Chemistry of Proteins II Biochemistry Slideshow

• Chemistry of amino acids
  • Lippincott’s Biochemistry
• Structure of Proteins
  • Lippincott’s Biochemistry
• Classification of Proteins
  • Biochemistry by Mushtaq Ahmed Vol I
• Plasma Proteins
  • Medical Biochemistry by Chatter Jea
• Techniques to study proteins
• Proteins are the most abundant macromolecule in the living cells and constitute 50% or more of their dry weight.
• They are found in all cells and all parts of the cells.
• Proteins also occur in great variety, hundreds of different proteins may be present in a single cell.  
• Proteins have many different biological roles. 

Definition of Proteins

Proteins are the nitrogenous compounds made of a variable numbers of amino acid residues joined to each other by a covalent bond called peptide linkage.

• Amino acid polymers are often differentiated according to their molecular weight or the number of amino acid residues they contain.
• Molecules with molecular weight ranging from several thousand to several million daltons (D) are called polypeptides.
• Those with low molecular, typically consisting of fewer than 50 amino acids are called peptides.
• The term protein describe molecules with more than 50 amino acids

Biomedical importance

• In addition to providing the monomer units from which polypeptide chains of proteins are synthesis, the L-α-amino acids and their derivatives participate in cellular functions as diverse as nerve transmission and the biosynthesis of porphyrins, purines, pyrimidines, and urea.
• Short polymers of amino acids called peptides perform prominent roles in the neuroendocrine system as hormones, hormone-releasing factors, neuromodulators, or neurotransmitters.
• Neither human nor any other higher animals can synthesize 10 of 20 common L-α-amino acids in amounts adequate to support .
• While proteins contain only L-α-amino acids, microorganisms elaborate peptides that contain both D- and L-α-amino acids.

Proteins are built from a repertoire of 20 amino acids

• Amino acids are the building blocks of proteins.
• An a– amino acid consists of a central carbon atom called the a carbon, linked to an amino group, a carboxylic group, ahydrogen atom, and a distinctive R group.
• The R group is often referred to as the side chain.
• With four different groups connected to the tetrahedral a- carbon atom,  a-amino acid are chiral; the two mirror-image forms are called the L-isomer and the D-isomer.

• Only L amino acid are constituents of proteins.
• Amino acids in solution a neutral pH exist predominantly as dipolar ions (also called zwitterions).
• In the dipolar form , the amino group is protonated ( -NH₃⁺ ) and the carboxyl group is deprotonated ( -COO^– ).
• The ionization state of an amino acid varies with pH.
• Amino acids are amphoteric molecules; that is , they can act as either an acid or a base.
• Such compounds are often called ampholytes.

Amino acids may have Positive, Negative, or Zero Charge

• Molecule that contain an equal number of ionizable groups of opposite charge and that therefore bear no net charge are termed zwitterions.
• Amino acids in blood and most tissues thus should be represented as in this form:

At its Isoelectric pH (pI), an Amino Acid Bears No Net Charge

• The isoelectric species is  the form of a molecule that has an  equal number of positive and negative charges and thus is electrically neutral.
• The isoelectric pH, also called pI, is the pH midway between pKa values on the either side of the isoelectric species.   

Classification of amino acids

• The 20 L-α-amino acids, present in proteins are genetically coded for proteins synthesis. These amino acids are classified into different classes on the basis of :

1. Structure of ‘R’ group.
2. Polarity of ‘R’ group and
3. Nutritional classification [need for growth and maintaining health]   

Structure of side chains

Classification of amino acids on the basis of charge on the side chain

NONPOLAR SIDE CHAINS

UNCHARGED POLAR SIDE CHAINS

BASIC SIDE CHAINS

ACIDIC AND BASIC PROPERTIES OF AMINO ACIDS

• Amino acids in aqueous solution contain weakly acidic α-carboxyl groups and weakly basic α-amino groups.
• In addition, each of the acidic and basic amino acids contains an ionizable group in its side chain.
• Thus, both free amino acids and some amino acids combined in peptide linkages can act as buffers.
• The quantitative relationship between the concentration of a weak acid (HA) and its conjugate base (A~) is described by the Henderson-Hasselbalch equation.

