Protein structures

continued

Proteins of ECM

Collagen of connective

tissue

Prepared by Iwona Kątnik-Prastowska

Topic: Structural fibrilar proteins of extracellular matrix and connective tissue

Fibrous proteins are distinguished from globular proteins by their filamentous or elongated form

Globular protein Fibrilar protein

Fibrous proteins play structural roles in animal cells

and tissues

They are the major proteins of: ECM skin,

connective tissue, animal fibers

like hair and silk

Fibrous proteins consist of particular kind, mainly nonpolar amino acids:

Collagens are structural fibrous proteins,

the most abundant in Vertebrates

Collagen makes the organic mass of skin, tendons, blood vessel, vitreous humor of the eye it is material in bone, and cartillage, is important constituent of dentine, Collagen fibers hold the animal tissue

together

Native collagen

is a relatively insoluble fibrous protein

Collagen Gelatin

Denaturation

The secondary structure of the collagen helix

is unique only to this protein

The helix is different from α-helix present in globular

proteins: it is left handed helical

twisted, and one polypeptide chain is

more expanded

Collagens form D-periodic fibrils

The basic unit of collagen is a tropocollagen: It consists of three helical polypeptide chains, each having over 1000 aa residues.

Mol.mass 300 000 Daltons.

Tropocollagen

The molecule of tropocollagen is stabilized by forming hydrogen bonds between chains Lenght: 300 nm

diameter: 1,4 nm

Two levels of the helical structure of tropocollagen fibers

Each polypeptide has a left handed helical sense, with 3,3 aa per turn and a total linear distance of 9.6 Å per turn.

Such three helical polypeptide chains are assembled into tropocollagen cables consisting of three intertwisted polypeptide chains, forming a superhelix with right handed sense. Each turn consists of 36 aa.

The structure is stabilized by hydrogen bonding formed among the polypeptide chains.

At each end of the rodlike peptide short segments of the peptides fold into globular domains which consist of 16 aa residues at N-termini and 25 aa at the C-termini

The most prominent amino acids of collagen are: Gly 33%, Pro-Hyp 22%, and Ala 11%.

Collagen contains hydroxylysine, which rarely, if ever, occurs in proteins.

The superhelix of the tropocollagen

The superhelices are arranged in the superhelix in a such way that the Gly are on the interior and in the contact with each other.

hydrogen bonding

Collagen is synthesized as a soluble precursor- procollagen, containing N-terminal and C-terminal propeptides. Cleavage by metallopeptidase initiate collagen fibrollogenensis

Procollagen

The immature form with terminal extensions crosslinked by S-S intracellular precursors. Three chains of procollagen are much longer than in mature proteins and have larger non-triple-helical ends

Proteolytic processing and modifications crosslinking

Self-assembly of fibrils

and covalent crosslinking of the fibrils

Collagens mature form

1.Translation on ribosomes 2.Hydroxylation of proline

(require Vit.C) and lysine residues; 3.Glycosylation to attach galactose and glucose 4. Formation of triple helix; 6. Removal of N- and C-terminal domains; 7. Oxidation and deamination of lysine residues to form aldehyde (allysine) and formation of crosslinks.

Collagen undergoes extensive posttranslational modification by:

Cross-linking at the ends of

polypeptide chains:

the following amino acids are known to form such

linkage: Lysine AlLysine Histidine 5-hydroxyllysine Such a collagen infrastructure provides a final product with tremendous amount of strength.

The cross-linking process continues through life, and the accumulating cross-links makes the collagen steadily less elastic and more brittle. As a result, bones and tendons in older individuals are more easy snapped, and the skin loses much of its elasticity.

Intensive cross-linking of polypeptide chains of collagen

have been associated with the aging process

High dose of some drugs such as penicillamine - a metal chelator,

will also inhibit cross-linking and cause the weakening of connective tissue.

There is a gap between one end of one unit and the start of another.

These gaps play an important role in bone and teeth formation.

The gaps in the quarter-stagger

arrangement are essential for the deposition of inorganic crystals of calcium hydroxyapatite Ca3 (PO4)3OH.

The gaps serve as the nucleation sites for

growth of these crystals. The combination of hydroxyapatite crystals and collagen creates a hard material that

still has some springiness.

