Protein Chemistry
Biological functions of Proteins:
1- Enzymes
Metabolic Regulations
2- Hormones (peptide)
3- Transport proteins
-
Oxygen transporter: Hb
-
Albumin
4- Structural proteins: examples
-
Collagen (bone/ connective tissues)
-
Muscle proteins (contraction/ movements)
5- Immunoglobulins: Defense
Flow of information:
One gene
mRNA
one polypeptide chain
One protein could have more than one polypeptide chain (P.P.C.). Therefore more
than one gene is needed.
Units of P.P.C.: Amino Acids (A.As.)
-
Structural backbone of A.A.
R
O
H+3N
C
C
O-
H
α-Amino group
α-Carbon atom
α-Carboxyl group
(asymmetric/chiral)
1
-
In mammals 20 a.as exist coded for by DNA.
-
At physiological pH (~ 7.4) both the carboxyl/amino groups exist in the
dissociated form: i.e -COO-/NH+3
-
All A.As are in the L-form.
Classification of A.As:
According to the properties of the “R” group (non-polar, polar, uncharged,
acidic, basic).
1- Amino Acids with non-polar “R” group:
-
“R” group does not bind/give off protons.
-
“R” group does not participate in H/ionic bonds.
-
“R” group participates in hydrophobic interactions.
-
aliphatic non-polar:

Glycine (Gly,G): R: H is the smallest a.a. does not favor the
formation of α helix. Carbon atom is “achiral”. (symmetric)..

Alanine (Ala, A): R: CH3

Valine (Val, V): R: CH. (CH3)2

Leucine (Leu, L): R: CH2.CH.(CH3)2

Isoleucine (I-Leu, I): R: CHCH3.CH2.CH3
Gly – Ala – Val - Leu/Ileu
Increasing non-polarity/hydrophobicity.
2
- Aromatic non-polar

Phenylalanine (Phe, F): Phenyl ring bound to CH group of Ala:
CH2
O
COO-
CH
NH+3

Tryptophan (Trp, W): R: indole group
C
O
CH2
CH
+
COO-
NH3
NH
-
S- containing non-polar:
Methionine (Meth, M): R : methylated thiol group
CH3 – S – CH2 – CH2 – CH – COONH+3
Finally:

Proline (Pro, P): is an imino rather than an amino acid
because it contains an imino group (NH+2).
CH2
H2C
CH – COOH2C
+
N
H2
3
-
Because of the rigidity of its 5-membered ring, Pro residues will kink a
polypeptide chain.
(See the importance of this property in P.P.C. secondary structure)
2- Amino Acids with uncharged Polar “R” group:

Cysteine (Cys, C): “R” group is a “Sulfhydryl” (SH).
Functions of SH group:
(1)
SH groups can bind to one another to from the disulfide bridges
that stabilize the structure of proteins
(2)
SH group presents in many enzymes: modulation of their
enzymic activities.

SH-CH2-~
Tyrosine (Tyr, Y): aromatic/polar “R” group (Hydroxy-Phe): “OH”
group makes it polar.
Both Cys and Tyr can lose a proton at an alkaline pH.

Serine (Ser, S): “R” group: CH2. OH

Threonine (Thr, T): “R” group: CH.CH3.OH
Ser, Thr and Tyr contain polar “OH” which can participate in
“hydrogen bond formation” and site of phosphate group attachment
(regulation of enzyme activity; glycoproteins: attachment of
oligosaccharide chains).