Amino acids and Neurotransmitters

• Two amino acids deserve some special notice because they are both key precursors to many hormones and neurotransmitters (substances involved in the transmission of nerve impulses).
• Two of the neurotransmitter classes are simple derivatives of the two amino acids tyrosine and tryptophan.
• Tryptophan → Serotonin.
• Phenylalanine → Epinephrine. 

The Genetic Code Specifies 20 L-a- Amino Acids

• Of the over 300 naturally occurring amino acids, 20 constitute the monomer units of proteins.
• This is true for proteins from all form of life on earth and reflects the universality of the genetic code.
• Specific proteins may, however, contain of these 20 amino acids that arise by a methylated, formylated, acetylated, carboxylated or other derivatives process known as Posttranslational modification.

Non-standard amino acids

• In addition to the 20 standard amino acids that are common in a proteins, other amino acids are have been  found as components of only certain types of proteins.
• 4-Hydroxyproline                Plant cell wall
• 5-Hydroxylysine                  Collagen
• N-Methylysine                      Myosin
• g-Carboxyglutamate             Blood clotting protein
• Desmosine                             Elastin
• Selenocysteine +

Some 300 aditional amino acids have been found in cells. They have a variety of functions but are not constituents of proteins.

Essential Amino acids

• Human body can not synthesize the essential amino acids, so they must essentially be present in our diet.
• Plant proteins
• Animal proteins
• Some essential amino acids are absent in plant proteins.  

Arginine and Histidine are synthesized in children in less amount for growth hence are called semi essential amino acids.

Peptide Bonds

• Amino acids can be polymerized to form chains.
• Polymers composed of two, three, a few (3-10), and many amino acid units are known, respectively, as dipeptides, tripeptides, oligopeptides,and polypeptides.
• After they are incorporated into a peptide, the individual amino acids (monomeric units) are referred to as amino acid residues.      

Classification of Proteins

• Proteins have been classified in several ways.
• Physiochemical properties.        
• Biological Functional bases.
• Nutritional basis Function
• Over all shape  (Structural basis).

Classification on the bases of physicochemical properties

1. Simple proteins
2. Compound or conjugated proteins
3. Derived proteins

Simple proteins

• On hydrolysis, these proteins yield only amino acids. These consist of the following types.
• Albumins
• Globulins
• Globins
• Prolamins
• Histones
• Protamines
• Albuminoids

Albumins

• These are water-soluble proteins and occur in both plant and animal kingdoms.
• Example are serum albumin, ovalbumin and lactalbumin in animals and legumelin in plants.
• They are coagulated by heat.
• They can be precipitated by full saturation with ammonium sulphate.     

Globulin

• These are insoluble in water but soluble in dilute salt solutions and are heat coagulable to various extent.
• They are found in animals, e.g. lactoglubulin, myosin in muscle , ovoglobulin, serum globulins and also in plants, e.g. legumin.
• Globulin are more easily precipitated than albumin and this can be done only half-saturation with ammonium sulphate. 

Globins

• These are rich in histidine but are not basic.
• They unite with heme to form hemoglobin.
• Hemoglobins of different species differ only with respect to globin and heme part is the same in all cases.

Prolamins

• These are soluble in 70 to 80 % ethanol but are insoluble in water and absolute alcohol.
• Example are gliadin of wheat and zein of maize.
• These are rich in amino acid proline but deficient in lysine

Histones

• These are very strongly basic proteins as they are rich in arginine.
• In combination with deoxyribonucleic acid (DNA) they form nucleoproteins of more correctly nucleohistone which occur in cell nuclei forming chromatin material.
• The association of DNA and histone gives rise to complexes called nucleosomes, 10nm in diameter, in which DNA strands wind around a core of histone molecule .
• Histones are soluble in water but not in ammonium hydroxide

Protamines

• These are present in sperm cells.
• They are of relatively smaller size.
• They are basic proteins and resemble histones but unlike them are soluble in ammonium hydroxide.
• Like histones, they form nucleoproteins with nucleic acids and are rich in arginine.
• These proteins lack in both tyrosine and tryptophan.     

