Proteins are large biological molecules, or macromolecules, consisting of one or more long chains of amino acid residues. Proteins perform a vast array of functions within living organisms, including catalyzing metabolic reactions, replicating DNA, responding to stimuli, and transporting molecules from one location to another. Proteins differ from one another primarily in their sequence of amino acids, which is dictated by the nucleotide sequence of their genes, and which usually results in folding of the protein into a specific three-dimensional structure that determines its activity. Proteins are also necessary in animals’ diets, since animals cannot synthesize all the amino acids they need and must obtain essential amino acids from food. Through the process of digestion, animals break down ingested protein into free amino acids that are then used in metabolism. Most proteins consist of linear polymers built from series of up to 20 different L-α-amino acids. Most microorganisms and plants can biosynthesize all 20 standard amino acids, while animals (including humans) must obtain some of the amino acids from the diet. The amino acids that an organism cannot synthesize on its own are referred to as essential amino acids. Key enzymes that synthesize certain amino acids are not present in animals — such as aspartokinase, which catalyzes the first step in the synthesis of lysine, methionine, and threonine from aspartate. If amino acids are present in the environment, microorganisms can conserve energy by taking up the amino acids from their surroundings and downregulating their biosynthetic pathways. In animals, amino acids are obtained through the consumption of foods containing protein. Ingested proteins are then broken down into amino acids through digestion, which typically involves denaturation of the protein through exposure to acid and hydrolysis by enzymes called proteases. Some ingested amino acids are used for protein biosynthesis, while others are converted to glucose through gluconeogenesis, or fed into the citric acid cycle. This use of protein as a fuel is particularly important under starvation conditions as it allows the body’s own proteins to be used to support life, particularly those found in muscle. Amino acids are also an important dietary source of nitrogen.
Protein-building amino acids
- Aspartic acid
- Glutamic acid
Essential amino acids
The biological importance of proteins is proven by the fact that they are involved in every cell process. Proteins are inevitable constituents of every living cell. Thanks to digestive enzymes, proteins which enter the body gradually break down into amino acids in the stomach and in the small intestine; these amino acids are already capable of absorption.With the help of an enzyme in the gastric acid, called pepsin, proteins fall apart. Milk protein is precipitated by another enzyme, called chymosin. The precipitated milk stays in the stomach for a while in order to be digested better. The gastric acid contains hydrochloric acid the task of which is to swell the proteins of nutriments and to initiate the function of the protein-degrading pepsin. The digestion of proteins continues in the duodenum. The duodenum’s own intestinal juice contains the protease, called erepsin; on the other hand, the protease, called trypsin, produced by the pancreas also goes into the duodenum. This is how digestion that started in the stomach continues in the duodenum; proteins break down into their smallest parts, amino acids.From the absorbed amino acids the body builds its own proteins in the liver during a short period of time again. This “protein building” is only possible if all the necessary amino acids are present at the same time. If any of the amino acids are less than necessary, the surplus amino acids cannot join anything and for this reason they degrade.
The major forms of protein supplements
These products usually contain a low level of cholesterol, fat, and carbohydrate, but compared to the other forms they have a higher biological value
Average protein content: 29-89%
By removing the fat and lactose, higher protein content can be attained besides a low biological value
Average protein content: above 90%
Predigested and partially hydrolyzed products whose price is generally higher, but they are easily digestible.
Average protein content: above 80%
For adults the protein requirement is 0.8-1.5 grams per kilogram of bodyweight. For athletes it is 1.6-2 grams per kilogram of bodyweight.Dietary supplements do not substitute a balanced diet, so it is not advisable to consume more than 50% of the daily requirements of protein supplements either.