biochem defense proteins

7/24/2019 BIOCHEM Defense Proteins

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DEFENSE PROTEINS

ANTIBODY AND STRUCTURE

Antibody

-produced by the white blood cells

-under the immunoglobulin family (immuno – immune reactions; globulin – group of proteins

within the blood)

-globular proteins - mobile within the cells

Two Main Functions:

1.

Bind specifically to foreign molecules (antigens)

2.

Recruit other cells to destroy the pathogens

do not destroy the antigens but inactivate or tag them for destruction

Defense Mechanisms

1. Agglutination – antibody bind the same determinant on one or than one antigen

2. Precipitation – soluble molecules are cross-linked into large insoluble molecule complexes and fall

out of solution and are phagocyized

3.

Neutralization – antibody bind to and block specific sites thus, antigen is prevented from binding to

receptors on tissue cells and is later destroyed

4.

Complete Fixation – main mechanism against cellular antigens; antibody bound to cells to change

shape and expose complement binding sites; triggers complement fixation on the antigenic cell

surface resulting in cell lysis

Antibody Structure



Heavy Chain –defines the effector function or determines the functional activity of an antibody

molecule

Light Chain – can either be lambda or kappa (no functional difference) but never one of each can be

present in an antibody molecule

Variable Region – accounts for specific antigen binding properties of the immunoglobulin

–

has specific shape that fit certain antigens

–

has two components:

1.

Hypervariable Region

– high ratio of different amino acids

– directly contact a portion of the antigen’s surface

2. Framework Region

– more stable amino acids

– form a beta sheet structure which serve as a scaffold to hold the the HV

regions in position to contact an antigen

Constant Region – determines the mechanism used to destroy the antigen

– ability to bind to complement and to certain nonspecific receptors on cells

– antibodies are grouped into five classes according to their constant region

IgG – γ (gamma) IgD – ∆ (delta)

IgA – α (alpha) IgE – ϵ (epsilon)

IgM – μ (mu)

Hinge Region – accounts for the antibody’s flexibility, opens and closes to allow better binding between

the antibody and the antigen

Disulfide Bonds – responsible for the protein folding and stability

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IMMUNOGLOBULINS

What is IgG?

*GAMMA immunoglobulin

*monomer

*play a major role in antibody-mediated defense mechanisms, including

complement activation, agglutination, opsonization and neutralization



*can leave blood vessls more easily than other Ig’s

*enables IgG to bind to invading pathogens in tissues before they get into circulatory system

Its structure and relationship

*antigen specificity is determined by Fab, effector functions triuggered by Fc

*effector are heavily dependent on N-linked glycan (residue below hinge region)

*Glycan- maintain two heavy hains in an open confirmation which is required for interaction with

activating Fc gamma receptors.

*The addition of sialic acid to the glycan reduce Fc gamma binding and converts IgG antibodies to anti-

inflammatory mediators through binding activities

Difference from other Ig

IgG is the only type of antibody can cross the placenta to protect the fetus

Agglutination

Because each basic antibody has two antigen binding siter, each can attach to two antigenic

determinants at once. The result of several Ig molecules binding with two microbial cells is

AGGLUTINATION/CLUMPING

Neutralization

Antibodies can neutralize a toxin by binding to a critical portion of the toxin so that it can no longer

function against the body. Similarly, antibodies can block attachment molecules on the surfaces of

bacteria and viruses such as they cannot adhere to target cells.

Opsonization

Antiubodies act as opsonins, molecules that stimulate phagocytosis

What is IgM?

*Mu immunoglobulin

*predominates in early primary immune response

*most common Ig expressed on the surface of B cells



*IgM is more efficient than IgG at complement activation, neutralization and agglutination because its

numerous antigen binding site


*IgM is more than five times larger than IgG because it is a pentamer, consisting of five Y-shaped

subnunits linked together in a circular fashion via disulfide bonds and as shore polypeptide Joining (J)

chain.

*Each IgM pentamer contains one copy of another polypeptide chain, called a J (joining) chain, which

has a molecular weight of !5kDa. This accessory polypeptide is produced by IgM-secreting cells. It is an

acidic glycoprotein with a high content of cysteine residues and thus is disulfide linked between two

adjacent IgM monomeric Fc regions at the carboxyl-terminal end. Presumably, oligomerization is

initiated at this site.


IgM is the first effective defense against bacteremia

What is IgD?

*Monomer

*Delta Immunoglobulin

*IgD are not secreted, but are membrane bound antigen receptors

on B cells that are often seen during the initial phases of a humoral

response

*The physiologic function of IgD is unknown. It is relatively labil to

degradation by heat or proteolytic enzymes. There are isolated

reports of IgD with antibody activity toward insulin, penicillin, milk

proteins, diphtherias toxoid, nuclear components or thyroid

antigens.


IgD and IgM, both classes of heavy chains are poduced by alternative splicing of a single RNA and have

identical antigen specificity. The IgD on these cells can bind antigen and transmit signals to the cell

interior, with consequences that appear identical to those produced by IgM. When such B cells become

activated, surface IgD expression ceases.


IgD does not bind complement, does not cross placenta and does not bind to cells thorugh its Fc region



What is IgE?

*Epsilon immunoglobulin

*monomer

*typical Y-shaped immunoglobulin with two EPSILON heavy chains

*IgE act as signal moecules- they attach to receptors found on a certain

WBC (basophils and mast cells) to trigger rapid release of histamine,

causing inflammation, nonspecific defense.

*like IgA, IgE produced mainly in the linings of the respiratory and

intestinal tracts and is part of the external secretory system of

antibody.


*The biological activity of IgE is accounted for by its property of binding through Fc region to basophils

and to mast cells.