Dentine, the main constituent of the internal part of a

tooth, contains a higher percentage (~75%)

of inorganic crystals, than does bone

and is therefore harder

TropoCollagen spontaneously assembles into fibrils,

easy undergo self-association

In the native collagen individual tropocollagens are assembled into

microfibrils,

Fibrillar collagens have ~50 known binding partners to generate the diversity of fibril patterns, which range from parallel bundles in tendon to othogonal lattices in cornea, and interlocking weaves in blood vessels, skin, and bone.

Collagen molecules might easily be sequestrated into

dead-end molecular interactions

Individual tropocollagens interact laterally, so are aligned in a linear head to tail fashion

Microfibrils of tropocollagen can associate into a fibril and several fibrils form the collagen fiber. „Staggered” arrangement

of tropocollagen:

Each molecule is ~300 nm long and overlaps its neighbour by about 64 nm, producing charactristic banded appearance of the fibers.

The collagen fibrils are of broad biomedical importance and have central roles in embryogenesis, tissue repair, fibrosis, arthritis, tumour invasion, and cardiovascular disease.

Collagens of different organs possess different types of polypeptide chains. They differ from each other on the basis of their amino acid compositions and other properties like carbohydrate content, types of chains, cross-linking. Collagen types are longer or shorten and aggregate in different ways. However, each containing the same motif of sequence Gly-Pro-Hyp

There are over than 30 types of collagens and collagen-related proteins

Collagen I consists of two chains of one kind and one of another while most of others collagens have 3 identical chains

Collagen type I is most abundant (accounts ~90%) in the human organism. It occurs in skin, tendon, and bone. It is synthesised by fibroblasts and is excreted into the ECM where it is polymerized into a durable long-lived material, form banded quarter-staggered fibrils

Collagen type II is found exclusively in cartilage and the vitreous humor of the eyes

Collagen form III is located in blood vessel and intestines and is prominent in embryonic tissues; form small banded fibrils

Collagen I- III forms fibrils with similar structures,

Collagen type V – abudant in interstitial tissues, form small fibrils, may contain an additional segment at the N-terminus

Collagen type IV is the major form of basement membranes

Collagen type XII with interrupted triple helices, with fibril assocated is present in embryonic tendon, periodental ligaments

Collagen type XIV with fibril associated occurs in fetal skin and tendon

Types XII, XIV, IX, and XVI contain interruptions in the helix which create bends, flexible sites, and sites of increased proteolytic susceptibility. They may link the fibrils to other components of the surrounding matrix.

Extensive collagen degradation

is seen in Paget’s disease,

rheumatoid arthritis, peridontal disease.

In advanced disease Hyp is often excreted

in large amounts into the urine.

The other examples of fibrous proteins Elastin, Fibroin, Keratins, Myosin and actin

Elastin is present in arterial blood

vessels, lungs, skin, nuchal ligament and form highly elastic fibers:

Is synthesised as tropoelastin and secreted into EC space where it form a rubber-like network

basic unit of elastin fiber, is a soluble protein (~65 kDa) which is primarily build from glycine, lysine and desmosine, valine and modified alanine and proline residues. Tropoelastin is highly cross-linked to form an insoluble complex.

Tropoelastin

The most common interchain cross-link in elastins is the result of the conversion of the amine groups of lysine to reactive aldehydes by lysyl oxidase. This results in the spontaneous formation of desmosine cross-links with lysine.

Desmosine

Fibroin is β sheet fibrous protein, with a polypeptide chain running parallel

to the fiber axis.

Fibroin is produced by silkworms and spiders.

Fibroin is β sheet fibrous protein. Almost half of its residues are glycines.

The α keratins

are predominantly α helical in structure

The α keratins are the major proteins of hair and fingernails and compose a major fraction of animal skin.

Keratin filaments are abundant in keratinocytes and in epithelial cells

Before you start laboratory classes You should be familiar with given lectures: Describe the primary, secondary , tertiary and quartenary structures of proteins,

Explain differences between globular and fibrillar proteins, Characterize modules, segments, repeats, domains of proteins. Give some examples, Characterize the structure of mosaic proteins. Give an example, List the components of extracellular matrix, Give some examples of globular and fibrillar proteins, Describe the structure of collagens and elastin.