Asparagine (Asn, N): an amide of Asp. “R” group: contains a
carbonyl and an amide group that neither protonate nor dissociate.
However, can participate in H-bonds.
4
H2N – C – CH2 – CH – COO-
Amide group
O
NH+3
Carbonyl group
-
Glutamine (Gln, Q): an amide of Glu.
H2N – C – CH2 – CH2 – CH – COOO
NH+3
Properties: similar to “Asn”
3- Amino Acids with acidic and Polar “R” group:
_
 Aspartic acid (Asp, D) and Glutamic acid (Glu, E)
β
Ɣ
α
_
α
β
OOC – CH2 – CH – COOOOC – CH2 – CH2 – CH - COO
NH+3
NH+3
Both Asp and Glu “R” groups bear a negative charge at neutral pH (7.0)
because they are strong proton donor.
Therefore, are called aspartate
glutamate.
4- Amino Acids with basic and Polar “R” group:
The “R” group in this class can accept protons.

Arginine (Arg, R) :
H2N – C- NH – (CH2)3 +

-
Lysine (Lys, K) :
NH2
H3+N – (CH2)4 –
At physiologic pH, the “R” group of Arg/Lys are fully ionised and
positively charged.
5

Histidine (His, H): “R” group: imidazole group
C – CH2 – CH – COO-
HC
+
HN
+
NH
NH3
C
H
Imidazole group
His is weakly basic and largely uncharged at physiologic pH. In P.P.C it can be either
positively charged or neutral depending on the ionic environment close to it.
Therefore, it is important in enzyme regulation and proteins function (e.g. Mb).
Detection and Quantitation of A.As
Ninhydrin (dye) reacts with the free α-NH+3 groups of amino acids to produce
a purple color (Pro produces yellow color).
Note:
-
All amino acids are optically active with the exception of Gly. Why?
-
All amino acids found in proteins are in the L-configuration.
-
Amino acids in aqueous solution have weak acid/base properties.
-
Amino acids can act as buffers (free a.as. / R groups in P.P.C.).
-
The numbers of “titratable” groups in “neutral” amino acids is” 2 (COOand NH+3).
e.g. Ala:
H
+
OH- H2O
H
H3N – C – COOH
CH3 (I)
OH- H2O
H+3N – C – COOCH3
H+
(II)
Ala in pH ~ 6.0
net charge = 0 (Isoelectric
point).
H+
Ala in pH < 2.0 PK,=2.3
Net charge = +1
6
H
H2N – C – COOCH3 (III)
PK2 = 9.1
Ala in basic soln. pH > 10.0 net charge = -1
Henderson – Hasselbalch equation
pH = pK1 + log [II]
[I]
pH = pK2 + log [III]
[II]
Buffer pairs: COOH/COOBuffer pairs: NH+3/H2N
When pH = pK1 (2.3), [II] = [I], pH = pK2 (9.2), [III] = [II].
The Isoelectric point (pI): At neutral pH, alanine exists in the dipolar form
where the net charge = 0. It follows the pI is the pH at which an a.a is
electrically neutral.
-
The number of “titratable” groups in acidic and basic amino acids are: 3.
-
All amino acids at physiologic pH are dipolar (amphoteric) and are
termed: ampholytes.
Structure and properties of polypeptides and proteins:
Levels of protein structural organization:
1- Primary.
2- Seconday. 3- Tertiary.
4- Quaternary.
1- Primary structure of Proteins:
Is designed as the number of amino acid residues and their sequence
(relative positions) in the P.P.C./protein molecule.
The primary structure of P.P.C. is generated by peptide bonds.
7
A. Peptide bond:
A type of amide linkage between the α-COO- of one a.a. and the αNH+3 of another:
+
H3N – CH – COO- + H3N – CH – COOR1
R2
H2O
O
+
H3N – CH –
C–N
H
– CH – COO-
R1
R2
Peptide bond
Peptide bond is covalent with partial double-bond properties that make it rigid and
prevent the adjacent group from rotating freely. C = O / N – H in the peptide bond
cannot dissociate.
C
H+3N
C
α
C
H
R
H
C
N
H
O
H
N
R
C
α
C
C
R
H
H
R
C
N
H
C
OH
C
Cα – N and Cα – C can freely rotate.
Therefore, a large number of angular
O
relationships may exist between planar units. The magnitude of Cα – N and Cα – C
