Prosthetic group lipoprotein

Proteins with molecular weights in the millions are the major constituents of all living cells. Simple proteins are hydrolyzed only to amino acids. Coqjugated proteins are hydrolyzed to amino acids and nonpeptide substances known as prosthetic groups. These prosthetic groups include nucleic acids of nucieoproteins, carbohydrates of glycoproteins, pigments (such as hemin and chiorophyli) of chromoproteins, and fats or lipids of lipoproteins. 

Some proteins also contain other compounds, apart from amino acids. These proteins are known as conjugated proteins and the non-amino acid part is known as a prosthetic group (Chap. 8) the protein part is termed the apoprotein. Glycoproteins (Chap. 6) and proteoglycans (Chap. 5) contain covalently bound carbohydrate, while lipoproteins (Chap. 6) contain lipid as the prosthetic groups. 

In the work of Loomeijer and work on similar lines carried out in the author s laboratory no evidence was found for the presence in important amounts of the common fatty esters, phospholipids, or steroids in purified preparations of elastin in these circumstances there seems to be little reason to classify elastin with the lipoproteins. The uptake of lipophilic dyes, which has been mentioned by several authors as indicating a lipoprotein character, can as well be explained by the presence of massive nonpolar radicals in the fluorescent prosthetic group, as described by Loomeijer, or to interactions with lipophilic sections of the peptide chain due to the large concentrations of nonpolar amino acids such as valine, the leucines, and phenylalanine. 

For example, globin (apoprotein) + heme (a prosthetic group that contains iron in a porphyrin ring) = hemoglobin. In a slightly different usage, apoprotein in association" with lipid = lipoprotein vehicles that transfer cholesterol and triglycerides in the blood stream. 

In addition to pure proteins, there are still several classes of what are known as conjugated proteins, that is, compounds of proteins with non-o-amino acids, i.e., with what are called prosthetic groups. Chromoproteins are such compounds of proteins with metals. Glycoproteins contain less than 4%, but mucoproteins more than 4%, polysaccharides. Compounds of proteins with lipids are called lipoproteins, those with nucleic acids nucleoproteins, and those with flavine derivatives flavoproteins. Hemoproteins are compounds of proteins and iron porphyrines. 

Conjugated proteins contain, in addition to amino acids, a non-protein moiety termed a prosthetic group. Some important examples of conjugated proteins are glycoproteins, lipoproteins, phosphoproteins and chromoproteins. 

The presence of non-protein prosthetic groups as in glycoproteins, phosphoproteins, lipoproteins, chromoproteins and nucleoproteins. 

Lastly, the complex proteins have been gathered into a separate classification. They are composed of a protein part and an additional, nonprotein, prosthetic group. It is rather difficult to maintain the distinction between proteins which adsorb metals and carbohydrates or incorporate them in small quantities, and proteins with a definite metal or carbohydrate component. The usual division of complex proteins comprises the following (1) metalloproteins, (2) phosphopro-teins, (3) lipoproteins, (4) nucleoproteins, (5) glycoproteins, and (6) chromoproteins. 

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