A protein is a large biological molecule (macromolecules) that consists of one or more chains of amino acid residues. For example, actin, casein_, collagen, cytokine, elastin, gelatin_, gluten_, hemoglobin_, keratin, lectin_, myoglobin, myosin, nebulin_, titin_, zein_.

Amino-acid sequences fold up into different shapes and can have different chemical properties. Proteins are typically hundreds or thousands of amino acids in length. The individual amino acids are connected with `covalent bonds`_, which hold them together tightly.

All proteins in all known species, including bacteria_ and eukaryotes, are constructed from the same set of 20 amino acids.

The simplest one is glycine, which has just a hydrogen atom as its side chain. With two hydrogen atoms bonded to the α-carbon atom, glycine is unique in being achiral. Alanine, the next simplest amino acid, has a methyl group (-CH3) as its side chain .


1   Etymology

1844, from French protéine, coined 1838 by Dutch chemist Gerhard Johan Mulder (1802-1880), perhaps on suggestion of Berzelius, from Greek proteios "the first quality," from protos "first" (see proto-) + -ine (2). This signifies its primary role in human nutrition.

Originally a theoretical substance thought to be essential to life, further studies of the substances he was working with overthrew this, but the words protein and proteid continued to be used in international work on the matter and also for other organic compounds; the modern use as a general name for a class of bodies arose in German. The confusion became so great a committee was set up in 1907 to sort out the nomenclature, which it did, giving protein its modern meaning and banishing proteid.

Many proteins end with the chemical suffix -in which indicates a neutral substance, antibiotic, vitamin, or hormone.

2   Function

Proteins acids have a number of crucial roles in the human body but most of them are structural (meaning the protein is used to build things). [6]_ Many hormones are made of protein. Your organs, muscles, skin and hair all contain protein. [6]_ Protein can also be used to produce energy in the body, usually by conversion to other nutrients (almost always glucose). [6]_

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.

3   Substance

Proteins consist of amino acids.

An amino acid is a molecule that has an amino group, a caroxylic acid group, and a characteristic side chain or residue each bound to a central carbon. Twenty amino acids are required for the production of the body's proteins. Most amino acids also have important non-protein functions. Nine min acid acids are considered essential since they are supplied by the diet, while the remaining amino acids are non-essential.

4   Classification

Some proteins are structural proteins, such as collagen. Other protetins are globular, such as enzymes_.

5   Examples

5.1   Myoglobin

Myoglobin is an iron- and oxygen-binding protein found in the muscle tissue of vertebrates in general and in almost all mammals. It is related to hemoglobin, which is the iron- and oxygen-binding protein in blood, specifically in the red blood cells. Myoglobin is only found in the bloodstream after muscle injury. It is an abnormal finding, and can be diagnostically relevant when found in blood. [2]

Myoglobin is the primary oxygen-carrying pigment of muscle tissues.[3] High concentrations of myoglobin in muscle cells allow organisms to hold their breath for a longer period of time. Diving mammals such as whales and seals have muscles with particularly high abundance of myoglobin.[2] Myoglobin is found in Type I muscle, Type II A and Type II B, but most texts consider myoglobin not to be found in smooth muscle.

5.2   Cytokine

Cytokine is a protein that induces inflammation in various tissues, including the brain. Cytokines are through to trigger cell death in the hippocampus.

6   Storage

Unlike carbohydrates and fats, there is no real storage form of protein. [1]

7   References

[1]Lyle McDonald. A Primer on Nutrition.

Proteins must first be broken down into amino acids (again by enzymes, called proteases). The rate at which that occurs depends on temperature, acidity, the type of protein, and what else is in your stomach. I would expect a protein in liquid form, as a sugar-free protein drink, on an empty stomach, should be broken down at a rate of perhaps 100mL every 5-10 minutes, so a 1L energy drink might reasonable be fully converted to amino acids and absorbed within an hour. The body can use those amino acids to build other proteins (anabolic metabolism), or they can be further broken down into alpha-keto acids (transamination) which can be used for energy, but that process can only occurs in the liver, so they must first be absorbed and transported there, and then the rate depends on their type (e.g., pyruvic acid vs. oxalo-acetic acid), the health of the liver, and what else is going on (e.g., alcohol metabolism). So in the absence of other energy sources, you might begin to extract usable energy from protein in perhaps 10-15 minutes, but that would be unusual.

If there's ready-to-use energy present, such as excess circulating glucose or liver glycogen, that will be used for energy before transamination, so for most people on a typical American diet, protein doesn't provide energy at all unless you are depleting your energy stores through hours of strenuous cardio; see "bonking" or glycogen depletion in the context of, e.g., marathon training.

Fats can also be turned into ketone bodies like alpha-keto acids, which is the basis of the ketogenic diet I mentioned elsewhere.

Proteins in blood and semen cause them to become flourescent with UV light.

Cows produce protein from grass.

Grass has a small percentage of crude protein. The nutrition comes from the microbes in their gut which are broken down by enzymes. Eat more grass, get more microbes, break em down. And on and on and on.

Exactly. Cattle are just using grass as a food for microbes.