High Yield Overview

PROTEOGLYCANS AND COLLAGEN

ECM Macromolecules | 28 Collagen Types | GAG Side Chains | Triple Helix Structure

28Collagen types identified

90%Type I collagen in bone/tendon

300nmTropocollagen length

67nmD-band periodicity

MAJOR COLLAGEN TYPES

Type I

PatternBone, tendon, ligament, skin

TreatmentMost abundant, fibril-forming

Type II

PatternHyaline cartilage, nucleus pulposus

TreatmentFibril-forming, smaller diameter

Type III

PatternBlood vessels, skin, healing tissue

TreatmentReticular fibers, with Type I

Type IV

PatternBasement membranes

TreatmentNetwork-forming, non-fibrillar

Critical Must-Knows

Collagen is most abundant protein in mammals (25-35% of total body protein)
Triple helix structure - Gly-X-Y repeat with glycine every third residue
Proteoglycans consist of core protein with GAG side chains
Aggrecan is major cartilage proteoglycan, decorin binds collagen
Hydroxylation requires Vitamin C - scurvy impairs collagen synthesis

Examiner's Pearls

"
Type I collagen mutations cause osteogenesis imperfecta
"
Type II collagen mutations cause chondrodysplasias
"
Lysyl oxidase creates crosslinks (requires copper)
"
GAG side chains on proteoglycans attract water via negative charge

Critical Collagen and Proteoglycan Exam Points

Triple Helix Structure

Collagen has unique triple helix with Gly-X-Y amino acid repeat. Glycine (smallest amino acid) must be every third residue to fit in center of helix. Proline and hydroxyproline common in X and Y positions. Mutations replacing glycine cause structural diseases (OI, EDS).

Post-Translational Modifications

Hydroxylation of proline and lysine requires Vitamin C (cofactor for prolyl and lysyl hydroxylases). Scurvy causes defective collagen. Lysyl oxidase (requires copper) creates crosslinks between collagen molecules, essential for tensile strength.

Proteoglycan Structure

Proteoglycans are core protein with GAG side chains (chondroitin sulfate, keratan sulfate, heparan sulfate, dermatan sulfate). GAGs are repeating disaccharides with sulfate/carboxyl groups creating negative charge. Attract water and cations.

Tissue-Specific Distribution

Type I collagen: bone (90%), tendon, ligament, skin. Type II: hyaline cartilage, nucleus pulposus. Type III: blood vessels, healing wounds (with Type I). Type IV: basement membranes. Know which type in which tissue.

At a Glance

Collagen is the most abundant protein in mammals (25-35% of total body protein), characterised by a unique triple helix structure with the Gly-X-Y amino acid repeat. Type I collagen predominates in bone/tendon (90%), while Type II is found in hyaline cartilage. Proteoglycans consist of core proteins with negatively charged glycosaminoglycan (GAG) side chains that attract water - aggrecan is the major cartilage proteoglycan. Key exam points: Vitamin C is essential for proline/lysine hydroxylation (scurvy causes defective collagen), and lysyl oxidase (copper-dependent) creates crosslinks essential for tensile strength.

Mnemonic

COLLAGENCOLLAGEN - Key Features

Common protein (25-35% body)

Most abundant protein in mammals

One-quarter is glycine

Gly-X-Y repeat, glycine every 3rd residue

Lysyl oxidase crosslinks

Creates covalent bonds between molecules

Long triple helix (300nm)

Tropocollagen molecule

Aligned in fibrils

Quarter-stagger 67nm D-band

Glycine must be central

Small enough to fit in helix center

Extracellular assembly

Procollagen processed outside cell

Nineteen fibril-forming types

Types I, II, III, V, XI form fibrils

Memory Hook:COLLAGEN is the common protein with glycine repeats forming triple helix

Mnemonic

TYPESTYPES - Major Collagen Types

Type I - Tendon, bone (most common)

90% of total body collagen

tYpe II - hYaline cartilage

Smaller fibrils than Type I

tyPe III - blooD vessels (reticular)

Found with Type I in skin, vessels

typE IV - basEmEnt mEmbranes

Network-forming, non-fibrillar

Scurvy affects all types

Vitamin C required for hydroxylation

Memory Hook:Know the TYPES of collagen: I in bone/tendon, II in cartilage, III in vessels, IV in basement membranes

