27: Bone Structure
- Page ID
- 143631
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\(\newcommand{\avec}{\mathbf a}\) \(\newcommand{\bvec}{\mathbf b}\) \(\newcommand{\cvec}{\mathbf c}\) \(\newcommand{\dvec}{\mathbf d}\) \(\newcommand{\dtil}{\widetilde{\mathbf d}}\) \(\newcommand{\evec}{\mathbf e}\) \(\newcommand{\fvec}{\mathbf f}\) \(\newcommand{\nvec}{\mathbf n}\) \(\newcommand{\pvec}{\mathbf p}\) \(\newcommand{\qvec}{\mathbf q}\) \(\newcommand{\svec}{\mathbf s}\) \(\newcommand{\tvec}{\mathbf t}\) \(\newcommand{\uvec}{\mathbf u}\) \(\newcommand{\vvec}{\mathbf v}\) \(\newcommand{\wvec}{\mathbf w}\) \(\newcommand{\xvec}{\mathbf x}\) \(\newcommand{\yvec}{\mathbf y}\) \(\newcommand{\zvec}{\mathbf z}\) \(\newcommand{\rvec}{\mathbf r}\) \(\newcommand{\mvec}{\mathbf m}\) \(\newcommand{\zerovec}{\mathbf 0}\) \(\newcommand{\onevec}{\mathbf 1}\) \(\newcommand{\real}{\mathbb R}\) \(\newcommand{\twovec}[2]{\left[\begin{array}{r}#1 \\ #2 \end{array}\right]}\) \(\newcommand{\ctwovec}[2]{\left[\begin{array}{c}#1 \\ #2 \end{array}\right]}\) \(\newcommand{\threevec}[3]{\left[\begin{array}{r}#1 \\ #2 \\ #3 \end{array}\right]}\) \(\newcommand{\cthreevec}[3]{\left[\begin{array}{c}#1 \\ #2 \\ #3 \end{array}\right]}\) \(\newcommand{\fourvec}[4]{\left[\begin{array}{r}#1 \\ #2 \\ #3 \\ #4 \end{array}\right]}\) \(\newcommand{\cfourvec}[4]{\left[\begin{array}{c}#1 \\ #2 \\ #3 \\ #4 \end{array}\right]}\) \(\newcommand{\fivevec}[5]{\left[\begin{array}{r}#1 \\ #2 \\ #3 \\ #4 \\ #5 \\ \end{array}\right]}\) \(\newcommand{\cfivevec}[5]{\left[\begin{array}{c}#1 \\ #2 \\ #3 \\ #4 \\ #5 \\ \end{array}\right]}\) \(\newcommand{\mattwo}[4]{\left[\begin{array}{rr}#1 \amp #2 \\ #3 \amp #4 \\ \end{array}\right]}\) \(\newcommand{\laspan}[1]{\text{Span}\{#1\}}\) \(\newcommand{\bcal}{\cal B}\) \(\newcommand{\ccal}{\cal C}\) \(\newcommand{\scal}{\cal S}\) \(\newcommand{\wcal}{\cal W}\) \(\newcommand{\ecal}{\cal E}\) \(\newcommand{\coords}[2]{\left\{#1\right\}_{#2}}\) \(\newcommand{\gray}[1]{\color{gray}{#1}}\) \(\newcommand{\lgray}[1]{\color{lightgray}{#1}}\) \(\newcommand{\rank}{\operatorname{rank}}\) \(\newcommand{\row}{\text{Row}}\) \(\newcommand{\col}{\text{Col}}\) \(\renewcommand{\row}{\text{Row}}\) \(\newcommand{\nul}{\text{Nul}}\) \(\newcommand{\var}{\text{Var}}\) \(\newcommand{\corr}{\text{corr}}\) \(\newcommand{\len}[1]{\left|#1\right|}\) \(\newcommand{\bbar}{\overline{\bvec}}\) \(\newcommand{\bhat}{\widehat{\bvec}}\) \(\newcommand{\bperp}{\bvec^\perp}\) \(\newcommand{\xhat}{\widehat{\xvec}}\) \(\newcommand{\vhat}{\widehat{\vvec}}\) \(\newcommand{\uhat}{\widehat{\uvec}}\) \(\newcommand{\what}{\widehat{\wvec}}\) \(\newcommand{\Sighat}{\widehat{\Sigma}}\) \(\newcommand{\lt}{<}\) \(\newcommand{\gt}{>}\) \(\newcommand{\amp}{&}\) \(\definecolor{fillinmathshade}{gray}{0.9}\)Bone Structure Cumulative Problem
Bone is a natural composite of collagen protein and the mineral hydroxyapatite.