angles defines the precise spatial relationship between planes.
The size and nature of “R” groups limit the magnitude of these angles.
8
O
O
The peptide bond is a trans bond, so is Cα, because does not cause steric interference
of the “R” groups while cis position does.
The C = O, and N – H groups of the peptide bond are not charged but polar to be
involved in H-bonds in α-helix and β-sheet structures.
Order of A.As and Naming of P.P.C
The free amino end (N-terminus)- left of P.P.C
The free carboxylend (C-terminus) –right of P.P.C.
(N – terminus) H+3N COO- (C – terminus)
- Amino acids are read from left to right.
Naming of P.P.C: - ine, - an, -ic/ate amino acids changed to –yl with the exception of
C-terminus. e.g. Val-Gly-Glu-Lys-Arg
“Valylglycylglutamyllysylarginine”
Determination of the Primary Structure:
1- Amino acid composition:
-
Prepare a pure/dry sample of P.P.C.
-
Determine M.Wt. (Methods?)
-
Hydrolysis of peptide bonds: (a) acid hydrolysis (heating at 110ºC for
24hrs in 6 N HCl). (b) alkaline hydrolysis (heating in NaOH).
Acid hydrolysis destroys: Trp, Cys and partially destroys Ser, Thr and Tyr.
Alkaline hydrolysis destroys: Cys, Ser, Thr. But does not damage Trp.
-
Amino acid analysis of P.P.C “hydrolysate” by “automated amino acid
analyser”. Each a.a. is eluted from ion-exchange column at a characteristic
pH and quantitated spectrophotometrically after addition of ninhydrin. A
second sample of P.P.C “alkaline hydrolysate” is run to determine Trp
content.
9
-
The exact no. of a.a. residues is calculated based on the P.P.C. M.Wt.
2- Amino acid sequence:
Edman’s reaction (50 a.a. residues and less):
- Phenylisothiocyanate combines with N-terminal a.a. residue under mild alkaline
conditions.
-
The N-terminal peptide bond is cleaved by acid hydrolisis yielding a
phenylthiohydantoin (PTH)- amino acid.
-
PTH – a.a. is identifed by chromatography.
-
The P.P.C is shortened by one a.a each cycle.
To
analyze
P.P.C.
>
50
a.a.
residues:
cleave
into
shorter
fragments
chemically/enzymatically.
* CnBr – cleaves peptide chains at carboxyl end of Meth.
* Trypsin – cleaves peptide bonds at carboxyl end of Arg/Lys.
*
Chymstrypsin - cleaves peptide bonds at carboxyl end of Phe/Tyr/Trp (less
specific).
The resulting fragments are separated by HPLC/mass spectrometry; analyzed and
sequenced.
New method: inject the whole peptide into mass spectrometer and read.
Sequencing of proteins composed of 2 or more P.P.Cs :
- cleave non-covalent bonds between chains by denaturing agent (e.g. 8 M Urea)
- each chain is then separated, analyzed and sequenced.
Separation of P.P.Cs containing S-S bonds:
- use performic acid to break S-S bonds.
10
Determination of primary structure by DNA sequencing
- sequence the coding region of DNA of P.P.C. using polymerase chain reaction
(PCR).
- read 3 bases (genetic code) for each a.a.
*limitations: - (1) does not predict positions of S-S bonds in P.P.C.
- (2) does not identify any a.a. modified post translationally.
2- Secondary structure of Proteins:
Definition: The spatial regular arrangements of amino acid residues close to one
another in the linear sequence of a P.P.C.
Types: α- helix, -pleated sheet, random coil, triple helix and -bends.
A. The α – helix
It is a spiral structure that forms spontaneously. Tight packed, coiled
polypeptide backbone core with the “R” groups extending outwards. Each turn in the
spiral contain 3.6 a.a. residues. The helix is stabilized by the weak H-bonds between