Mnemonic

PROTEOGLYCANSPROTEOGLYCANS - Structure and Function

Protein core with GAG chains

Core protein covalently linked to GAGs

Repel water into tissue

Negative charges attract water

Osmotic pressure created

Swelling pressure from hydration

Types: aggrecan, decorin, perlecan

Tissue-specific proteoglycans

Each GAG is disaccharide repeat

Repeating sugar units

Oh-so-negative charge (SO4-, COO-)

Sulfate and carboxyl groups

GAGs: CS, KS, HS, DS, HA

Five main GAG types

Large aggregates form

Aggrecans bind to hyaluronan

Year-long half-life (aggrecan)

Slower turnover than proteins

Cartilage has highest content

4-7% of cartilage wet weight

Attract cations (Na+, Ca2+)

Donnan equilibrium

Non-sulfated: hyaluronan

Only GAG without sulfate

Sulfate groups key to function

Provide negative charge

Memory Hook:PROTEOGLYCANS have protein core with negatively-charged GAG chains attracting water

Overview

Collagen and proteoglycans are major components of extracellular matrix in connective tissues including bone, cartilage, tendon, ligament, and skin.

Why collagen and proteoglycans matter clinically:

Genetic Collagen Disorders

Mutations in collagen genes cause osteogenesis imperfecta (Type I), chondrodysplasias (Type II), vascular Ehlers-Danlos syndrome (Type III), and Alport syndrome (Type IV). Understanding collagen structure explains disease mechanisms.

Wound Healing and Tissue Repair

Collagen synthesis is essential for fracture healing, tendon repair, and wound closure. Type III collagen appears early in healing, later replaced by Type I. Factors impairing collagen synthesis (vitamin C deficiency, medications) delay healing.

Degenerative Diseases

Osteoarthritis involves proteoglycan loss and collagen network disruption. Intervertebral disc degeneration involves aggrecan degradation. Understanding normal structure is essential to understand pathology.

Biomaterial Design

Collagen-based scaffolds are used in cartilage repair and tissue engineering. Understanding collagen structure, crosslinking, and degradation informs biomaterial design.

Collagen as Structural Protein

Collagen is the most abundant protein in mammals (25-35% of total body protein). It provides structural framework and tensile strength in connective tissues. The unique triple helix structure with Gly-X-Y repeat is essential for collagen function.

Concepts and Molecular Structure

Collagen Triple Helix

Primary structure - Gly-X-Y repeat:

Glycine every third residue (Gly-X-Y)
X position: Often proline (28%)
Y position: Often hydroxyproline (38%)
Glycine is smallest amino acid, fits in helix center
Any other amino acid at glycine position disrupts helix

Secondary structure - Left-handed helix:

Each alpha chain forms left-handed helix (polyproline II type)
3 residues per turn
Extended structure (not compact like alpha-helix)

Tertiary structure - Right-handed superhelix:

Three alpha chains wrap around each other
Right-handed triple helix
Glycine residues at central axis (every third residue)
Proline and hydroxyproline stabilize helix
No internal hydrogen bonds within chains
Hydrogen bonds between chains

Tropocollagen molecule:

300 nm length
1.5 nm diameter
Molecular weight: ~300 kDa
Three chains: Type I has two alpha-1(I) and one alpha-2(I)
Type II has three identical alpha-1(II) chains

Glycine Mutations

Glycine substitutions in Type I collagen cause osteogenesis imperfecta. Glycine is the only amino acid small enough to fit in the center of the triple helix. Replacing glycine with larger amino acids (e.g., serine, cysteine) disrupts helix structure, causing brittle bones. Location of mutation affects severity.

The triple helix structure is fundamental to collagen function and stability.

Major Collagen Types

28 collagen types have been identified. The major types relevant to orthopaedics are Types I, II, III, IV, V, IX, X, and XI.

