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Protein classification based on chemical composition
On the basis of their chemical composition, proteins may be divided into two classes: simple and complex.
Simple proteins
Also known as homoproteins, they are made up of only amino acids.
Examples are plasma albumin, collagen, and keratin.
Conjugated proteins
Sometimes also called heteroproteins, they contain in their structure a non-protein portion.
Three examples are glycoproteins, chromoproteins, and phosphoproteins.
glycophorin, the best known among erythrocyte membrane glycoproteins;
fibronectin, that anchors cells to the extracellular matrix through interactions on one side with collagen or other fibrous proteins, while on the other side with cell membranes;
all blood plasma proteins, except albumin;
immunoglobulins or antibodies.
hemoglobin and myoglobin, which bind, respectively, one and four heme groups;
chlorophylls, which bind a porphyrin ring with a magnesium atom at its centre;
rhodopsins, which bind retinal.
Protein classification based on shape
On the basis of their shape, proteins may be divided into two classes: fibrous and globular.
Fibrous proteins
They have primarily mechanical and structural functions, providing support to the cells as well as the whole organism.
These proteins are insoluble in water as they contain, both internally and on their surface, many hydrophobic amino acids. The presence on their surface of hydrophobic amino acids facilitates their packaging into very complex supramolecular structures.
In this regard, it should be noted that their polypeptide chains form long filaments or sheets, where in most cases only one type of secondary structure, that repeats itself, is found.
In vertebrates, these proteins provide external protection, support and shape; in fact, thanks to their structural properties, they ensure flexibility and/or strength.
Some fibrous proteins, such as ?-keratins, are only partially hydrolyzed in the intestine.
Here are some examples.
Globular proteins
Most of the proteins belong to this class.
They have a compact and more or less spherical structure, more complex than fibrous proteins. In this regard, motifs, domains, tertiary and quaternary structures are found, in addition to the secondary structures.
They are generally soluble in water but can also be found inserted into biological membranes (transmembrane proteins), thus in a hydrophobic environment.
Unlike fibrous proteins, that have structural and mechanical functions, they act as:
Examples of globular proteins are myoglobin, hemoglobin, and cytochrome c.
At the intestinal level, most of the globular proteins of animal origin are hydrolyzed almost entirely to amino acids.
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Protein classification based on biological functions
The multitude of functions that proteins perform is the consequence of both the folding of the polypeptide chain, therefore of their three-dimensional structure, and the presence of many different functional groups in the amino acid side chains, such as thiols, alcohols, thioethers, carboxamides, carboxylic acids and different basic groups.
From the functional point of view, they may be divided into several groups.
hemoglobin, that carries oxygen from the alveolar blood vessels to tissue capillaries;
transferrin, which carries iron in the blood;
membrane carriers;
fatty acid binding proteins (FABP), that is, the proteins involved in the intracellular transport of fatty acids;
proteins of plasma lipoproteins, macromolecular complexes of proteins and lipids responsible for the transport of triglycerides, which are otherwise insoluble in water;
albumin, that carries free fatty acids, bilirubin, thyroid hormones, and certain medications such as aspirin and penicillin, in the blood.
Many of these proteins also play a protective role, since the bound molecules, such as fatty acids, may be harmful for the organism when present in free form.
ferritin, that stores iron intracellularly in a non-toxic form;
milk caseins, that act as a reserve of amino acids for the milk;
egg yolk phosvitin, that contains high amounts of phosphorus;
prolamins and glutelins, the storage proteins of cereals.
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Collagen:
Collagen is a hard, insoluble, and fibrous protein that makes up one-third of the protein in the human body.
In most collagens, the molecules are packed together to form long, thin fibrils.
These act as supporting structures and anchor cells to each other.
They give the skin strength and elasticity.
Collagen is secreted by various cells, but mainly by connective tissue cells.
It is found in the extracellular matrix. This is an intricate network of macromolecules that determines the physical properties of body tissues.
There are at least 16 different types of collagen, but 80 to 90 percent of them belong to types 1, 2, and 3. These different types have different structures and functions.
The collagens in the human body are strong and flexible.
Type 1 collagen fibrils are particularly capable of being stretched.
Collagen is found in the bones, muscles, skin, and tendons.
Collagen holds the body together. Collagen forms a scaffold to provide strength and structure.
Functions :
Skin
One of the most noticeable functions of collagen is the support it provides for skin. collagen makes up approximately 80 percent of the dry weight of skin. Collagen provides structure to the skin, and works hand in hand with another protein, called elastin, to allow skin the flexibility it needs to stretch and return to its original state as body moves.
Bone
Bone is made up of a mixture of collagen and a mineral called hydroxyapatite. The two work together to form the structure, flexibility and strength of bones.
Muscles
Collagen fibers provide muscles with the strength and structure needed to move and function throughout the day. Collagen fibers not only make up skeletal muscle fibers, but they also make up smooth muscles (such as those in the bladder and reproductive system) and cardiac (heart) muscles.
Blood Vessels
Collagen makes up the walls of the veins, arteries and capillaries of the body. This gives the vessels strength, structure and flexibility, all of which are needed to effectively transport blood throughout your entire body.
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https://www.livestrong.com/article/358255-what-fruits-contain-collagen/
https://www.medicalnewstoday.com/articles/262881.php