What amino acids make up collagen?
Introduction
Collagen is a fibrous protein that forms the connective tissue, and in mammals and birds it constitutes a very important proportion of total proteins, of the order of a quarter of the total, which makes it the majority.
There are several types of collagen, designated by Roman numerals. In type I collagen, the most abundant, the constituent structural unit is the tropocollagen, a protein of around 300,000 molecular weight, made up of three chains of the same size, two of them identical, called 1, and another slightly different, the a 2. The three chains are joined together by hydrogen bonds between the amino and carboxyl groups of the glycine residues, and by hydrogen bonds with the hydroxyproline side chains, forming a triple helix, peculiar structure of the Collagen This propeller only breaks at the ends.
Collagen type III is formed by three identical chains to 1, and has the peculiarity that at the carboxyl terminal end the three chains are not grouped in helical form, but joined together by disulfide bridges. This type of collagen, located in the perimyo and endomysium of the muscle, seems to be especially important in terms of conferring hardness on the meat.
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The tropocollagen units, which have a length of about 2,800 amstrongs, are grouped in parallel, joined together to form the collagen fibers. Within the fibers, the units are displaced approximately a quarter of their length with respect to the adjacent ones, and each unit is about 400 amstrongs from the next one.
The amino acid composition of collagen is quite particular. Glycine represents, in moles, approximately 1/3 of the amino acids present. They also contain very high amounts of proline, and of hydroxyproline (up to 10%), and it is one of the few proteins that contains hydroxylysine. The presence of hydroxyproline is often used as an analytical criterion to evaluate the amount of collagen (connective tissue) present in minced meat products.
Both hydroxylysine and hydroxyproline are produced after the synthesis of the polypeptide chain, by modification of the non-hydroxylated amino acids by specific enzymes.
The primary sequence of collagen chains is also very peculiar, since, except at the ends of the chain, glycine is distributed regularly, occupying the position of one in three minoacids.
The peculiarities of the collagen sequence make possible its peculiar secondary structure. The small size of the glycine allows three chains to cross each other, while the rigidity produced by the proline ring favors the angle of rotation they adopt.
As the age of the animal increases, collagen has a higher number of crosslinks by covalent bonds between the chains. These bridges are formed by the initial action of the enzyme lysine oxidase, which transforms aldehydes into lysine or hydroxylysine, aldehydes that can later be condensed by spontaneous chemical reactions with other groups.
The heating of the collagen produces its denaturation, whose first consequence is an enormous shortening of the fibers, shortening that occurs at a temperature characteristic for the collagen of each animal species. The triple helical structure is also destroyed. The temperature at which the collagen is denatured depends on the content of proline and hydroxyproline: The higher the content, the higher the temperature required. In this sense, there is a great difference in proline content and in the temperature of denaturation between the collagen of warm-blooded animals and that of fish, both much smaller in the latter. In some cold-water fish, the collagen is denatured at less than 20 ° C. In cooking foods, part of the collagen is solubilized and forms the "gelatin", which imparts viscosity to the stews.
Collagen is an abundant component in skin, tendons, vascular system and other waste materials, from which commercial gelatin can be obtained, which is a product of partial degradation of collagen, extracted by heating after treatment in an acidic or alkaline medium. Depending on the type of treatment, a different type of gelatin is obtained. Gelatine is a very useful material in food technology, to obtain thermally reversible gels, with a very low "fusion" point. These gels are formed by cooling by the binding of chains by partial reconstruction of helices of the collagen type, but with large disorganized zones.
From a nutritional point of view, collagen and even gelatin are very unbalanced proteins in terms of their amino acid composition. Collagen is very deficient in tryptophan, and gelatine practically lacks it, since the little that existed is usually destroyed in its preparation. The ease with which it protects itself depends a lot on its state. Native collagen is quite resistant to proteoliis by most proteinases, while denatured collagen is easily hydrolyzed.