Major Collagen Types in Orthopaedic Tissues

Type	Structure	Tissue Distribution	Function	Clinical Significance
I	Fibril-forming [α1(I)]₂α2(I)	Bone, tendon, ligament, skin, dentin	Tensile strength, structural support	OI: mutations in COL1A1, COL1A2
II	Fibril-forming [α1(II)]₃	Hyaline cartilage, vitreous humor, nucleus pulposus	Compression resistance in cartilage	Chondrodysplasias, early OA
III	Fibril-forming [α1(III)]₃	Blood vessels, skin, reticular fibers, healing tissue	Elastic recoil, early wound healing	Vascular EDS (Type IV EDS)
IV	Network-forming [α1(IV)]₂α2(IV)	Basement membranes (all)	Filtration barrier, cell attachment	Alport syndrome (kidney, ear, eye)
V	Fibril-forming [α1(V)]₂α2(V)	Bone, cornea, with Type I	Regulates Type I fibril diameter	Classical EDS (with Type I)
IX	FACIT [α1(IX)]α2(IX)α3(IX)	Cartilage, vitreous, with Type II	Links Type II fibrils, resists shear	Multiple epiphyseal dysplasia
X	Network-forming [α1(X)]₃	Hypertrophic cartilage (growth plate)	Endochondral ossification	Schmid metaphyseal chondrodysplasia
XI	Fibril-forming [α1(XI)]α2(XI)α3(XI)	Cartilage, vitreous, with Type II	Regulates Type II fibril diameter	Stickler syndrome (with Type II)

Type I vs Type II Collagen

Type I collagen (bone, tendon, ligament) forms larger diameter fibrils (50-200 nm) and has high tensile strength. Type II collagen (cartilage) forms smaller diameter fibrils (20-40 nm) optimized for compression resistance. Type I has two alpha-1(I) and one alpha-2(I) chain. Type II has three identical alpha-1(II) chains (homotrimer).

Collagen classification:

Fibril-forming: Types I, II, III, V, XI (form D-banded fibrils)
FACIT (Fibril-Associated Collagens with Interrupted Triple helices): Types IX, XII, XIV
Network-forming: Types IV, VIII, X (basement membranes, specialized networks)
Anchoring fibrils: Type VII (epidermis-dermis junction)
Transmembrane: Types XIII, XVII, XXIII

Understanding tissue-specific collagen types is essential for orthopaedic basic science.

Clinical Relevance and Applications

Proteoglycans consist of core protein with covalently attached GAG side chains.

Proteoglycan Structure

Components:

Core protein: Synthesized on ribosomes, varies in size
GAG chains: Synthesized in Golgi, attached to core protein
Link protein: Stabilizes aggrecan-hyaluronan binding (aggrecan only)

GAG attachment:

GAGs attached to serine residues on core protein
Tetrasaccharide linker: xylose-galactose-galactose-glucuronic acid
Except keratan sulfate (attached to serine or threonine via N-acetylgalactosamine)

Size range:

Small: Decorin (40 kDa core, 1 GAG chain, total 90-140 kDa)
Medium: Perlecan (470 kDa core, multiple HS chains)
Large: Aggrecan (220 kDa core, ~100 CS chains, total 2-3 million Da)

Proteoglycans vary in size and GAG composition depending on tissue function.

Evidence Base

Collagen Structure and Biosynthesis

Shoulders MD, Raines RT • Annu Rev Biochem (2009)

Key Findings:

Gly-X-Y triplet repeat is essential for triple helix formation
Hydroxyproline stabilizes triple helix through stereoelectronic effects
Vitamin C is required as cofactor for prolyl and lysyl hydroxylases
Lysyl oxidase creates crosslinks essential for tensile strength

Clinical Implication: Understanding collagen biosynthesis explains scurvy (vitamin C deficiency), lathyrism (lysyl oxidase inhibition), and osteogenesis imperfecta (structural mutations).

Proteoglycan Structure and Function

Iozzo RV, Schaefer L • Matrix Biol (2015)

Key Findings:

Proteoglycans are core protein with GAG side chains
GAGs create fixed negative charge attracting water
Aggrecan provides compressive stiffness in cartilage
Small leucine-rich proteoglycans (decorin, biglycan) regulate collagen assembly

Clinical Implication: Proteoglycan loss is earliest change in osteoarthritis. Understanding proteoglycan function explains MRI changes and mechanical property loss in early OA.

Collagen Type I Mutations in Osteogenesis Imperfecta

Marini JC, Forlino A, Bächinger HP, et al • Nat Rev Endocrinol (2017)

Key Findings:

OI caused by mutations in COL1A1 or COL1A2 (Type I collagen genes)
Glycine substitutions disrupt triple helix (most severe)
Quantitative defects (reduced collagen) cause milder disease
Severity depends on mutation location and type

Clinical Implication: Understanding collagen structure explains genotype-phenotype correlation in OI. Glycine substitutions in C-terminal region cause most severe disease.