Albuminoids

• They are also called scleroproteins and occur only in animals; they do not occur in plants.
• These proteins include collagen and elastin which occur in the connective tissue and keratin which is found in ectodermal tissue such as nails, hair, hoofs ,horns, etc.
• These proteins include
• Collagen               3. Keratin
• Elastin

Compound or conjugated proteins

• In these molecules the protein is attached or conjugated to some non-protein group which are called prosthetic groups.
• Following types of proteins belong to this group.
• Nucleoproteins.                               DNA, RNA
• Phosphoproteins                             phosphoric acid

• Lipoproteins                                       Lipids
• Glycoproteins                          Carbohydrates
• Chromoproteins                               colour sub.
• Metalloproteins                               Metal   

Nucleoproteins

• These proteins are the result of the conjugation of certain basic protein (histones) with nucleic acids (DNA and RNA).
• These proteins are most abundant in tissue having a large proportion of nuclear material , e.g. yeast  and other glands and sperm.

Phosphoproteins

• These are proteins conjugated with phosphoric acid and include casein of milk and vitellin of egg yolk.

Lipoproteins

• These are conjugated proteins containing lipid substances like lecithin, cholesterol, triglycerides, and fatty acids.
• These occur in blood plasma, nervous tissue, egg yolk, milk and cell membranes.
• Bacterial antigens and viruses also contain lipoproteins.
• Chylomicroms.
• HDL.
• LDL.
• VLDL.             IDLP

Glycoproteins or  Mucoproteins

• These are proteins to whose polypeptide backbone are covalently attached oligosaccharide (glycan) chains.
• This term also includes mucoproteins.
• The carbohydrates contents of these proteins ranges from 1% to over 85% by weight.
• They are found in serum, egg white, human urine, tendons (tendomucoid), bones (osseomucoids) and cartilage (chondroproteins).
• They are generally present in all kinds of animal mucins which they make slippery and suitable for lubrication and in the blood group substances.  
• Hormones such as interstitial cell stimulating hormone (ICSH), follicle stimulating hormone (FSH), human chorionic gonadotropin (hCG) and thyroid stimulating hormone (TSH) are carbohydrates-containing proteins.
•   Plasma proteins that transport iron and copper        ( transferrin and ceruloplasmin) and proteins that act as antibodies (immunoglobulins) are also glycoproteins.
• Mucoproteins having more than 4% carbohydrates and glycoproteins containing less than 4% carbohydrates.

Chromoproteins

• These are compounds of proteins with pigments such as heme and include hemoglobin and cytochromes.
• Other examples are the enzymes flavoproteins and rhodopsin (visual purple in retinal rods); in these cases the prosthetic groups are riboflavin and 11-cis vitamin A aldeyhde respectively.

Metalloproteins

• These are proteins which are in combination with metallic atoms,e.g. ferritin (iron), carbonic anhydrase (Zinc), and ceruloplasmin (Copper).  

Derived proteins

• This class of proteins includes substances which are derived from simple and conjugated proteins.
• These are formed from naïve proteins by the action of heat, physical forces or chemical factors and X-rays etc.  
• These proteins are subdivided into primary and secondary derived proteins

Primary derived proteins [PDP]

•                    Heat, U.V.& X-rays     
• Proteins                                           PDP
•                      Acids or alkalis    

Fibrin from fibrinogen, myosan from myosin and edstan from edestin   

Secondary derived proteins [SDP]

• Proteoses, peptones and peptides are SDP
• SDP are formed by progressive hydrolysis of proteins by enzymes.
• Proteins → proteans → Metaproteins → proteoses → peptones→ peptides → aminoacids. 