Nair, Gautieri, Chang & Buehler, “Molecular mechanics of mineralized fibrils in bone”, Nature Comm., 2013, 2720, 1-9.
Collagen
Collagen is a protein primarily made from glycine and proline amino acids.
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Proline in collagen strands is S. Draw S-proline. Clearly indicate the chirality with wedges and dashes.
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Draw the following short peptide:
Gly-Gly-Pro-Gly-Gly-Asp-Pro-Gly
This protein forms α-helices.
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What IMF holds α-helices in that secondary structure?
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Why aren’t α-helices made from prolines strong?
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What about the shape of proline helps form α-helices?
Tropocollagen
Tropocollagen is a rod-shaped helical trimer.
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What is holding the three strands together in this structure?
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Clearly circle the atoms involved in this interaction.
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Label the type of interaction.
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Hydroxyapatite
Hydroxyapatite is made up of Ca2+, PO43- and OH-.
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Draw a Lewis structure for PO43-.
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What is the molecular geometry of this phosphate ion?
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Provide an electron configuration for Ca2+.
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In a unit cell, there are 3 P atoms and 13 O. Provide an empirical formula for hydroxyapatite consisting of Ca2+, PO43- and OH-.
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Draw one Ca2+ ion bound to 3 bidentate phosphate ions.
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What is coordination geometry of this structure?
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Show the enantiomer.
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In the actual structure, the phosphates ‘bridge’ between 2 Ca2+ ions. Draw a structure of a phosphate binding to two calcium ions.
Salt bridges in bone structures
In collagen, the number of salt bridges (ionic bonds) per unit cell is ~260, due to interactions between charged side chains of collagen: lysine, arginine, glutamic acid and aspartic acid.
lysine:
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Draw neutral lysine.
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Is the neutral side chain of lysine (acidic OR basic)?
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Using curved arrows, show the acid-base reaction of the side chain of lysine.
Aspartic acid:
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Draw neutral aspartic acid.
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Is the neutral side chain of aspartic acid (acidic OR basic)?
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Using curved arrows, show the acid-base reaction of the side chain of aspartic acid.
Salt Bridges within collagen:
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(4 pts) Draw an ionic interaction between the two newly charged side chains.
Salt bridges between collagen and hydroxyapatite:
In the mineralized collagen, there is a high number of mineral–mineral salt bridges, due to the fact that the moieties forming HAP are highly charged: Ca2+, PO43- and OH-. Salt bridges are also found relevant between collagen and mineral phase (about 220 and 520 in 20% HAP and 40% HAP case, respectively), contributing to the increase in mechanical properties of mineralized fibrils.
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Show an example of a salt bridge between a lysine side chain and mineral PO43- group.
Bones
Mineral crystals carry more stress, whereas the collagen protein carries more deformation.
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Ionic Compounds are:
rigid/brittle OR flexible/soft
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Proteins are:
rigid/brittle OR flexible/soft
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Would a bone made out of ionic compounds be more likely to break?
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Would a bone made out of proteins have much strength?
A collagen molecule at high stress levels gets stretched and starts to uncoil, and eventually slides on the HAP surface.
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At a catastrophic level of stress (think of a jackhammer on a sidewalk), what would happen to any material?
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What happens to the intermolecular forces under stress?
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How does the collagen slipping mechanism prevent catastrophe?
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In your own words explain why bones are made from a mixture of tropocollagen and hydroxyapatite.