LG 2.4 Biochemistry of Bone

Question 1

What is Tropocollagen? What is it made of and what is it a precursor for?

Answer 1

A molecule of collagen in a triple helical form = building block of collagen fibrils

Question 2

Why are myofibrils intracellular modular?

Answer 2

Because it needs ATP.

Question 3

What two phase nano composites make up bone? Which makes bone more flexible?

Answer 3

Mineral (Carbonated hydroxyapatite)- 65%- this makes bone stiffer 
Organic phase (collagen) 45%, This makes bone more flexible

Question 4

What is makes up 1/3 of human protein?

Answer 4

Collagen

Question 5

How much of the human proteins are made of Collagen Type I?

Answer 5

1/4 of human protein

Question 6

Draw out Glycine. Where is it located the triple helix?

Answer 6

See Slide 1 for drawing. It is located in the core.

Question 7

Draw out Proline. Where is it the triple helix and in what conformation?

Answer 7

See slide 1. The ring of proline sticks out of the helix, and it is in the endo form.

Question 8

Draw out Hydroxyproline.Where is it the triple helix and in what conformation?

Answer 8

See Slide 2.The ring sticks out of the helix, and it is in the exo form.

Question 9

How does Proline affect peptide backbone in comparison to other amino acids?

Answer 9

the amino acid that most constrains peptide backbone flexibility, because it puts a closed ring in the backbone that allows no rotation around that bond.

Question 10

How does Glycine affect peptide backbone in comparison to other amino acids?

Answer 10

the amino acid that least constrains peptide backbone flexibility because it has the smallest of all side chains (a hydrogen atom)

Question 11

Explain why a collagen fibril has a banding pattern that repeats every 67 nm.

Answer 11

The gap region and overlap is made up of 27nm and 40 nm regions, which equals 67 nm

Question 12

Describe the overall biosynthesis and assembly collagen with in context of the endoplasmic reticulum, golgi bodies, and extracellular space.

Answer 12

See Slide 3

a. Procollagen a-chains are translated into the endoplasmic reticulum, where three of them are simultaneously posttranslationally modified and assembled to form triple helical procollagen
b. procollagen triple helices are exported. Then the N- and C-termini are cleaved leaving tropocollagen, the basic building block which is assembled into collagen fibrils.
c. Tropocollagen is expressed and assembled intracellulary. Protocollagen peptides are translated by ribosomes directly into the endoplasmic reticulum
i. Tropocollagen pack together to form wide collagen fibrils
d. Procollogen Strand→ Procollagen triple helix→Tropocollagen triple helix

Question 13

Where does posttranslational modification of collagen occur?

Answer 13

Endoplasmic Reticulum

Question 14

What is hydroxylation? What is an example of this?

Answer 14

See slide 4 and 5.
Hydroxylation is the addition of an hydroxyl group to an amino acid residue, is prevalent in collagen.

Example: Proline–>Hydroxyproline
Lysine–>Hydroxylysine

Question 15

What is the function of the hydroxyapatite?

Answer 15

Hydroxyapatite crystals nucleate within and grow beyond the gap regions of collagen fibrils thereby cement the tropocollagen building blocks together, further stabilizing collagen collagen fibrils, and contributing the the 67 nm banding pattern.

Question 16

Describe the Hierarchial structure of collagen

Answer 16

Amino Acids→Tropocollagen (Brick)→Hydroxyapatite (Mortar,crystal on top of bricks) causes mineralization, causing stiffness→ Collagen fibril (Tropcollagens packed together)→Collagen Fiber (Through mineralization)→ Tissues bone (Packing together of fibers)

Question 17

What occurs during Vitamin C deficiency?

Answer 17

vitamin C is able to accept or donate electron in order to support biological reactions.
The donation of an electon from vitamin C to reduce iron in proline or lysine hydroxylase is essential to the production of collagen.–If no Vitamin C–tropocollagen building blocks are defective, then 1/3 of the human protein is defective and fails to adequately glue human connective tissue together resulting in skin lesions, degraded gums, etc.

Question 18

What type of bond links collagen peptides?

Answer 18

Disulfide bonds

Question 19

Explain why a mutation that leads to a substitution for glycine in collagen leads to osteogenesis imperfecta.

Answer 19

Glycine fits into tropcollagen core at every third position
In order to form the triple helix. If a new AA is added it will disrupt the triple helix formation.

Osteogenesis imperfecta, “brittle bone disease” is usually caused by mutations that affect which amino acid in collagen.
i. 80% is due to substitution of glycine in a1 or a2 chains of type I collagen

Question 20

Glycosylation

Answer 20

enzymatic linking of sugars to protein to serve as recognition or linkage sites for other proteins.

Question 21

Glycation

Answer 21

non-enzymatic random linking of sugars to protein, excessive crosslinking in diabetics or in elderly leads to brittle bones and to stiff bones, ligaments, and cartiledge.
i. Harmful, faster bone aging

Question 22

How the buildup of advanced glycation end products that occurs over time and that is accelerated by hyperglycemia makes bones more brittle, ligaments more stiff, and skin more wrinkled.

Answer 22

b. Glucose can form a Schiff base between lysine and glucose especially in the case of aging or diabetics making an alternative Schiff base .
i. This will cause change crosslinking (extra)→ Decrease stability
ii. Bones will age faster

Question 23

Why is collagen described as modular?

Answer 23

Collagen can be described as a structural motif that is a ring one amino acid wide and three amino acids around. That motif is translated by one amino acid along the length of the tropocollagen molecule and rotated by 120o, this pattern is repeated 1000 times which corresponds to each amino acid in length of tropocollagen.

Just as tropocollagen is built by repeating an extremely modular structure, so tropocolagen molecules stack together into a higher level of modularity. About 1000 tropocollagen building blocks stack together to form a 100 nm wide collagen fibril.