Bone proteins mainly consist of collagen and other non-collagen proteins.
Collagen is the most abundant protein and is found all over the body in bones, tendons, and ligaments.
It is important to mention about bone morphogenic proteins in passing. Bone morphogenic proteins are different entities and not structural bone protein.
Bone morphogenetic proteins or popularly called BMPs are named so because they were originally found to induce bone formation, but they are more widespread and perform many other functions in the cartilage, muscle, kidney, and blood vessels.
These are members of the transforming growth factor.
In the following text, we discuss structural bone proteins.
There are over 11 types of collagen proteins in the body.
Bone mainly has type 1 which is also present in skin and ligaments. The synovial membrane is rich in type II collagen.
Collagen is the major constituent of the bone matrix. Collagen is also the major extracellular protein of the body and comprises 30 percent of today’s body protein.
The collagen molecule is first synthesized as a precursor, procollagen, which consists of three pro-alfa chains.
A normal ligament consists of about 90 percent fibrillar types I collagen with less than 10 percent being type III collagen. Other collagen types are present in smaller quantities.
The most prevalent collagen in the growth plate is type II.
Although type X collagen is thought to be required in cartilage calcification, its relation to other matrix constituents remains unclear.
Type VI collagen is found in the pericellular capsule and matrix around the chondrocytes.
The skeleton, the vertebral column, and the pelvis are formed by endochondral ossification. Endochondral bone development begins as a condensation of mesenchymal cells derived from mesoderm, which form an extracellular matrix. Collagen I is the major extracellular matrix (ECM) molecule of bone.
Non-Collagen Bone Proteins
Non-collagenous bone proteins are a heterogeneous group that varies from entrapped serum protein to glycoproteins, which are unique to the bone. These bone proteins probably play a role in mineralization and are formed exogenously or locally.
Osteocalcin or bone Gla protein (BGP) is the best characterized of the non-collagen bone proteins and it makes up between 10 and 20 percent of them.
Osteocalcin is produced only by osteoblasts and a proportion of the newly synthesized protein escapes into the serum.
Raised serum levels of osteocalcin have been reported in diseases that are associated with increased bone turnover such as Paget’s disease, renal osteodystrophy, and primary hyperparathyroidism.
Osteocalcin as a biochemical marker of bone formation
Matrix Gla Protein
As much as 50 percent of the total glutamic acid-containing bone proteins are distinct from osteocalcin. The best characterized of these is matrix Gla protein (MGP), a 10,000 Da protein found in association with bone morphogenetic protein (BMP).
A number of phosphoproteins and glycoproteins are found in bone and the best characterized of these bone protein is osteonectin.
It binds collagen and hydroxyapatite through separate areas of its molecule, is found in relatively large amounts in immature bone and promotes mineralization of collagen.
It is possible that osteonectin plays a crucial role in mineralization.
Bone proteoglycans constitute approximately 10 percent of the non-collagen bone proteins. Their role is unclear.
The sulphated glycosaminoglycans in connective tissue are chondroitin sulphate, keratan sulphate and dermatan sulphate. They are bound at one end to a protein core.
The elasticity and resilience of cartilage results from this matrix of proteoglycans, collagen, and water. They have demonstrated how the compressive stiffness of cartilage over short intervals directly correlates to the presence of proteoglycans when measured as glycosaminoglycans.
Because of this, as a load is applied to cartilage there is an increase in the fluid pressure and water is driven out, but the cartilage deforms only slowly.
The proteoglycans have a very significant role in controlling swelling, pressure and the movement of water molecules when cartilage is placed under load.
In addition to their contribution to the matrix structure, proteoglycans in the growth plate may play a role in mineralization. Focal concentrations of proteoglycan at the sites of mineralization are well documented.
Because the chondroitin sulfate chains of proteoglycans bind calcium and because phosphate can displace this calcium from the proteoglycans, proteoglycan may serve as the medium within which calcium release by ion exchange could raise the Ca X PO¬4 product above the threshold for hydroxyapatite precipitation.
These are glycoproteins containing the sugar N-acetylneuraminic acid (sialic acid). The make up approximately 7.5 percent of the total non-collagen bone proteins and their function is unclear.
These constitute the largest number of non-collagen bone proteins. They include serum albumin and some immunoglobulins. They constitute approximately a quarter of the total non-collagenous protein and their function is unknown.