PROTEINS i. FIBROUS PROTEINS Collagen Elastin Keratin ii. GLOBULAR PROTEINS Myoglobin Hemoglobin.
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Transcript of PROTEINS i. FIBROUS PROTEINS Collagen Elastin Keratin ii. GLOBULAR PROTEINS Myoglobin Hemoglobin.
PROTEINSi. FIBROUS PROTEINS
CollagenElastinKeratin
ii. GLOBULAR PROTEINSMyoglobin
Hemoglobin
STRUCTURE Triple helix Has 3.3 residues per
turn and a rise per residue nearly twice that of an -helix
Every 3rd amino acid residue is a glycine residue
Gly-Pro-X or Gly-Hyp-X
COLLAGEN
Hydroxyproline (Hyp) Result from
hydroxylation of proline residue
Stabilizes the triple helical structure of collagen
Ascorbate
Biosynthesis of Collagen
Biosynthesis of Collagen
1. Synthesis of a chains of pre-procollagen on ribosomes. A signal protein directs them to the RER .
m-RNA
Signal protein
2. Cleavage of signal protein forms procollagen
3. Hydroxylation of lysine and proline
OHOH OH
OH OHOH
Ascorbic acid is necessary to activate the hydroxylases
Lysine Hydroxylysine Peptidyl lysine hydroxylase
Proline Hydroxyproline
Peptidyl proline hydroxylase
4. Glycosylation•Addition of galactose and glucose to some hydroxylysine residues. •The enzymes galactosyl transferase and glycosyl transferase are required for this process.
OH OH OH
OH OHOHOH
5. Assembly of three - chains to form procollagenThis involves the formation of disulfide bonds between parts of the polypeptide chains known as registration peptides, which occur at both ends of the pre-procollagen.
SS
Registration peptides
SS
SS
SS
SS
SS
•The assembly process aligns the three a - chains relative to one another. •The three alpha chains are wound around one another in the form of a triple helix. •The assembly process occurs in the Golgi apparatus.
6. Secretion of procollagen molecules by exocytosis into the extra cellular space
7. Cleavage of registration peptides
•Occurs in the extra cellular space, and is catalysed by procollagen peptidases. •The resulting molecule is called tropocollagen.
Procollagen peptidase
Procollagen peptidase
8. Self-assembly or polymerization of tropocollagen molecules form collagen fibrils
• Cross-linkage between adjacent tropocollagen molecules stabilizes the fibrils. • It involves the removal of an amino group (NH2), which has a net oxidative effect and the formation of covalent cross-links.• It is catalyzed by lysine oxidase (or catalase).
Ehlers-Danlos Syndrome
Inheritable defects in collagen molecule
Characterized by stretchy skin and loose joints
Due to defect in genes that encode -collagen-1, procollagen N-peptidase, or lysyl hydroxylase
COLLAGEN DISEASESOsteogenesis
imperfecta Brittle bone
syndrome Bones easily
bend and fracture
Menke’s syndrome Characterized by kinky hair
and growth retardation Due to dietary deficiency
of copper required by lysyl oxidase, which catalyzes a key step in the formation of the covalent cross-links that strengthen collagen fibers.
Less than 1 in 1,250,000 children will be born with Menkes Disease, otherwise known as Kinky Hair Syndrome.
Menkes is a genetic disorder caused by a mutation usually primarily in the Y Chromosome (boys) it affects the copper levels and metabolism in the body causing seizures, brain damage, weakened bones and muscles, organ shutdown and failure to thrive.
There is no cure and unless caught within days of birth medication and treatment is only symptomatic.
Children with the disease will die before their first decade of life, most dying within infancy or toddlerhood.
COLLAGEN DISEASES
Scurvy Best known defect in
collagen biosynthesis Deficiency of Vit.C
(required by prolyl and lysyl hydroxylases)
Bleeding gums, swelling joints, poor wound healing, and ultimately death.
Elastin
•Connective tissue protein with rubber-like properties
•Found in lungs, walls of large blood vessels, and elastic ligaments
•Can be stretched to several time their normal length, but recoil to their original shape when relaxed
STRUCTURE OF ELASTIN
Composed primarily of small non polar amino acid residues (e.g. G, A, V)
Also rich in proline and lysine, but contains little hydroxyproline and hydroxylysine
Interchain cross-links form desmosine residues
DESMOSINE CROSS-LINK
An extensively interconnected, rubbery network that can stretch and bend in any direction when stressed, giving connective tissue elasticity
Role of -1 antitrypsin in elastin degradation
Inhibit neutrophil elastase (protease that degrades elastin of alveolar walls)
Deficiency of -1 antitrypsin leads to destruction of the alveolar walls of the lungs resulting to EMPHYSEMA
Treatment :
Administration of -1 AT
Role of -1 antitrypsin in elastin degradation
As we breath, we not only take in oxygen, but we also take in our environment.