Aggrecan Structure and Degradation

Roughley PJ, Mort JS • Matrix Biol (2014)

Key Findings:

Aggrecan aggregates with hyaluronan via link protein
ADAMTS aggrecanases cleave in interglobular domain (IGD)
MMP cleavage sites differ from aggrecanase sites
Aggrecan loss is earliest change in osteoarthritis

Clinical Implication: Aggrecan degradation biomarkers can detect early OA. ADAMTS inhibitors are therapeutic targets for preventing cartilage degeneration.

Basic Science Viva Scenarios

Practice these scenarios to excel in your viva examination

VIVA SCENARIOStandard

Scenario 1: Collagen Triple Helix (~3 min)

EXAMINER

"Describe the structure of the collagen triple helix. What is the significance of the Gly-X-Y repeat?"

EXCEPTIONAL ANSWER

The collagen triple helix consists of three left-handed polyproline II-type helical alpha chains that wrap around each other to form a right-handed superhelix. The fundamental feature is the Gly-X-Y amino acid repeat where glycine appears every third residue. Glycine is the smallest amino acid and is the only one that fits in the center of the triple helix where the three chains come together. In the X and Y positions, proline is commonly found in the X position (28%) and hydroxyproline in the Y position (38%). Hydroxyproline, formed by post-translational hydroxylation of proline residues requiring Vitamin C, stabilizes the triple helix through stereoelectronic effects. The triple helix is stabilized by hydrogen bonds between chains rather than within chains. Each tropocollagen molecule is 300 nm long and 1.5 nm in diameter. The importance of the Gly-X-Y repeat is that any mutation replacing glycine with a larger amino acid disrupts the triple helix structure. This occurs in osteogenesis imperfecta where glycine substitutions in Type I collagen cause unstable triple helices resulting in brittle bones.

KEY POINTS TO SCORE

Three alpha chains form right-handed triple helix

Gly-X-Y repeat: glycine every third residue

Glycine is small enough to fit in helix center

X position: often proline, Y position: often hydroxyproline

Hydroxyproline stabilizes helix (requires Vitamin C for synthesis)

Tropocollagen: 300 nm length, 1.5 nm diameter

Glycine mutations disrupt helix causing OI

Hydrogen bonds between chains stabilize structure

COMMON TRAPS

✗Confusing left-handed (individual chains) with right-handed (superhelix)

✗Not explaining why glycine must be every third residue

✗Forgetting hydroxyproline role in stability

✗Missing Vitamin C requirement

LIKELY FOLLOW-UPS

"What enzyme hydroxylates proline and what does it require?"

"What happens in scurvy and why?"

"What is the difference between Type I and Type II collagen chains?"

VIVA SCENARIOChallenging

Scenario 2: Collagen Biosynthesis (~4 min)

EXAMINER

"Describe the steps of collagen biosynthesis from translation to fibril formation. Where are crosslinks formed and why are they important?"

EXCEPTIONAL ANSWER

Collagen biosynthesis involves intracellular and extracellular steps. First, pro-alpha chain mRNA is translated on ribosomes and the chains enter the rough ER. Second, hydroxylation occurs in the ER: prolyl hydroxylase converts proline to hydroxyproline, and lysyl hydroxylase converts lysine to hydroxylysine. Both require Vitamin C as cofactor. Hydroxyproline stabilizes the triple helix while hydroxylysine provides sites for glycosylation and crosslinking. Third, hydroxylysine residues are glycosylated with glucose and galactose in the ER and Golgi. Fourth, three pro-alpha chains assemble into procollagen triple helix, initiated by C-terminal propeptides. Fifth, procollagen is secreted from the cell and N- and C-terminal propeptides are cleaved by procollagen peptidases, creating tropocollagen (300 nm). Sixth, tropocollagen molecules self-assemble into fibrils with quarter-stagger arrangement creating 67 nm D-band periodicity. Finally, lysyl oxidase, a copper-dependent enzyme, oxidizes lysine and hydroxylysine residues to aldehydes which spontaneously condense to form covalent crosslinks. Immature crosslinks form initially (Schiff bases), which mature over time to pyridinoline and deoxypyridinoline. Crosslinks are essential for tensile strength. Without crosslinks, as in lathyrism from lysyl oxidase inhibition or copper deficiency, collagen fibrils lack strength and tissues are fragile.

