Phosphoproteins

It is compound peptides without prosthetic group. The role of prosthetic group plays residues of phosphoric acid connected by ester bounds of hydroxiaminoacids: serine, threonine, tyrosine. Some fragments of phosphoric acid could be connected to one residue amino acid (F).

Phosphoproteins are value proteins with a great molecular mass. They are not coagulated by higher temperature (that they are termostable), they are not not soluble in the water, but soluble in solution of light salts and alkalis. They have acid nature (4,7) because residues of amino acids.

They are used as a plastic material and played an important role in growth. Therefore phosphoproteins need for children. The specimens:

1) milk caseinogen (P ~ 1%),

2) vitellin and phosphovitin of yolk of eggs (P~ 10%),

3) ihtullin of caviar (P~ more than 10%).

Caseinogen is the most important protein of milk which amounted about 80% from all milk peptides. Caseinogen are used in production of cheese for turning of milk.

Chromoproteins are coloured proteins (greek word chroma - paint). Molecules consist of protein and prosthetic group. They are colored because metal or vitamin which connected with protein. Amount of chromoproteins have proteins and respiratory enzymes, which compound the group of hemoproteins. Pigments (rodopsin, melanin), magnesium-porphirins (chlorophyll), yellow enzymes - flavin’s enzymes ( respiratory ferments are included in this group of complex proteins.

One chomoprotein which has the most important role for organism is hemoglobin. Hemoglobin is tetrameric protein composed of pairs of two different polypeptides (termed α, β, γ and δ, S, etc.). While similar in overall length, the α (141 -residue) and β (146 resudue) polypeptides of hemoglobin A (HbA) are encoded be different genes and have different primary structures. The tetrameric structures of common hemoglobins are follows: HbA (normal adult hemoglobin)= α₂β₂, HbF (fetal hemoglobin) = α₂γ₂, HbS (sickle cell hemoglobin) = α₂S₂, and HbA₂ (a minor adult hemoglobin )= α₂δ₂.

Hemoglobins of vertebrate erythrocytes perform two major transport functions:1) transport of O₂ from the respiratory organ to peripheral tissues and 2) transport of CO₂ and protons from peripheral tissues to the respiratory organ.

Heme consists of pyrrole rings and methylene bridge carbons are coplanar, and their iron atom (Fe²⁺) resides in almost the same plane. The fifth and sixth coordination positions of Fe²⁺ are directed perpendicular to - and directly above and below - the plane of the heme ring. Observe the nature of the substituent groups on the β carbons of the pyrrole rings, the central iron atom, and the location of the polar side of the heme ring (at about 7 o’clock) that faces the structure of myoglobin molecule.

Myoglobin - red muscle tissue myoglobin stores oxygen that under conditions of oxygen deprivation (e.g., serve exercise) is released for use by muscle mitochondria for oxygen-dependent synthesis of ATP.

Myoglobin is a single polypeptide chain of molecular weight 17000. Clear different, however, are apparent in their spatial distribution. The surface is polar and the interior nonpolar, a pattern characteristic of globular proteins. Residues with both polar and nonpolar regions (eg Thr, Trp, Tyr) orient their nonpolar regions inward. Apart from two His of myoglobin contains only nonpolar residues (eg, Leu, Val, Phe, Met).

Metalloproteins are compound proteins. Specimens:

Transferrin is a β₁-globulin with a molecular mass of approximately 76 kDa. It is a glycoprotein and is synthesized in the liver. About 20 polymorphic forms of transferrin have been found. Trasferrin plays a central role in the body’s metabolism of iron because it transports iron (2 mol of Fe²⁺ per mole of transferrin ) in the circulation to sites where iron is required from the gut to the bone marrow and other organs. Iron is important in the human body because of its occurrence in many hemoproteins, such as hemoglobin, myoglobin, and the cytochromes. Iron is ingested in the diet, and its absorption as Fe²⁺ is tightly regulated at the level of the intestinal mucosa. Under normal circumstances a healthy adult male loses about 1 mg/d, which is replaced by absorption. Adult females are more prone to states of iron deficiency because some may lose excessive blood during menstruation. Approximately 200 billion red blood cells (about 20 ml) are catabolized per day, releasing about 25 mg of iron into the body. Free iron is toxic, but association with transferrin diminishes its potential toxicity and also directs iron to where it is required in the body.

Ferritin is another protein that is important in the metabolism of iron. Under normal conditions, it stores iron that can be called upon for use as conditional require. In conditions of excess iron (e.g., hemochromatosis), body stores iron in liver and spleen as ferritin. Ferritin contains approximately 23% of iron, and apoferritin (the protein moiety of free of iron) has a molecular mass of approximately 440 kDa. Ferritin is composed by 24 subunits of 18/5 kDa, which surround in a micellar form some 3000-4500 iron atoms. Normally , there is a little ferritin in human plasma. However, in patients with excess iron, the amount of ferritin in plasma is markedly elevated. The amount of ferritin in plasma can be conveniently measured by a sensitive and specific radioimmunoassay and serves as an index of body iron stores.

Hemosiderin is a somewhat ill-defined molecule; it appears to be a partly degraded form of ferritin but still containing iron. It can be detected by histologic stains (eg, Prussian blue) for iron, and its presence is determined histologycally when excessive storage of iron occurs.

Primary hemochromatosis is a common genetic disorder characterized by excessive storage of iron in tissues, leading to tissue damage. Secondary hemochromatosis can occur after repeated transfusions, excessive oral intake of iron.

Ceruloplasmin (about 160 kDa) is an α₂-globuline. It has a blue color because of its high copper content and carries 90% of the copper present in plasma. Each molecule of ceruloplasmin binds six atoms of copper very tightly, so that the copper is not readily exchangeable. Albumin carries the other 10% of the plasma copper but binds the metal less tightly than ceruloplasmin. The amount of ceruloplasmin in plasma is decreased in liver disease. In particular, low levels of ceruloplasmin is found in Wilson disease (hepatolenticular degeneration), a disease due to abnormal metabolisms of copper.

Lipoproteinsare multicomponents complex of proteins and lipids that form distinct molecular aggregates. Each type of lipoprotein has a characteristic molecular mass, size, composition, density, and physiological role. The protein and lipid in each complex are held together by noncovalent forces.

Plasma lipoproteins have been extensively characterized. Their roles in blood include transport of lipids from tissue to tissue and participating in lipid metabolism. Four classes of plasma lipoproteins exist in normal fasting humans: in the postabsorptive period a fifth type, chylomicrons, is also present (Tab.2) The classes are identified by their density, which is determined by ultracentrifugation and by electrophoresis. The protein components of a lipoprotein particle are the apolipoproteins. Each type of lipoprotein has a characteristic apolipoprotein composition, the different apolipoproteins often being present in set ratio. The most prominent apolipoproteins in high density lipoproteins (HDLs) is apolipoprotein A-1 (apo A-1). Low density lipoproteins (LDLs) contain apoB, which is also present in intermediate density lipoproteins (IDLs)and very low density lipoproteins (VLDLs).

<== попередня сторінка	\|	наступна сторінка ==>
	\|	Table 2 Composition of the free (transport) lipoproteins in plasma of human

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