Pollen, sawdust, car exhaust, dander, paint fumes and over-spray, radon gases, hair sprays, perfumes, household cleansing fumes, tobacco smoke and many other pollutants
Elastase - the lungs natural defense against the irritants
Elastase (shown as white dots) attach themselves to the foreign material in the sac and consumes it as well as bacteria in the lungs
Role of -1 antitrypsin in elastin degradation
Once the lungs has been cleaned, antitrypsin deactivates elastase
Role of -1 antitrypsin in elastin degradation
Without the antitrypsin enzyme, the elastase continues to consume anything in its path, including healthy lung tissue
Role of -1 antitrypsin in elastin degradation
As the healthy lung (sac) tissue is destroyed, the once stretchy walls of the sac become stiff and enlarged with air.
The enlarged sacs no longer have the ability to exchange oxygen and carbon dioxide with the bloodstream.
The lungs poor elasticity causes the lungs not to deflate properly trapping air leading to over-inflation of the lungs.
This is known as emphysema.
Role of -1 antitrypsin in elastin degradation
Pathogenesis of Empysema
- KERATIN
Proteins that forms tough fibers Found in the hair, nails, and outer
epidermal layers of mammals Constituents of intermediate filaments
of cytoskeleton in certain cells Rich in Cys, -S-S- cross-links between
adjacent polypeptide chains resulting to fibers that are insoluble and resistant to stretching
GLOBULAR HEMEPROTEINS
Group of specialized protein that contains heme
Maintain a supply of oxygen essential for oxidative metabolism
MYOGLOBIN HEMOGLOBIN
HEME Fe+2- protoporphyrin
IX A cyclic tetrapyrrole A planar network of
conjugate double bonds absorbs visible light and colors heme deep RED.
MYOGLOBIN (Mb) A monomeric protein
(153 amino acid residues) of the red muscles
Used in some tissues, notably muscle, – as a storage reserve of
O2 and
– for intracellular transport of O2
Myoglobin Structure– 78% helical (the other 22%
in turns and short loops, no sheet at all)
– 8 - helices designated by letters A-H, in an N to C terminal direction, and connections between helices are referred to as "AB", "CD", etc.
– Polar amino acid residues are found at the surface
– Non polar amino acid residues (L, V, F, & M) are found at the interior except His E7 and HisF8
The heme of myoglobin lies in crevices of helices E and F
Distal histidine
Proximal histidine
Myoglobin structure
HEMOGLOBIN Found exclusively in
red blood cells Its main function is to
transport oxygen from the lungs to the capillaries of the tissues
The oxygen-binding properties of hemoglobin are regulated by allosteric effectors
Hemoglobin Heterotetramer: 2 -subunits (gray and
light blue) + 2 - subunits (pink and dark blue)
(NOTE: designation of individual polypeptide chains with greek letters has nothing whatever to do with their
secondary structural elements!)
Subunit composition of principal hemoglobin
22 - Hemoglobin A (HgA) is the major hemoglobin in adults
22 - Fetal hemoglobin (HbF)
2S2 - Sickle cell hemoglobin (HbS)
22 - Minor adult hemoglobin (HbA2) Myoglobin and the polypeptide of
hemoglobin A have almost identical secondary and tertiary structures
Quaternary structure of hemoglobin
Hgb tetramer is composed of two identical dimers, ( )1 and ()2, dimers 1 and 2 respectively
Dimers are held together by hydrophobic interactions
Ionic and H-bonding also occur between the members of the dimer
Conformational Changes resulting from oxygenation
and deoxygenation of hemoglobin
T form or “Taut” (tense) form
Deoxy form of Hgb Two dimers
interact through a network of ionic bonds and hydrogen bonds
Low oxygen-affinity form of hemoglobin
Conformational Changes resulting from oxygenation
and deoxygenation of hemoglobin
R form or Relaxed form Binding of oxygen to Hgb
(oxyhemoglobin) Some ionic bonds and H-
bonds between the dimers are ruptured thus, the polypeptide chains have more freedom of movement
High oxygen-affinity form of hemoglobin
Shift from deoxy to oxy conformation
Fe lies out of plane of heme ring
Fe moves into plane of heme ring
OXYGEN BINDING TO MYOGLOBIN
Can bind only one molecule of oxygen (one heme group only)
The oxygen dissociation curve for Mb is a hyperbolic shape
Mb reversibly binds a single molecule of oxygen
Mb + O2 MbO2
HEMOGLOBIN 4 O2 binding sites
per molecule The oxygen