CO and NH group situated four residues ahead in the P.P.C. The S-S bridges stabilize
the α - helix as seen in fibrous proteins of hair and skin.
Examples: Keratin – 100% α - helix (rigid)- 2/more α helices combine.
Hemoglobin – 80% α - helix (flexible)
α - helix could be: left or right- handed , but the most stable is the : Right – handed
(natural).
11
Factors Affect the Stability of α - Helix:
1- Proline: The presence of “Pro” disrupts the α - helix and forms a “kink” which
change P.P.C growth by 90. This is because the imino group (part of the
ring structure) is incapable of forming a resonating peptid bond.
Secondly, the ring structure of “Pro” “R” group is incompatible with the α
helix.
2- The presence of large number of charged a.a resudies e.g. Asp,Glu,Lys,Arg,His.
Ionic bonds or electro-static repulsion disrupts α - helix (Lys+ - Arg+ Asp –
Glu- , Lys+ - Asp-, Arg+ - Glu- )
3- Glycine: is a non -α- helix forming a.a because of its small size.
4- The presence of a.a residues with bulky “R” group (Trp/Val/Lleu/I-leu). All cause
steric interference and therefore disrupt the α- helix.
B. The  - Pleated Sheet
The P.P.C is almost fully extended (c.f. α- helix: tight coil). All CO/NH groups of
peptide bonds are involved in H- bonding. The surfaces of the  - sheets appear:
pleated. The - pleated sheet is composed of 2 or more P.P.Cs. or segments of P.P.C
(fully extended). The H- bonds are perpendicular () to P.P. backbone (axis) (c.f. α helix are parallel to P.P. backbone). The H-bond could be: interchain (between 2
P.P.Cs.) - or intrachain (between 2 segments of the same folded P.P.C).
Types: Parallel: The 2 P.P.Cs. / segments run in the same direction (side by side), so
their N-termini are at the same end:
12
N
N
C
N
C
C
Anti – Parallel: The 2 P.P.Cs. / Segments run in the opposite direction (The Nterminus of one is next to the C-terminus of the other).
C
N
N
C
E.g α- chymotrypsin has extensive  - pleated sheets with only 2 small regions of α helix.
C. Random Coil (Non-repetitive Secondary Structure):
Less regular structure that has a loop or coil conformation. There is no
consistent relationship between planes and therefore H - bonds are not regular. The
random coil structure is found in globular proteins.
Random Coil
13
D.  - Bends (Reverse turns)
-
A sequence of 4 a.a. residues (Pro/Gly frequently found) that joins (connects)
successive strands of anti – parallel - pleated sheets. It is stabilized by H-and
ionic bonds.
Function: It reverses the direction of P.P.C in order to form 3-dimensional structure.
E. Triple – helix : See collagen
3. Tertiary structure: (essential for biological activity of the P.P.C)
Definition: The spatial arrangement of a.a residues widely separated in the linear
sequence of a P.P.C.
1  2  3 .
Features: 1- Hydrophobic side chains (R-groups) are buried in the interior.
2- Hydrophilic groups generally found in the exterior (surface) of
globular proteins.
3- All interior hydrophilic groups (including CO /NH groups of peptide
bonds) are involved in H- bonds or electrostatic interactions.
This ensures the total exclusion of water from interior (functional site
of protein).
The factors stabilizing the tertiary Structure
The partial double-bond of peptide bonds restricts the folding of P.P.C.
However, the 3-dimensional structures may be stabilized by:
14
1- Disulfid bonds:
Covalent linkage between 2 SH from Cystein  Cystine. Types: intrachain or
interchain. They confer stability to tertiary structure. S-S bonds found in secretory