*IgE may also be important in the humoral response to parasite disease because it it often found at high

levels in the serum of the patients with helminthic infections.

*The Fc portion of the molecules binds to the target cells,wereas the Fab portion bind the allergen


IgE is present at lowest serum concentration. IgE does not cross the placenta and IgE-antigen complexesdo not bind complement by the classic pathway.

What is IgA?

-

The most hydrophobic of all the immunoglobin

-

In serum, IgA is usually a monomer but in secretions it is a dimer or a tetramer

-

Secreted into saliva, into tears, and onto mucuous membranes to protect body surfaces, found

in colostrum and milk to provide immune protection to the new born

-

Protects the places where we have saliva, tears, and mucus like our mouth, nose, lungs, andintestines

Total antibody serum: 15% (2nd

most abundant Ig)

Molecular weight: 320,000 (2nd

heaviest Ig)

H- chain type: Alpha



Alpha, delta, and gamma chains – have hinge region for added flexibility

Two subclasses:

IgA1 – mostly found in serum and made by bone marrow B-cells

IgA2 – mostly found in mucosal secretions, colostrum and milk and is made by B-cells in the mucosae

Both are found in external secretions like colostrum, and milk, tears, and saliva, where IgA2 is more

prominent in the blood

*secretory IgA – have a j-chain like found in IgM and a polypeptide chain called a secretory piece. It is

the first line of defense in the intestinal epithelium against pathogenic substances.

J chain – facilitates polymerization

Secretory piece – produced by epithelial cells of the mucuos membranes and helps IgA to be

transported across the mucosa and also protects in from degredation

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ENKEPHALINS

Enkephalins are pentapeptides that are the smallest molecules with pain killing or opiate activity. They

are found in neurons in the central nervous system and by cells in the adrenal medulla.

Their differentiating characteristics are on the C-terminal.

Amino acids that are crucial to its function are Tyr1, Gly3, and Phe4. Tyrosine is analogous to the 3-

hydroxyl group on morphine, a pain reliever, which is why it has its analgesic effects. If Tyrosine is

hydrolyzed, the peptide is non-functional.



1.

Met-Enkephalin

Tyr glyglyphemet --- met enkephalin

Met-enkephalin, also known as opioid growth factor , produces analgesic and antidepressant-like

effects.

They have Beta-pleated sheet held together by hydrogen bonds. The terminal NH3 group is also highly

shielded by the hydrophobic side chains of the peptide. They are asymmetrical and the Charge-Charge

dipole between Oxygens and terminal NH3(ammonia) also helps stabilize the structure.

2.

Leu-Enkephalin

Tyr glyglypheleu --- leuenkephalin

Leu-enkephalin is an endogenous agonist for the receptors that are stimulated by opiate alkaloids.

Hydrogen bonding between carbonyl oxygen in Tyr and the amide nitrogen in Phe stabilizes Beta-turn,

forming a pseudo ten membered ring. Surprisingly, The rigid morphine and the highly flexible leu-

enkephalin bind to the same receptor and elicts pain relief because of the stabilization of leu-

enkephalin.

The analgesic potency of met enkephalin is twice that of leuenkephalin as a consequence of the

interaction of the receptor site due to the R group at the C terminal. Leu encephalin also offers steric

effects which hinders reactivity.

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PEPTIDE ANTIBIOTICS

Gramicidin S



Gramicidin S or Gramicidin Soviet is a peptide antibiotic for topical use and is found to be very

effective against gram-positive bacteria infection because it is produced by the gram positive

bacterium Bacillus brevis.

Used in treating genital ulcer caused by sexually transmitted disease and for superficial wound.

It is a cyclic decapeptide that is composed of five different amino acids that is used twice.

It is composed of two amino acids that are uncommon in peptides. These are ORNITHINE and D-

PHENYLALANINE.

Antibacterial Activity: The gramicidin S as an antibiotic will attack and destroy the cell

membrane of the bacteria.

Exchange of VAL, LEU, PRO to other amino acids will not affect the activity instead it will just

reduce the activity.

Exchange of ornithine to lysine will also not affect the antibacterial activity. Because ornithine

and lysine have the same use.

Ornithine- is used for improving athletic performance reducing glutamine

poisoning in the brain condition due to liver disease and for wound healing.

Lysine- is used for improving athletic performance, for wound healing and used

for treating and preventing cold sores.

But if the D-phenylalanine is replace by other amino acid, the antibacterial activity will be

inactive because D-phe is the one who blocks enkephalinase, who deceases the level of pain we

can feel.

Gramicidin A

It is a linearpentadecapeptide antibiotic (pentadecapeptide = it is made up of 15 amino acids.

Alternate L- and D- amino acid residues has been shown to effect Ionic Flux across lipid bilayer

membranes of bacterial cells by the formation of transmembrane channel.



Two molecules of Gramicidin A is required per transmembrane channel.

These combine in such a way as to span the length of the lipid portion of the bilayer.

Its primary structure is such that all side chains are Hydrophobic

except Ethanolamine at the C-terminal.

The N-terminal amino acid is blocked by a Formyl group.

Head to head coupling is favored by the formylated group which possesses uninterruptedstructural continuity.

When formyl group was removed from Gramicidin A (deformyl Gramicidin A ) activity was lost.

Without the N-formyl group head to head attachment would not be possible.

Ions of Sodium, Potassium , and Ammonium can interact with acyl oxygens of ethanolamine

oxygen. There is alternating direction of carbonyl group allowing the interaction to exchange

coordination in a directional manner and thereby allow flow along the transmembrane channel

This ion flux explains the antibacterial effect of Gramicidin A.

biochem defense proteins

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