KEY POINTS TO SCORE

Step 1: Translation in RER, pro-alpha chains

Step 2: Hydroxylation (proline, lysine) requires Vitamin C

Step 3: Glycosylation of hydroxylysine

Step 4: Triple helix formation (procollagen) in ER/Golgi

Step 5: Secretion, propeptide cleavage → tropocollagen (300 nm)

Step 6: Fibril self-assembly in quarter-stagger (67 nm D-band)

Step 7: Crosslink formation by lysyl oxidase (requires copper)

Crosslinks provide tensile strength (pyridinoline, deoxypyridinoline)

Scurvy: vitamin C deficiency → unstable collagen

Lathyrism: lysyl oxidase inhibition → no crosslinks, weak collagen

COMMON TRAPS

✗Missing intracellular vs extracellular steps

✗Not explaining cofactors (Vitamin C, copper)

✗Forgetting propeptide cleavage step

✗Not connecting to diseases (scurvy, lathyrism, OI)

LIKELY FOLLOW-UPS

"What is the D-band and why is it 67 nm?"

"What is the role of procollagen peptidases?"

"Why are pyridinoline crosslinks measured in urine?"

VIVA SCENARIOStandard

Scenario 3: Proteoglycan Structure (~3 min)

EXAMINER

"Describe the structure of aggrecan. How does it provide compressive stiffness in articular cartilage?"

EXCEPTIONAL ANSWER

Aggrecan is the major proteoglycan in articular cartilage, consisting of a core protein (220-250 kDa) with approximately 100 chondroitin sulfate chains and 60 keratan sulfate chains covalently attached, giving a total molecular weight of 2-3 million Da. Multiple aggrecan molecules (50-100) bind to a central hyaluronan backbone via their G1 domain, stabilized by link protein (40-45 kDa), forming massive aggregates of 50-200 million Da. The GAG side chains (chondroitin sulfate and keratan sulfate) have high fixed negative charge density from sulfate groups (SO₄⁻) and carboxyl groups (COO⁻). These negative charges attract cations (Na⁺, Ca²⁺) and water via Donnan osmotic equilibrium, creating swelling pressure of 0.1-0.3 MPa. This hydration and swelling pressure provides compressive stiffness. The collagen network in cartilage constrains this swelling, creating prestress in the tissue even without external load. When cartilage is compressed, water is forced out of the matrix and flows through the porous collagen-proteoglycan network. This biphasic behavior provides shock absorption and load distribution. The high proteoglycan content in the deep zone of cartilage (50-60 mg/mL) provides the greatest compressive stiffness where compressive loads are highest.

KEY POINTS TO SCORE

Aggrecan: core protein (220-250 kDa) + ~100 CS + ~60 KS chains

Total molecular weight: 2-3 million Da

Aggregates: 50-100 aggrecans bind to hyaluronan via link protein

Aggregate size: 50-200 million Da

GAG chains have high fixed negative charge (SO₄⁻, COO⁻)

Negative charges attract water via Donnan osmotic pressure

Swelling pressure (0.1-0.3 MPa) provides compressive stiffness

Collagen network constrains swelling (prestress)

Biphasic behavior: water flows under load (shock absorption)

COMMON TRAPS

✗Not explaining aggregation with hyaluronan

✗Missing link protein role

✗Not connecting negative charge to water attraction

✗Forgetting collagen constrains swelling

LIKELY FOLLOW-UPS

"What are the GAG types on aggrecan?"

"What is the link protein and what does it do?"

"How is aggrecan degraded in osteoarthritis?"

MCQ Practice Points

Exam Pearl

Q: What is the predominant proteoglycan in articular cartilage and its function?

A: Aggrecan is the major proteoglycan, attached to hyaluronic acid via link protein forming large aggregates. Contains glycosaminoglycan (GAG) side chains (chondroitin sulfate, keratan sulfate). Highly negatively charged, attracting water creating osmotic swelling pressure that resists compressive loads. Loss of aggrecan is early OA feature.

Exam Pearl

Q: What is the distribution of collagen types in articular cartilage?

A: Type II collagen: 90-95% of cartilage collagen, provides tensile strength. Type IX: Cross-links Type II fibrils. Type XI: Regulates fibril diameter. Type VI: Pericellular matrix around chondrocytes. Fibrocartilage (meniscus, labrum) contains Type I collagen. OA involves shift from Type II to Type I.