dissociation curve for Hgb is sigmoidal in shape
Subunits cooperate in binding oxygen
OXYGEN BINDING TO MYOGLOBIN
Cooperative binding of Oxygen
It means that the binding of an oxygen molecule at one heme group increases the oxygen affinity of the remaining heme group in the same Hgb molecule
This is referred to as heme-heme interaction
Cooperative Interactions (allosteric interactions)
[Greek: "allos"="other", "stereos" = space] occur when binding of one ligand at a specific site is influenced by binding of another ligand, which is called an "allosteric effector" or "modulator"
The second site is also called an allosteric site
Homotropic interaction/effect – the interacting sites all bind the
same ligand (e.g., binding of O2 at one site on Hb influences the binding affinity for O2 of another site)
Heterotropic interaction/effect – the interacting sites bind
different ligands
Cooperative Interactions (allosteric interactions)
Homotropic Interaction/Effect
– Positive homotropic effect – the homotropic effector increases
the binding affinity for the same kind of ligand at other sites
– O2 in the hemoglobin system increases the O2 binding affinity of other sites
Heterotropic Interaction/Effect
– Negative heterotropic effector or allosteric inhibitor the effector decreases the binding affinity for the
primary ligand protons, or CO2, or 2,3-BPG; all are negative
heterotropic effectors of O2 binding to hemoglobin
– Positive heterotropic effector, or allosteric activator effector increases the binding affinity for the
primary ligand
The Bohr Effect- effect of binding of protons (H+)
and CO2 on O2 binding affinity of Hb
HbO2 + H+ HbH+ + O2
Oxyhemoglobin
Deoxyhemoglobin
Increase protons or a lower pO2
Decrease protons or an increase pO2
Binding of Carbon dioxide
CO2 is a negative heterotropic effector (allosteric inhibitor) of O2 binding to Hb
Presence of CO2 in the tissues reduces affinity of Hb for O2 (favors deoxy, T state) in two ways: 1. CO2 lowers the pH (Bohr effect) 2. CO2 participates in formation of carbamates by the N-terminal a-amino groups of Hb:
– Formation of carbamate releases H+, which contributes to the Bohr effect
– Carbamate formation (CO2 binding) favors the deoxy state.
Binding of CO
CO binds tightly (but reversibly) to the Hb iron, forming carbon monoxyhemoglobin, HbCO
CO binding to one or more of the 4 heme sites of Hb shifts to relaxed conformation
The affinity of Hb for CO is 220X greater than for oxygen
Binding of CO
60% of HbCO are fatal
CO poisoning is treated with Oxygen therapy, which facilitates the dissociation of CO.
2,3-bisphosphoglycerate (2,3-BPG) or "2,3-diphosphoglycerate" (DPG)
Most abundant organic phosphate in the red blood cells
Binds strongly to the deoxy form of Hb (T state), but only very weakly to oxy form (R state)
2,3-bisphosphoglycerate (2,3-BPG)
or "2,3-diphosphoglycerate" (DPG)
Favors/stabilizes the T form, reducing the O2 binding affinity of Hb (shifts binding curve to the right)
Increased concentration of 2,3-BPG, reduce affinity of hemoglobin for O2which increases the efficiency of O2 delivery (enhances RELEASE) to tissues
2,3-bisphosphoglycerate (2,3-BPG) or "2,3-diphosphoglycerate" (DPG)
Increase concentration of 2,3-BPG is observed in:
•Chronic hypoxia (observed in Obstructive Pulmonary Emphysema)
•High altitudes
•Chronic anemia
MUTANT HUMAN HEMOGLOBINS
METHEMOGLOBIN
– The heme iron is ferric than ferrous – Oxidation of ferrous to ferric is caused by the
side effects of drugs such as sulfonamides, or endogenous substance like hydrogen peroxide
– Can neither bind nor transport oxygen HEMOGLOBIN M
– HisF8 has been replaced by tyrosine HEMOGLOBIN S
– V has replaced Glu6 of the -subunit
BIOMEDICAL IMPLICATIONS
MYOGLOBINURIA – Dark red coloration of urine following
massive crush injury because myoglobin is released from damaged muscle fibers
A N E M I A S– Reduction in the number of RBC or of Hb
in the blood, or impaired production of erythrocytes (Vit.B12 deficiency)
BIOMEDICAL IMPLICATIONS
THALASSEMIA– Partial or total absence of one or more (-
thalassemia) or (-thalassemia) chains of hemoglobin
GLYCOSYLATED HEMOGLOBIN (HbA1c)– Glucose enters the erythrocytes and
glycosylates the -group of lysine residues and the amino terminal of Hb
– Reflects the mean blood glucose concentration thus provides valuable information for management of Diabetes Mellitus