Classification of Proteins According to Biological function

Class  Example

Enzymes   Ribonuclease

  Trypsin

Transport proteins  Hemoglobin

  Serum albumin

  Myoglobin

  b₁-Lipoprotein

Class  Example

Nutrient and  Gliadin (wheat)

Storage proteins   Ovalbumin  (egg)

  Casein (milk)

  Ferritin

Contractile or  Actin

motile proteins  Myosin

  Tubulin

  Dynein

Class   Example

Structural proteins  Keratin

  Fibroin

  Collagen

  Elastin

  Proteoglycans

Defense proteins  Antibodies

  Fibrinogen

  Thrombin

  Diphtheria toxin

  Snake venom

Class   Example

Regulatory proteins  Insulin

  Growth hormone

  Carticotropin

  Repressors 

Nutritional bases

• An optimal diet includes, in additional to sufficient water, adequate calories, proteins, fat, minerals and vitamins.
• Caloric intake & Distribution

                The caloric values of the dietary intake must equal the energy expended as heat and work if body weight is to be maintained.

• When the caloric intake is insufficient, body store of protein and fat are catabolized, and when intake is excessive obesity result.
• In addition to the 2000 kcal/d necessary to meet basal needs, 500-2500 or more kcal/d are required to meet the energy demands of daily activities.
• A daily protein intake of at least 1 g/kg body weight to supply 8 essential amino acids and other amino acids is now regarded as desirable.
• The source of protein is also important.

Grade I proteins

• The animal proteins of meat, fish, and eggs, contain amino acids in approximately the proportion required for protein synthesis and other uses.
•  Some of the plant proteins are also grade I

Grade II proteins

• Most plant proteins are grade II because they supply different portion of amino acids, and some lack one or more of essential amino acids.
• Proteins needs can be met with a mixture of grade II proteins, but the intake must be large because of the amino acid wastage.

Proteins can also be Classified according to Shape.
Two Types of Protein Conformations

• Axial ratios (the ratio of their shortest to longest dimensions).
• Globular.
• Fibrous.

Globular proteins

• Globular proteins are compact, are roughly spherical or ovoid in shape, and have axial ratios not over 3 or 4.
• Most enzyme are globular proteins, whose large internal volume provides ample space in which to construct cavities of the specific shape, charge, and hydrophobicity or hydrophilicity required to bind substrates and promote catalysis.
• Insulin, Albumin, Globulin, Enzyme, & cytochrome c.   

Fibrous proteins

• By contrast many structural proteins adopt highly extended conformation. These fibrous proteins possess axial ratios of 10 or more.
• Fibrous proteins typically contain high proportion of regular secondary structure, such as α-helix or β-pleated sheet. 
• Example of fibrous proteins include            α-keratin, collagen and silk fibroin.

• Collagen and elastin are examples of common, well-characterized fibrous proteins that serve structural functions in the body.
• For example, collagen and elastin are found as components of skin, connective tissue, blood vessel walls, and sclera and cornea of the eye.

• Each fibrous protein exhibits special mechanical properties, resulting from its unique structure, which are obtained by combining specific amino acids into regular, secondary structural elements.
• This is in contrast to globular proteins, whose shapes are the result of complex interactions between secondary, tertiary, and, sometimes, quaternary structural elements.

Three-dimensional structure is determined by X-ray crystallography or by NMR spectroscopy

• Another technique that supplements the results of X-ray diffraction has come into wide use in recent years. It is a form of nuclear megnatic resonance (NMR) spectroscopy. 

Protein structure

• Primary
• Secondary
• Tertiary
• Quaternary

Primary structure of a protein

• The number & sequence of amino acid residues along the peptide chain is called the primary structure.
• The primary structures of a large numbers of polypeptides have been elucidated

• Insulin (51)
• ribonuclease (124)
• α chain of hemoglobin (141)
• β chain of hemoglobin (146)
• oxytocin (8)
• ACTH (38)
• calcitonin (32).