Exam Pearl

Q: What is the water content of articular cartilage and its significance?

A: Articular cartilage is 65-80% water by weight. Water content highest in superficial zone, lowest in deep zone. Creates biphasic viscoelastic behavior - fluid pressurization under load. Water bound to proteoglycans (fixed charge density). Dehydration decreases compressive stiffness. OA shows increased water content paradoxically.

Exam Pearl

Q: What are the structural zones of articular cartilage?

A: Superficial zone (10-20%): Collagen parallel to surface, highest water, flattened chondrocytes, lubricin secretion. Middle/transitional zone (40-60%): Random collagen orientation. Deep zone (30%): Collagen perpendicular, highest proteoglycan, columns of chondrocytes. Calcified zone: Anchors to subchondral bone via tidemark.

Exam Pearl

Q: What is the triple helix structure of collagen?

A: Three polypeptide chains (α-chains) wind into right-handed triple helix. Stabilized by glycine at every third position (smallest amino acid fits helix center). Proline and hydroxyproline provide rigidity. Hydroxyproline requires Vitamin C (scurvy causes collagen defects). Cross-linking between molecules provides tensile strength.

Australian Context

Australian Epidemiology and Practice

Proteoglycans and Collagen in Australian Orthopaedic Practice:

Collagen and proteoglycan biochemistry is essential FRACS Basic Science examination content
Understanding ECM composition explains osteoarthritis pathophysiology and cartilage repair strategies
AOANJRR tracks cartilage repair procedures and arthroplasty outcomes related to cartilage failure

RACS Orthopaedic Training Relevance:

Collagen triple helix structure, biosynthesis steps, and crosslinking are core Part I examination topics
Proteoglycan structure, GAG types, and aggrecan function frequently examined
Understanding OI, EDS, and scurvy demonstrates clinical correlation with basic science

Clinical Practice in Australia:

Cartilage repair procedures (MACI, microfracture, ACI) performed at major Australian centres
Understanding collagen and proteoglycan biology essential for cartilage regeneration research
Osteoarthritis management requires understanding of proteoglycan degradation mechanisms

PBS Considerations:

Collagen-based wound dressings and scaffolds used in Australian surgical practice
Glucosamine and chondroitin supplements (proteoglycan precursors) available over-the-counter
Hyaluronic acid injections available for OA management (not PBS-subsidised)

eTG Recommendations:

Vitamin C supplementation recommended in deficiency states to support collagen synthesis
Wound healing protocols include nutritional optimisation for collagen production

Management Algorithm

PROTEOGLYCANS AND COLLAGEN

High-Yield Exam Summary

Collagen Triple Helix

•Gly-X-Y repeat: glycine every 3rd residue (small enough for helix center)
•X = proline (28%), Y = hydroxyproline (38%)
•Three alpha chains: right-handed superhelix, 300 nm length, 1.5 nm diameter
•Type I: [α1(I)]₂α2(I), Type II: [α1(II)]₃ homotrimer

Collagen Biosynthesis

•Hydroxylation: Prolyl/lysyl hydroxylase (requires Vitamin C cofactor)
•Triple helix: C-terminal propeptides initiate assembly
•Propeptide cleavage: Procollagen → tropocollagen (300 nm)
•Fibril assembly: Quarter-stagger creates 67 nm D-band
•Crosslinks: Lysyl oxidase (requires copper) creates pyridinoline, deoxypyridinoline

Major Collagen Types

•Type I: Bone, tendon, ligament (90% of body collagen), OI mutations
•Type II: Hyaline cartilage, nucleus pulposus, chondrodysplasia mutations
•Type III: Blood vessels, skin, healing tissue, vascular EDS
•Type IV: Basement membranes (network-forming), Alport syndrome

Proteoglycan Structure

•Core protein + GAG side chains (chondroitin sulfate, keratan sulfate, etc)
•Aggrecan: 2-3 MDa, ~100 CS + ~60 KS chains
•Aggregates: 50-100 aggrecans bind hyaluronan via link protein (40-45 kDa)
•Decorin: Small (90-140 kDa), binds collagen, regulates fibril diameter