proteins (prevent denaturation in the extracellular environment).
2- Hydrophobic interactions:
The size and nature of R- groups help to govern tertiary structure. Small size “R”
groups allow tight folding of P.P.C, whereas bulky “R” groups prevent close
approach of other groups.
In globular proteins: a.a residues with non-polar “R” groups tend to be buried in the
interior of P.P.C. The stability of the tertiary structure is enhanced by the
“hydrophobic” interactions of these “R” groups. Whereas, “polar” “R” groups are
located in the surface (reason for globular protein solubilites, e.g. plasma proteins).
In cell membrane proteins: the “non-polar" “R” groups are located on the surface of
the molecular in contact with the hydrophobic lipid bi-layer of cell membrane.
3- Hydrogen bonds:
Amino acid residues containing OH/NH in their “R” groups can form H-bonds
with COO- or C = O of peptide bonds. Also, H-bonds exist between polar groups on
the surface of protein molecule and water. This enhances the solubility of protein
molecule.
4- Ionic interactions:
-COO- (Asp/Glu) interacts with –NH3 (Lys,Arg). The presence of like charged
a.a residues decreases the 3-dimensional structure of the protein molecule.
Note: Natural folding of P.P.C
(1) occurs during synthesis. This ensures only one type of folding.
15
(2) Chaperons (special types of proteins) ensure proper folding of P.P.C by
increasing the rates of final stage in folding process. Many proteins
contain chaperons “signals” (specific a.a. sequence).
4. Quaternary structure:
Definition: It is the arrangement of P.P.Cs in relation to one another in
multiple-chains (subunits) protein (multi-meric). The subunits are linked by “noncovalents” interactions such as H-bonds, electrostatic bonds (ionic), and
hydrophobic interaction. The subunits may function independently or cooperatively
(e.g Hb).
Denaturation of Proteins
It is apparent that the 1º, 2º, 3º structures are the basis for the chemical,
physical, and biological properties of proteins. When normal conformation is altered
(unfolding and disorganization) but not accompanied by a rupture (hydrolysis) of
peptide bonds, the protein is denatured, and its distinctive properties and biological
activity may be changed.
Forms of denaturation: (a) unfolding of 3º-sturcture. (b) 2º structure change to
varying degree of random coil. (c) S – S bonds cleavage.
Denaturaion is always irreversible, but some cases may be reversible (i.e.
when limited unfolding and refolding occurs).
Denaturating agents: (a) Temperature (50 – 60º C). (b) Extremes in pH (< 4, >10).
(c) High conc. of organic solvents e.g. alcohol, acetone, ether and urea or guanidine –
HCl. (d) Detergents. (e) Heavy metal ions e.g. Pb, Hg.
Reversal of denaturaturation is renaturation (e.g. Ribonucleese)
16
8 M urea – 2 – Mercaptoethanol (2M.E.)
“Denaturaion”
“Renaturation”
Dialysis (- 8 M urea/2M. E.)
Air oxid. to reform S – S bonds
Native P.P.C. (Lowest energy, greatest
stability and biologically active)
Structure-Function Relationship of Proteins (Conjugated/Simple proteins)
1- Globular Hemeproteins: Examples: Cytochromes (oxid/red.),catalase (breaking
down H2O2), Mb/Hb (reversible O2 carriers). The heme part is tightly bound
(Prosthetic group). Hemeproteins is an example of conjugated proteins.
Structure of Heme:
- Conjugated ring system of protoporphyrin IX +Fe2+.
- 6 coordinations of Fe2+Fe 2+ - bonded to “4N” of porphyrin rings
- in Mb/Hb : one bond is formed by coordination to “R” group of His