Amino acid sequence determines  Primary structure

Secondary structure

• a- Helix
• b-Pleated sheets

a-Helix

• The alpha Helix is a coiled Structure stabilized by intrachain hydrogen bonds.
• It is a clockwise, rod like spiral and is formed by intarchain H bonding between the C = O group of each amino acid and the  –NH₂ group of the amino acid that is situated 4 residues ahead in the linear sequence.

Beta pleated sheets

• The β pleated sheet ( or, more simply , the β sheet) differ markedly from the rodlike α- helix.
• A polypeptide chain, called a β strand, in a β sheet is almost fully extended rather than being tightly coiled as in the α helix.
• A rang of extended structures are sterically allowed.    

A Parallel b-pleated sheet

Adjacent β strands run in the same direction. Hydrogen bonds connect each amino acid on one strand with two different amino acids on the adjacent strands.

An antiparallel b-pleated sheet

Adjacent β strands run in opposite direction. Hydrogen bonds between NH and CO groups connect each amino acids to a single amino acid on an adjacent strand, stabilizing the structure. 

Loops & Bends

•Roughly half of the residues in a “typical” globular protein reside in α-helices and β- sheets and half in loops, turns, bends and other extended configurations features.  

Supersecondary structure

• Many globular proteins contain combination of α-helix and β-pleated sheet secondary.
• These patterns are called supersecondary structure. 
• In the βαβ unit, two parallel β-pleated sheets are connected an α- helix segments.
• In the β-meander pattern, two antiparallel β-sheets are connected by polar amino acids and glycines to effect an abrupt change in direction of the polypeptide chain called reverse or β-turn.
• In αα-units, two successive α-helices separated by a loop or nonhelical  segment become enmeshed because of compatible side chains.
• Several β-barrel arrangements are formed when various b-sheet configurations fold back on themselves.
• When an antiparallel b-sheet doubles back on itself in a pattern that resemble a common Greek pottery design, the motif is called the Greek key.

Tertiary structure

• The overall three –dimensional structure of protein is the tertiary structure of protein.
• The shape of globular proteins involve interaction between amino acid residues that may be located at considerable distance from each other in the primary sequence of the polypeptide chain and includes a-helices and b-sheet.
• Noncovalent interactions between the side chains of amino acid residues are important in stabilizing the tertiary structure and include (i) hydrophobic (ii) electrostatic interaction as well as (iii) hydrogen bond.
• In addition, covalent linkages may occur involving disulfide bond formation between cysteine residues. 
• Proteins tend to fold so that the atoms are packed closely together.
• Therefore, van der Waals forces between the atoms play an important role in stabilizing the structure of proteins.
• Occur between nonpolar side chains.

Tertiary Structure

• Strong bonds
• Peptide bond
• Disulfide bond
• Weak bonds
• Hydrophobic
• Electrostatic
• Hydrogen

Example of tertiary structure of protein. The enzyme triose phosphate isomerase.

Note the elegant and symmetric arrangement of alternating β sheets and α helices.

Domains

• A domain is a section of protein structure sufficient to perform a particular chemical or physical task such as binding of a substrate or other ligand.
• Other domains may anchor a protein to a membrane or interact with a regulatory molecule that modulates its functions.

Two-domain structure of the subunit of a homodimeric enzyme, a bacterial class II HMG-CoA reductase.

As indicated by the numbered resides, the single polypeptide begins in the large domain, enters the small domain, and ends in the large domain.

Folding of protein

The three-dimensional structure of a protein, a linear polymer of amino acids, is dictated by its amino acid sequence.   

Quaternary Structure

• Quaternary structure is a property of proteins that consists of more than one polypeptide chain.
• Each chain is called a subunit.
• Quaternary structure is the three-dimensional structure of a protein composed of multiple subunits.
• Commonly occurring example are dimers, trimers, and tetramers, consisting of two, three, and four polypeptide chain 
• These subunits are held together by some types of non-covalent interactions involved in tertiary structure, that is, hydrophobic and electrostatic interactions and hydrogen bonds. 