GAG Types

•Chondroitin sulfate (CS): GlcUA-GalNAc, 4- or 6-sulfate, cartilage/bone
•Keratan sulfate (KS): Gal-GlcNAc, 6-sulfate, cartilage (increases with age)
•Dermatan sulfate (DS): IdoUA-GalNAc, decorin GAG, regulates collagen
•Heparan sulfate (HS): High sulfation, basement membranes, growth factor binding
•Hyaluronan (HA): No sulfate, not protein-bound, aggregation backbone

Proteoglycan Function

•Fixed negative charge (SO₄⁻, COO⁻) attracts water via Donnan equilibrium
•Swelling pressure (0.1-0.3 MPa) provides compression resistance
•Collagen network constrains proteoglycan swelling (prestress)
•Aggrecan in cartilage: highest in deep zone (50-60 mg/mL)

Clinical Correlations

•Scurvy: Vitamin C deficiency → no hydroxyproline → unstable collagen
•Lathyrism: Lysyl oxidase inhibition (or Cu deficiency) → no crosslinks
•OI: Type I collagen mutations (glycine substitutions) → brittle bones
•OA: Aggrecanase (ADAMTS-4,5) cleaves aggrecan IGD → proteoglycan loss
•Urinary PYD/DPD: Bone resorption markers (crosslinks released)

Collagen Triple Helix

Primary structure - Gly-X-Y repeat:

Glycine every third residue (Gly-X-Y)
X position: Often proline (28%)
Y position: Often hydroxyproline (38%)
Glycine is smallest amino acid, fits in helix center
Any other amino acid at glycine position disrupts helix

Secondary structure - Left-handed helix:

Each alpha chain forms left-handed helix (polyproline II type)
3 residues per turn
Extended structure (not compact like alpha-helix)

Tertiary structure - Right-handed superhelix:

Three alpha chains wrap around each other
Right-handed triple helix
Glycine residues at central axis (every third residue)
Proline and hydroxyproline stabilize helix
No internal hydrogen bonds within chains
Hydrogen bonds between chains

Tropocollagen molecule:

300 nm length
1.5 nm diameter
Molecular weight: ~300 kDa
Three chains: Type I has two alpha-1(I) and one alpha-2(I)
Type II has three identical alpha-1(II) chains

Glycine Mutations

The triple helix structure is fundamental to collagen function and stability.

Type

Structure

Tissue Distribution

Function

Clinical Significance

Fibril-forming [α1(I)]₂α2(I)

Bone, tendon, ligament, skin, dentin

Tensile strength, structural support

OI: mutations in COL1A1, COL1A2

Fibril-forming [α1(II)]₃

Hyaline cartilage, vitreous humor, nucleus pulposus

Compression resistance in cartilage

Chondrodysplasias, early OA

III

Fibril-forming [α1(III)]₃

Blood vessels, skin, reticular fibers, healing tissue

Elastic recoil, early wound healing

Vascular EDS (Type IV EDS)

Network-forming [α1(IV)]₂α2(IV)

Basement membranes (all)

Filtration barrier, cell attachment

Alport syndrome (kidney, ear, eye)

Fibril-forming [α1(V)]₂α2(V)

Bone, cornea, with Type I

Regulates Type I fibril diameter

Classical EDS (with Type I)

FACIT [α1(IX)]α2(IX)α3(IX)

Cartilage, vitreous, with Type II

Links Type II fibrils, resists shear

Multiple epiphyseal dysplasia

Network-forming [α1(X)]₃

Hypertrophic cartilage (growth plate)

Endochondral ossification

Schmid metaphyseal chondrodysplasia

Fibril-forming [α1(XI)]α2(XI)α3(XI)

Cartilage, vitreous, with Type II

Regulates Type II fibril diameter

Stickler syndrome (with Type II)

GAG	Disaccharide Repeat	Sulfation	Location	Function
Chondroitin sulfate (CS)	GlcUA-GalNAc	4-sulfate or 6-sulfate	Cartilage, bone, tendon	Hydration, compression resistance
Keratan sulfate (KS)	Gal-GlcNAc	6-sulfate	Cartilage, cornea	Hydration, increases with age
Dermatan sulfate (DS)	IdoUA-GalNAc	Variable sulfation	Skin, blood vessels, heart valves	Collagen fibril organization
Heparan sulfate (HS)	GlcUA/IdoUA-GlcNAc	High sulfation (N, 2-O, 6-O)	Basement membranes, cell surface	Growth factor binding, anticoagulation
Hyaluronan (HA)	GlcUA-GlcNAc	None (no sulfate)	Synovial fluid, cartilage, vitreous	Aggregation backbone, lubrication