(proximal). The other is available for O2 – binding close to “His” (distal).
Permenant oxidation at the 6th coordination leads to the formation of “hemin”
(Fe3+) and Mb/Hb are converted to: “metmyoglobin and methemoglobin (cannot bind
to O2; contain H2O at 6th coordination). Alleviation of this oxidation by “RBC”
NADH-cytochrome b5 - reductase.
17
Structure and Function of Myglobin: (Mb)
Site: Heart and skeletal muscle.
Function: Reservoir of O2 and O2 carrier.
Structure: Single P.P.C (similar to Hb α/ chain).
1˚- Structure : 146 a.a residues.
2˚- Structure: ~80% of its P.P.C is in the α- helix (8- Segments joined by non-helical
random coil strucures). The segments labelled A-H, terminated by
either Pro or β- bends and loops.
3˚- Structure: - Non-polar a.a. residues found almost entirely in the interior of Mb
molecule, packed closely together and stabilized by hydrophobic
interactions.
-charged a.a residues found almost exclusively on the surface,
forming H-bonds with H2O.
Heme (Prosthetic group): in the hydrophobic pocket of Mb. His (proximal): binds
directly to Fe2+, while distal “His” does not, but helps stabilize the binding of O2 to
Fe2+.
Therefore, microenvironment is ideal for heme to reversibly bind to O2.
Structure and Function of Hemoglobin: (Hb)
Site: Red blood cells (RBCs)
Function: Transport of O2 from lungs to tissues.(CO2 from lungs to tissues).
18
Types: Adult Hb (HbA) : major type (2 α + 2β P.P.Cs. ).
Structure: 1˚, 2˚, 3˚, similar to Mb.
Quaternary structure: Hb dimers (2 α β) held together by hydrophobic
interactions.(α1 β2 contact). Ionic, H-bonds and hydrophobic interactions stabilize the
α β structure (α1- β1 contact). α1- β2 contact is less stronger than α1-β1 contact,
making the dimers able to move with respect to each other.
Tense (taut ) and relaxed forms of Hb:
-
α1 β2 contact: all ionic and H-bonds are involved making the dimer molecule
movement with respect to each other very difficult.
This is the deoxygenated form of Hb and has low oxygen affinity. Also termed
tense or taut: T-form
-
In the relaxed form, (R) : some of the ionic and H-bonds are broken in α1β2
contact of Hb dimers. The “R” forms occur when O2 binds to Hb.more freedom of
movement of Hb dimers at the α1β2 contact. The “R” form has high oxygen
affinity.
Oxygen Binding to Mb and Hb:
-
Oxygen dissociation curves:
Mb: One site available for O2 binding
simple equilibrium binding
Mb + O2
MbO2
- P50 = 1 mm Hg. Shape: Hyperbolic
- Higher O2 affinity in tissue than Hb. Hence
acts as O2 acceptor from Hb. During muscular exercise:
it serves as O2 donor.
Hb: Cooperative O2 binding (4 binding sites). Hence, shape is sigmoidal.
19
-P50 ~26 mmHg.
Comments on: Heme-Heme Interactions:-
Binding of the first O2 to “α” chain of Hb is difficult. Once happens it breaks
some of the ionic and H- bonds in “α1β2” contact. This makes the second O2
binding easier at the second heme, and so on. This is known as heme-heme
interactions (i.e binding of O2 at one heme site facilitates the binding of O2 on the
other site).
Sigmoidal O2 dissociation curve of Hb:
-
Very efficient O2 carrier from lungs (PO2 ~40 mmHg), and O2 deliverer to tissues
(PO2 < 40 mmHg).
Factors affecting “O2” binding of Hb (Allosteric efectors):
General rule: Any substance that increases the stability of the “T” form of Hb will
decrease the “Hb” affinity for O2.
Examples: 1- CO2: Hb – NH2 + CO2
Hb – NH – COO- + H+
(Lowers Oxygen affinity of Hb).
20
2- pH:  pH (in tissues) (  H+: increases “T” form stability)
(lowers O2 affinity: therefore, shift of d-curve to right).