Hemoglobin

Denaturation & Renaturation of Proteins

Isolation of Proteins from cells

• To study a proteins in any detail it must be separated from all other proteins.
• The source of a protein is generally tissue or microbial cells.
• Homogenization
  • Grinding the tissue in a blender with a suitable buffer.
  • Potter-Elvejhem homogenizer
  • Sonication.
  • Cycle of Freezing and thawing.

• Fractionation.
• Localization
• Differential centrifugation.
• Salting out . Ammonium sulphate [ (NH₄)₂SO₄]
• Dialysis might be used, for example, to remove ammonium sulphate from protein preparation.  
• Column Chromatography
• A modern refinement in chromatographic method is HPLC, or high-performance liquid chromatography.  

Three chromatographic methods used in protein  purifications

• Ion-exchange chromatography.
• Size-exclusion chromatography.
• Affinity chromatography.

Ion-exchange chromatography

1. Cation exchangers
2. Anoin exchangers

Blood

• The specific gravity of blood is 1.055 to 1.065 its viscosity is approximately four  to five times that of water.
• If blood is drawn from a vein and measures are taken to prevent clotting, the suspended cellular elements can be separated by centrifugation.
• The normally clear, slightly yellow  supernatant fluid is termed blood plasma.
• Should the blood be allowed to clot, there separates from the clot a clear yellowish fluid , the blood serum. 
• The yellow color is due to the presence of small quantities of bilirubin, a bile pigment, and of carotenoids.
• The total volume of blood in the vascular system approximates 8 percent of the body weight, about 5 to 6 liters of blood in the adults. 

Plasma proteins

• Normal plasma contains 69 to 85 g / L of proteins
• Of this, 30 to 50 g is albumin and 15 to 30 g is composed of a mixture of globulins.
• A/G ratio  2:1
• The plasma proteins usually are classified on the basis of solubility and electrophoretic separation into six categories.

Synthesis of Proteins

• The liver is the principal site of synthesis of all circulating proteins apart from γ-globulins, which are produced in the reticuloendothelial system.

Serum Albumin

• Albumin is one of the few plasma proteins that is not a glycoprotein, and it is the lowest molecular weight of almost all plasma proteins.
• The major function of albumin is its role in osmotic regulation.
• Albumin is responsible for 75 to 80 percent of the osmotic effect of plasma.
• Only about 40 percent of the albumin is contained in the circulation, the remaining being  in the extra vascular space of tissue, principally in muscle, skin, and the intestine.
• In addition to its osmotic functions, albumin serves an important role in transport of diverse substances, many of which are sparingly soluble in water.     
• Normal lipid metabolism require albumin.
• Of particular importance is the transport of fatty acids, which bind tightly to albumin and are transported by this means from liver to peripheral tissues.
• Albumin also binds bilirubin and aids in its transport to liver, where it is excreted in the bile.
• A major defect in analbuminemia, a rare disorder characterized by very low blood level of albumin, is impaired lipid transport with elevated level of cholesterol, phosphoglycerides, and lipoproteins.
• It is interesting that these patients do not present with edema as a sign due to an osmotic compensation by globulins which are mildly elevated. 
• Albumin level decreases in Nephrotic syndrome, hepatic cirrhosis, glomerulonephritis, edema, kwashiorkor, and malnutrition.
• Albumin level increases in Dehydration.

Bence Jones proteins

• Bence Jones proteins are low molecular weight proteins made up of immunoglobulin light chains which coagulate between 40-60° C but redissolve on boiling.
• These have been found in the urine of patients of multiple myeloma, and the test is used for diagnosing the same.  

