and the reference contains 100 mg, then the chemical score would be (45 mg/100 mg) x 100 = 45%. The PDCAAS is calculated by multiplying the chemical score by the actual percentage of food that was digested. For example, if the chemical score is 45% and the percentage digestibility is 87%, then PDCAAS = 0.45 x 0.87 = 0.39.
Reference: Potter, N. N. & Hotchkiss, J. H. (1998). Food Science, 5th edition. New York, NY: Springer Science+Business Media LLC.