PROTEINS i. FIBROUS PROTEINS Collagen Elastin Keratin ii. GLOBULAR PROTEINS Myoglobin Hemoglobin.

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PROTEINS i. FIBROUS PROTEINS Collagen Elastin Keratin ii. GLOBULAR PROTEINS Myoglobin Hemoglobin

Transcript of PROTEINS i. FIBROUS PROTEINS Collagen Elastin Keratin ii. GLOBULAR PROTEINS Myoglobin Hemoglobin.

Page 1: PROTEINS i. FIBROUS PROTEINS Collagen Elastin Keratin ii. GLOBULAR PROTEINS Myoglobin Hemoglobin.

PROTEINSi. FIBROUS PROTEINS

CollagenElastinKeratin

ii. GLOBULAR PROTEINSMyoglobin

Hemoglobin

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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

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Hydroxyproline (Hyp) Result from

hydroxylation of proline residue

Stabilizes the triple helical structure of collagen

Ascorbate

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Biosynthesis of Collagen

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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

  

  

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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

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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

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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.

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6. Secretion of procollagen molecules by exocytosis into the extra cellular space

   

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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

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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).

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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

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COLLAGEN DISEASESOsteogenesis

imperfecta Brittle bone

syndrome Bones easily

bend and fracture

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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

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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.

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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

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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

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DESMOSINE CROSS-LINK

An extensively interconnected, rubbery network that can stretch and bend in any direction when stressed, giving connective tissue elasticity

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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

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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

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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

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Once the lungs has been cleaned, antitrypsin deactivates elastase

Role of -1 antitrypsin in elastin degradation

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Without the antitrypsin enzyme, the elastase continues to consume anything in its path, including healthy lung tissue

Role of -1 antitrypsin in elastin degradation

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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

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Pathogenesis of Empysema

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- 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

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GLOBULAR HEMEPROTEINS

Group of specialized protein that contains heme

Maintain a supply of oxygen essential for oxidative metabolism

MYOGLOBIN HEMOGLOBIN

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HEME Fe+2- protoporphyrin

IX A cyclic tetrapyrrole A planar network of

conjugate double bonds absorbs visible light and colors heme deep RED.

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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

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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

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The heme of myoglobin lies in crevices of helices E and F

Distal histidine

Proximal histidine

Myoglobin structure

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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

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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!)

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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

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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

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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

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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

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Shift from deoxy to oxy conformation

Fe lies out of plane of heme ring

Fe moves into plane of heme ring

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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

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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

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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

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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

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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)

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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

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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

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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

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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.

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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

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Binding of CO

60% of HbCO are fatal

CO poisoning is treated with Oxygen therapy, which facilitates the dissociation of CO.

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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)

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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

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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

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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

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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)

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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