γ-Globulins

• This fraction of plasma contains the immunoglobulins.
• Included in the group are the so-called cryoglobulins, immunoglobulin that precipitate on cooling serum.
• Cryoglobulins are often found during inflammatory illnesses, e.g., rheumatoid arthritis, as well as in multiple myeloma.
• The protein designated cold insoluble globulin, behave as a cryoglobulin
• The term immunglobulins refers not only to the normal classes of antibodies but also to vast numbers of Pathological proteins, generally called myeloma proteins.
• These proteins are synthesized in large amount in multiple myeloma, a malignant disease in which single, specific cells of the antibody forming system have transformed neoplastically and have proliferated.
• Neoplasm ; any new and abnormal growth.
• Proliferation ; reproduction or multiplication of  similar forms specially of cells.

Immunoglobulins  (Antibodies)

• These are a group of proteins (gama globulins) produced by the body (from B lymphocytes and plasma cells) in response to the presence of foreign compounds.
• They are five basic types :G, A, M, E & D.
• All of immunoglobulins have a similar basic structure.
• PLASMA IMMUNOGLOBULINS PLAY AMAJOR ROLE IN THE BODY’S DEFENSE MECHANISMS
• The immune system of the body consists of two major components: B lymphocytes and T lymphocytes.
• The B lymphocytes are mainly derived from bone marrow cells in higher animals.
•  
• The B cells are responsible for the synthesis of circulating, humeral antibodies, also known as immunoglobulins.
• The T cells are involved in a variety of important cell-mediated immunologic processes such as graft rejection, hypersensitivity reactions, and defense against malignant cells and many viruses.
• This section considers only the plasma immunoglobulins, which are synthesized mainly in plasma cells.
• These are specialized cells of B cell lineage that synthesize and secrete immunoglobulins into the plasma in response to exposure to a variety of antigens.

Immunoglobulins

• Antibodies are plasma proteins known as immunoglobulins.
• There are three major classes of immunoglobulins :IgG, IgA, and IgM; minor classes immunoglobulins found in human plasma are designated as IgD and IgE.

Fab (antigen binding )

Fc (crystallizable)

IgM

• IgM is the first type of antibody produced in response to an antigen.
• It is most effective against invading microorganism; although its binding to antigen may be weak, it contains multiple binding sites and its overall avidity for an antigen is high.
• Thus, it agglutinates cell with surface antigens extremely well.
• IgM is found primarily in the blood plasma; it enter interstitial fluids only slowly, and does not cross the placenta of the pregnant female to enter the fetal circulation.

• IgG is produced after IgM and is the major immunoglobulin of blood and interstitial fluids.
• It is the only type of antibody that can cross the placenta and provide immunity to the human fetus.

IgG  antibodies consist of four chains, two heavy (blue) and two light chains (red), linked by disulfide bonds. The heavy and light chains come together to form F_ab domains, which have the antigen-binding sites at the ends. The two heavy chains form the F_c domsin. The F_ab domains are linked to the F_c domain by flexible linkers.

• IgA is also produced later than IgM and acts as a protective barrier against invasion of microorganisms at several potential sites of entry, e.g., the digestive and respiratory tracts.
• IgA is also the primary antibody of milk and colostrum and serves to protect the gastrointestinal tract of nursing infants in the first day of life from invasion by pathogenic organisms.  

IgD and IgE

• IgD is normally present in blood in only very small concentrations. Found on the surface of B cells as well as in serum
• IgE is also present in very small amount and acts primarily on mast cells and basophils.

• Another kind of defense against infection organisms is provided by the presence of the enzyme lysozyme in tears nasal secretions, saliva, spleen, and leukocytes

Major functions of Blood

• Respiration—transport of oxygen from the lungs to the tissuesand of CO₂ from the tissues to the lungs.
• Nutrition—transport of absorbed food materials.
• Excretion—transport of metabolic waste to the kidneys, lungs, skin, and intestines for removal.
• Maintenance of the normal acid-base balance in the body.
• Regulation of water balance through the effects of blood on the exchange of water between the circulating fluid and the tissue fluid.
• Regulation of body temperature by the distribution of body heat.
• Defense against infection by the white blood cells and circulating antibodies.
• Transport of hormones and regulation of metabolism.
• Transport of metabolites.
• Coagulation.

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