 pH (shift of d-curve to left). e.g in lungs
This is called the “Bohr Effect” (change of O2 binding due to increase in [H+](pH).
Significance of Bohr effect: allows the unloading of O2 from HbO2 in metabolically
active tissue where H+ is high.
HbO2 + H+
HbH+ + O2
3 - 2, 3 – bisphosphoglycerate (2, 3 – BPG): It forms ionic bonds with negatively
charged a.a residues located between the 2β globin chains. Hence, stabilizing the
deoxy-Hb structure, which decrease its oxygen affinity. Shifting the d-curve to the
right.
-
2, 3 - BPG found in RBCs. at 1:1 (2,3-BPG:Hb)
-
[2,3 - BPG] “increase” in response to hypoxia. Example: high altitude and
emphysema.
-
Stored blood: add inosine to prevent 2, 3-BPG depletion.
-
HbF(2α + 2Ɣ): has a higher O2 affinity than HbA: because weaker interaction with
2, 3-BPG leading to decrease stability in the “T” Hb form. Therefore increasing its O2
affinity.
Other minor Hbs:
- HbF (Fetal Hb): > 60% of Hb in fetal /newborn life, ~ 2% in adult life. (2α + 2Ɣ
chains)
- HbA2: 2% of total Hb in adult. (2α + 2δ chains)
- HbA1c: glycosylated Hb (diagnostic in D.M).
21
Genetics: 2 α genes located on chromosome 16, -plus α – globin like genes.
1β + β globin like genes at chromosome 11.
A. Fibrous Proteins
Physical properties:
* Simple structure (primary and secondary) specific to each fibrous protein (c.f.
globular proteins: complex).
* Water insoluble.
Function: Structural (mecahnical):
Collagen and elastin : Skin, conective tissue, sclera cornea, blood vessel walls.
Keratin: skin and hair.
Collagen (extracellular protein):
- Most abundant protein in human body.
- Rigid, and insoluble proteins.
- Location: throughout the body.
- Shape: differ according to tissue e.g. gel (stiffen structure: e.g. vitreous humor), fibers
(provide strength: e.g. tendon), stacks (transmission of light, as in cornea). Collagen of
bone as highly organized fibers at an angle to each other.
- Cellular site of synthesis:
Fibroblasts (e.g skin), Osteoblasts (bone) Chondroblasts (cartilage).
-
Collagen synthesis:
pre-collagen
enzymes
polymerization
Mature collagen
monomers aggregates and cross-linked fibrils.
Collagen Structure
22
collagen
A: Types of collagen: three polypeptide chains of the “α” chains which differ in
sequence slightly.
-
Amino acid sequence: Glycine is found in every third position of the P.P.C (fits in
restricted space where three chains meet). (Gly-X-Y) n. n= 333
X – frequently is Pro, Y: often OH – pro /OH-lys.
-
Secondary Structure:
Triple helix: R groups extending outside the molecule (P.P.C.) allow
interactions between the triple-heilcal molecules
(monomer)
-
aggregates
fibers.
Hydroxylation and Glycosylation:
Hydroxylation of pro/Lys occurs after synthesis of the pro-collagen (posttranslational modification Hydroxylation is important for the stabilization of
the triple helix.
OH-Lys may be glycosylated prior to the triple helix formation (glucose or
galactose).
OH-Lys
+
OH (glu)
Lys
o
Lglu/gal
glycosidic link
Hydroxylation requires prolyl/Lysyl hydroxylase, α KG, and ascorbate (vit.C).
-
Cross-linking: OH-Lys/Lys by OH-Lysl/Lysl oxidase (oxidative deamination):
allysine/OH-allysine
Fibrils (tensile).
Allysine (reactive aldehyde) reacts with lysine/OH-lysine.
Most abundant types of collagen
Type
I
Chain Composition
α1 (I)2 α2 (I)
23
Location
Skin, bone, tendon,blood
vessel walls, cornea
II
[α1 (II)]3
cartilage, disk, vitreous
humor
III
[α1 (III)]3
Blood vessel walls, fetal
skin
IV
α1 (IV)2 α2 (IV)
Basement membrane
24