Dental Biochemistry 2 – (Lec. 4)Connective tissue and muscle protein
Collagen• The major structural protein found in connective tissue is the collagen.• Collagen is a Greek word which means the substance to produce glue.• It is the most abundant protein in the body.• About 25-30% of the total weight of protein in the body is collagen.• It serves to hold together the cells in the tissues.• It is the major fibrous element of tissues like bone, teeth, tendons, cartilage and blood vessels.• When a solution of collagen is boiled, the viscosity of the solution decreases, which indicates that the native rod like structure is altered and a protein, with random coil structure results. It is then called gelatin.
Structure of Collagen• The tropocollagen is made up of three polypeptide chains.• There are 6 types of collagen, out of which type I is the most abundant form; it contains 2 chains of alpha-1 and one chain of alpha-2.• Each polypeptide chain of collagen has about 1000 amino acid residues.• The amino acid composition of collagen is quite unique.
• About 33% of the amino acids is glycine, that is, every third residue is glycine.• The repetitive amino acid sequence may be represented as Gly – X – Y – Gly – X - Y; where X and Y are other amino acids, most commonly proline, hydroxyproline and hydroxy lysine are found in fairly large proportions in collagen.• The hydroxylated amino acid residues are of special functional significance.
Synthesis of Collagen• The collagen is synthesized by fibroblasts intracellularly, as a large precursor, called procollagen.• It is then secreted.• The extracellular procollagen is cleaved by specific peptidases to form tropocollagen.
Hydroxylation of Proline and Lysine• The hydroxylation of proline and lysine residues of collagen is a post-translational modification taking place intracellularly.• Prolyl hydroxylase and lysyl hydroxylase enzymes contain ferrous iron at the active site and require a reducing agent like ascorbic acid.• So, vitamin C deficiency leads to poor hydroxylation.• It is the major biochemical defect in scurvy.
Triple Standed Helix• The collagen is a rod like structure.• Each of the 3 polypeptide chains is held in a helical conformation by winding around each other.• The resulting cable is made in a manner that 3.3 amino acid residues make one turn and each turn is separated by 2.9 A°.• The three strands are hydrogen bonded to each other.• Glycine, because of its small size can fit into.• For the same reason, glycine also produces a shallow groove into which other polypeptide strands are interwined.
Quarter Staggered Arrangement• The Tropocollagen molecules are arranged in a ‘quarter staggered array’ to form collagen fibers(Molecules in each row separated by 400 A°and adjacent rows by 680A°).• The structure repeats after fifth row.• Thus the collagen fiber has triple stranded, quarter staggered arrangement.• This arrangement helps in mineralization.
Cross Linked in Collagen Fibers• The collagen fibers are strengthened by covalent cross-links between lysine and hydroxy lysine residues.• The cross links are formed by lysyl oxidase.• It is a copper containing enzyme, the copper ion being located at its active site.• In copper deficiency, collagen synthesis is abnormal.• The older the collagen, the more the extent of cross linkages.• The process continues, especially in old age, so that the skin, blood vessels and other tissues become less elastic and more stiff, contributing a great extent to the medical problems of the old people.
Function of Collagen1. To give support to organs.2. To provide alignment of cells, so that cell anchoring is possible. This in turn, helps in proliferation and differentiation of cells.3. In blood vessels, if collagen is exposed, platelets adhere and thrombus formation is initiated.
Abnormalities in Collagen1. Osteogenesis imperfecta:• It is inherited as a dominant trait.• It is the result of a mutation which results in the replacement of a single glycine residue by cysteine.• This change disrupts the triple helix near the carboxy terminus, hence the polypeptide becomes excessively glycosylated and hydroxylated.• So, unfolding of the helix takes place and fibrillar array cannot be formed.• This results in brittle bones leading to multiple fractures and skeletal deformities.
2. Ehlers-Danlos syndrome:• It is due to defective collagen formation.• It is characterized by loose skin, hypermobile and lax joints.3. Deficiency of Ascorbic Acid:• It is characterized by defective hydroxylation of collagen.• The collagen formed is weak, leading to fragility of blood vessels, poor wound healing, bleeding gum, etc.
4. Copper deficiency:• Copper deficiency blocks the lysyl oxidase, resulting in reduced formation of cross linking.• The elastic nature of elastin fibers are due to these different cross links.
Elastin• Elastin is a protein found in connective tissue and is the major component of elastic fibers.• The elastic fibers can stretch and then resume their original length.• They have high tensile strength.• They are found in the ligaments as well as in the walls of the blood vessels, especially large vessels like aorta.
Keratins• Keratine are fibrous proteins present in hair, skin an nail, horn, hoof, etc.• They mainly have the alpha helical structure.• Each fibril has 3 polypeptide chains and each bundle has about 10-12 fibrils.• The matrix has cysteine-rich polypeptide chains which are held together by disulfide bonds.• The more the number of disulfide bonds, the harder the keratin is.
Muscle Proteins• Striated muscle is made up of multinucleated cells bound by plasma membrane called Sarcolemma.• Each muscle cell contains myofibrils about 1mm in diameter.• The functional unit of a myofibril is a sarcomere.• The dark A bands and light I bands alternate regularly.
• These bands are formed by variable combination of thick and thin filaments.• The thick filament is primarily myosin and thin filament contains actin, tropomyosin and troponin.• Thick and thin filaments slide past each other during the muscle contraction, so that the muscle shortens by as much as a third of its original length.• However the length of the thick and thin filaments do not change during muscle contraction.
Myosin• Myosin molecules are large (about 540 KD), each with 6 polypeptide chains.
Actin• It is the major protein of the thin filaments.• It is a monomeric protein often referred to as G- actin due to its globular shape.• It can polymerize into a fibrous form, called F-actin, which is a helix of actin monomer.• The muscle contraction results from interaction of actin and myosin, to form actomyosin, with energy provided by ATP.• When the two thin filaments that bind the cross bridges of a thick filament are drawn towards each other, the distance between Z lines becomes shorter.
• This could result in the process of contraction of muscle fibers.• This needs energy from hydrolysis of ATP, effected by the ATPase activity of myosin.• The contractile force is generated by conformational changes, leading to cyclic formation and dissociation of actin and S1 heads of myosin.• There is a reversible attachment and detachment of myosin S1 head to actin.• This is due to the hinge like movements between the domains of myosin.• The action of calcium is brought about by 2 proteins, troponin complex and tropomyosin located in the thin filament.
• The troponin complex has 3 different polypeptide chains.• Out of this, troponin-C (TnC) binds calcium.• Troponin-I (TnI), binds to actin and inhibits binding of actin to myosin.• Troponin I is a marker for myocardial infarction.• Its level in serum is increased within 4 hours of myocardial infarction.
Troponin-T (TnT) binds to tropomyosin.• Two isomers of cardiac TnT, called TnT1 and TnT2 are present in adult human cardiac tissue.• Serum levels of TnT2 increases within 4 hours of myocardial infarction, and remains high for up to 14 days.• The TnT2 is 100% sensitive index for myocardial infarction.• The reservoir of high energy phosphateis skeletal muscle is creatine phosphate.• The reaction (Lohman’s reaction). CK• Creatine phosphate + ADP -------→ ATP + Creatine
• During muscle contraction, the ATP level remains high as long as creatine phosphate is present.• But following contractile activity, the level of ADP and Pi rises.• The reduced energy charge of active muscle stimulates glycogen breakdown, glycolysis, TCA cycle and oxidative phosphorylation, so that energy is derived from aerobic metabolism.• Hence, only aerobic exercise is useful for weight control.