Bone Composition

Bone is a kind of specialized connective tissue providing a supportive framework for the body. Bones are continually being remodeled during the life of the body so they can be used to determine the age of a body.

About 70% of bone is made up of calcium hydroxyapatite, 3Ca₃(PO₄)₂.Ca(OH)₂.
The crystals of calcium hydroxyapatite in bone are very small, about 5nm x 5 nm x 40nm.
The total surface area of calcium hydroxyapatite in bone is extremely large, about 40 hectares in the skeleton of a 70kg man.
About 10% of bone is made up of water, and the other 20% is made up of organic compounds such as collagen. The term collagen refers to a group of proteins, and is the most abundant protein found in mammals, about 30% of all the protein in the body is collagen.

The bone of very young people, less than 4 years of age, is called woven bone because the collagen fibres are interwoven and run in all directions just like a piece of felt, and the bone contains large holes like Swiss cheese.

The bones of bones of adults include lamellar bone which has collagen fibres organized into layers or sheets known as lamellae. These are usually built up around blood vessels like the layers of an onion. This produces compact bone.
Adult bones also include trabecular (cancellous, spongy) bone. The bone itself is in the form of little beams (trabeculae) which are about 200μm thick. The spaces in the bone can contain marrow.

Osteoporosis refers to a decrease in the amount of bone tissue which results in an increased tendency for the bone to fracture. The shape and size of the bone doesn't change, but the little beams, the trabeculae, become thinner and some may disappear altogether.
Astronauts in space can also display a decrease in the amount of bone tissue because there is less load, and therefore less stress, on their bones in space.

Aerobic and resistance training increase the density. Bone is deposited when there is load, and stress, applied to the bone.

Scientists at the Rensselaer Polytechnic Institute have developed a new technique to analyze the bone matrix. This technique provides information about the concentration of different proteins in the bones, which can be used to determine how the bone was formed, how it has been modified over time, and, if the bone is prone to fracture.

Reference
G. E. Sroga, L. Karim, W. Colon, D. Vashishth. Biochemical characterization of major bone-matrix proteins using nanoscale-size bone samples and proteomics methodology. Molecular & Cellular Proteomics, 2011; DOI: 10.1074/mcp.M110.006718

Further Reading
Molecular Mass (Formula Weight)
Percentage Composition
Unit Conversions
Density
Mass-Mole Calculations

Study Questions

1. Calculate the molecular mass (formula weight) of calcium hydroxyapatite.
2. Calculate the percentage composition of calcium hydroxyapatite.
3. 1 hectare is 1,000m², what is the surface area in m² of calcium hydroxyapatite crystals in a 70kg man?
4. If you laid out all the calcium hydroxyapatite crystals in the bones of the 70kg man above onto a football field, how many football fields would you need? (Assume a football field is about 6,000m²)
5. The little beams in trabecular bone are 200μm thick. Convert this to a thickness in
   
   • meters
   • millimeters
   • nanometers
   • angstroms
6. The density of a bone in the leg is about 2g/cm³. 70% of bone is made up of calcium hydroxyapatite. Calculate:
   • the mass of calcium hydroxyapatite present in a 500cm³ bone sample
   • the moles of calcium hydroxyapatite present in the same bone sample
   • the percentage of calcium present in the same bone sample
   • the mass of calcium present in the same bone sample
   • the mass of water present in the same bone sample
   • the mass of organic compounds present in the same bone sample
     
7. Which type of bone, woven bone or lamellar bone, has the greatest density? Explain your answer.
8. Will the bones of a person suffering from osteoporosis be more or less dense than the bones of an athletic young adult? Explain your answer.
   
   

Mammoth Collagen

University of York and Manchester scientists have extracted protein from the bones of a 600,000 year old mammoth. The scientists used an ultra-high resolution mass spectrometer to produce an almost complete sequence of amino acids for the collagen protein.

About 30% of all the protein found in mammals is collagen, making it the most abundant protein found in mammals. It is the main component of connective tissue and is found in muscles, tendons, ligaments and skin as well as in the cornea, cartilage, bone, blood vessels, the gut and intervertebral discs.

A collagen molecule can be about 300nm long and 1.5nm wide and is made up of 3 polypeptide chains in the structure of a left-handed helix. These helices twist together into a right-handed coil forming a triple helix which is stabilized by hydrogen bonds.
In each of the three polypeptide chains there is a regular arrangement of amino acids, often following the sequence Gly-Pro-X or Gly-X-Hyp where X is another amino acid.

Bio-archaeologists are excited about this because it is believed that protein can last in a useful form ten times longer than DNA. So, while DNA is useful in discoveries up to 100,000 years old, collagen could be used in identifying extinct animals up to 1,000,000 years old.

Reference
M. Buckley, N. Larkin, M. Collins. Mammoth and Mastodon collagen sequences; survival and utility. Geochimica et Cosmochimica Acta, 2011; 75 (7): 2007 DOI: 10.1016/j.gca.2011.01.022

Further Reading
http://www.ausetute.com.au/aminoacid.html
http://www.ausetute.com.au/proteins.html
http://www.ausetute.com.au/dna.html

Study Questions

1. What is the general name given to the smaller units that make up a protein?
2. Name the four elements found in all proteins.
   
3. What is meant by the term polypeptide?
4. Explain why proteins are considered to be biological polymers.
   
5. What is a peptide bond?
6. Amino acids are often represented by a three letter code. Give the name for each of the following amino acids:
   
   • gly
   • ala
     
   • hyp
   • pro
7. Draw a structure for each of the following tripeptides:
   
   • gly-ala-hyp
   • gly-pro-ala
8. Identify the peptide bond(s) in each of the tripeptides above.
9. What is meant by each of the following terms with respect to proteins:
   
   • primary structure
   • secondary structure
   • tertiary structure
10. Describe the primary, secondary and tertiary structures for collagen.