GAG	Disaccharide Repeat	Sulfation	Location	Function
Chondroitin sulfate (CS)	GlcUA-GalNAc	4-sulfate or 6-sulfate	Cartilage, bone, tendon	Hydration, compression resistance
Keratan sulfate (KS)	Gal-GlcNAc	6-sulfate	Cartilage, cornea	Hydration, increases with age
Dermatan sulfate (DS)	IdoUA-GalNAc	Variable sulfation	Skin, blood vessels, heart valves	Collagen fibril organization
Heparan sulfate (HS)	GlcUA/IdoUA-GlcNAc	High sulfation (N, 2-O, 6-O)	Basement membranes, cell surface	Growth factor binding, anticoagulation
Hyaluronan (HA)	GlcUA-GlcNAc	None (no sulfate)	Synovial fluid, cartilage, vitreous	Aggregation backbone, lubrication

Proteoglycan	Main GAG	Primary Tissue	Key Function
Aggrecan	Chondroitin sulfate, keratan sulfate	Hyaline cartilage	Compression resistance
Decorin	Dermatan sulfate	Tendon, ligament, bone	Regulates collagen fibril diameter
Biglycan	Dermatan sulfate	Bone, tendon	Collagen organization, growth factor binding
Perlecan	Heparan sulfate	Basement membranes	Filtration, growth factor reservoir

Proteoglycans and Collagen in Connective Tissue

Proteoglycans and Collagen in Connective Tissue

PROTEOGLYCANS AND COLLAGEN

MAJOR COLLAGEN TYPES

Critical Must-Knows

Examiner's Pearls

Critical Collagen and Proteoglycan Exam Points

Triple Helix Structure

Post-Translational Modifications

Proteoglycan Structure

Tissue-Specific Distribution

At a Glance

COLLAGENCOLLAGEN - Key Features

TYPESTYPES - Major Collagen Types

PROTEOGLYCANSPROTEOGLYCANS - Structure and Function

Overview

Genetic Collagen Disorders

Wound Healing and Tissue Repair

Degenerative Diseases

Biomaterial Design

Concepts and Molecular Structure

Collagen Triple Helix

Collagen Biosynthesis Steps

Collagen Synthesis and Assembly

Vitamin C Deficiency - Scurvy

Copper Deficiency - Lathyrism

Collagen Fibril Structure

Collagen Crosslinks

Major Collagen Types

Major Collagen Types in Orthopaedic Tissues

Clinical Relevance and Applications

Proteoglycan Structure

Glycosaminoglycan (GAG) Types

Major GAG Types

Major Proteoglycans

Proteoglycan Tissue Distribution

Aggrecan Aggregation

Evidence Base

Collagen Structure and Biosynthesis

Proteoglycan Structure and Function

Collagen Type I Mutations in Osteogenesis Imperfecta

Aggrecan Structure and Degradation

Basic Science Viva Scenarios

Scenario 1: Collagen Triple Helix (~3 min)

Scenario 2: Collagen Biosynthesis (~4 min)

Scenario 3: Proteoglycan Structure (~3 min)

MCQ Practice Points

Australian Context

Australian Epidemiology and Practice

Management Algorithm

PROTEOGLYCANS AND COLLAGEN

Collagen Triple Helix

Collagen Biosynthesis

Major Collagen Types

Proteoglycan Structure

GAG Types

Proteoglycan Function

Clinical Correlations

Proteoglycans and Collagen in Connective Tissue

Proteoglycans and Collagen in Connective Tissue

PROTEOGLYCANS AND COLLAGEN

MAJOR COLLAGEN TYPES

Critical Must-Knows

Examiner's Pearls

Critical Collagen and Proteoglycan Exam Points

Triple Helix Structure

Post-Translational Modifications

Proteoglycan Structure

Tissue-Specific Distribution

At a Glance

COLLAGENCOLLAGEN - Key Features

TYPESTYPES - Major Collagen Types

PROTEOGLYCANSPROTEOGLYCANS - Structure and Function

Overview

Genetic Collagen Disorders

Wound Healing and Tissue Repair

Degenerative Diseases

Biomaterial Design

Concepts and Molecular Structure

Collagen Triple Helix