Proteins Hand Out

7/27/2019 Proteins Hand Out

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PROTEINS Proteins are high molecular weight (5000 to 25,000,000) substances containing the elements C, H, O, and N. They are

macromolecules which also contain other elements like S, P, and a few, such as hemoglobin, contain some other

elements.

Other than water, proteins are the chief constituents of all cells of the body. The word protein is derived from the Greek word proteioswhich means primary or of first importance. About half the dry weight of living materials is protein. They are the source to replace N as almost 15 g of N is lost

everyday by an adult chiefly as urinary urea. Sources of proteins:

Plants synthesize proteins from inorganic substances present in the air and in the soil. Animals cannot synthesize

proteins from such sources. Animals obtain proteins from plants or from other animals who have obtained them from

other plants.

Animals excrete waste materials containing many N compounds. These N cpds along with decaying animal and plant

matter are converted into soluble N cpds by soil bacteria. Plants, then use these soluble N cpds to manufacture more

protein thus completing a cycle.

Functions of Proteins:(Most impt type of cpd in the body.)1. Building of new cells

2. Maintenance of existing cells3. Replacement of old cells hormones

4. Valuable source of energy

5. Involved in the regulation of metabolic processes

6. Involved in the catalysis of biochemical reactions (enzymes)

7. Involved in the transport of oxygen (hemoglobin)

8. Involved in the bodys defense against infection (antibodies)

9. Involved in the transmission of hereditary characteristics (nucleoproteins)

10. Involved in the transmission of impulses (nerves)

11. Involved in muscular activity (contraction)

12. Components of skin, hair, and nails, and connecting and supporting tissues

Composition of proteins:Proteins are polymers built up from smaller units called amino acids. Hydrolysis of proteins yields

amino acids. Polymers are chain-like molecules produced by the linking together of a number of

small units, chiefly -amino acids.

O

RCHCOH -amino acid where R can be

NH2 many different groups

Amino acids:Most proteins produce approximately 20 -amino acids. Alpha amino acids have both an amino and a carboxylic acid

group attached to the same -carbon, that is the carbon atom next to the acid group. The 2nd C from the acid group iscalled *+ carbon; then come the *+ gamma and *+ delta carbons.

There are 20 known amino acids that can be produced by the hydrolysis of proteins. All these amino acids, except

glycine, which has no chiral C, have the L-configuration.

The body can synthesize some, but not all, of the amino acids it needs. Those that it cannot synthesize must be

supplied from the diet. These are called the nutritionally essential amino acids. They are the ffg:

1. leucine

2. isoleucine Mnemonic

3. threonine Pvt. Mat Hill

4. tryptophan or Pvt. Tim Hall H-histidine5. phenylalanine 5 6 3 42 7 18 a-arginine

6. valine Adequate amounts of amino

7. methionine acids are required to maintain the

8. lysine the proper N balance in the body.

Semi-essential amino acids

Arginine and histidine are synthesized partially by

the body but not at the rate to meet the requirement in growing children, pregnant and lactating women.


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Non-essential amino acids

These can be synthesized by the body and may not be required in the diet. These amino acids are derived from the

carbon skeletons of lipids and carbohydrates during their metabolism or from the transamination of essential amino

acids.

Deficiency in one or more essential amino acids in the diet reduces protein synthesis leading to failure in growth of

the child, negative nitrogen balance in adults and fall in plasma proteins and hemoglobin levels.

Amino acids can be divided into two groups, polar and nonpolar, depending on the polarity of the R group attached to

the -carbon. If the R-group is non-polar, then the amino acid will be less soluble in water than the one containing a

polar group.

An R-group that is polar, such as OH, SH, NH2, or COOH, produces an amino acid that is polar. Such amino

acids are soluble in water.

Polar A.A.

COOH COOH COOHH2NCH H2NCH H2NCH O

H CH2COOH CH2C

Glycine (gly) aspartic acid (asp) NH2asparagine (asn)

COOH

H2NCH NHCH2CH2CH2NHC arginine (arg)

NH2COOH COOH COOH

H2NCH H2NCH H2NCHCH2 CH2CH2COOH CH2OH

SH glutamic acid (glu) serine (ser)

Cysteine (cys)

COOH COOH COOH

H2NCH H2NCH O H2NCHHCOH CH2CH2C CH2

CH3 NH2 C=CH

threonine (thr) glutamine (gln) HN N

C


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COOH H

H2NCH histidine (his)

CH2

Tyrosine (tyr)

OH

Non-polar A.A.

COOH COOH COOH

H2NCH H2NCH H2NCH

CH3 CH CH2

alanine (ala) CH3 CH2CH3 CH

isoleucine (ile) CH3 CH3

leucine (leu)

COOH COOH

H2NCH CHCH2CH2SCH3 HN CH2 proline(pro)

methionine (met) H2C

COOH CH2

COOH H2NCH

H2NCH CH COOHCH2 H3C CH3 H2NCH

Valine (val) CH2

Phenylalanine (phe) NTryptophan (trp)These units are joined together through the peptide bonds (CONH). The peptide linkage is formed

between two amino acids by the release of one water molecule. The amino group of the first amino acid and the

carbonyl group of the next amino acids are involved in the formation of peptide bonds.

Amphoteric Nature

Amino acids contain theCOOH group, which is acidic, and theNH2 group, which is basic. In solution, the carboxyl

group can donate a H+ to the amino group, forming a dipolar ion, called a zwitterion.


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RCHCOOH RCHCOO-

NH2 NH3+

Amino acid zwitterion form of an amino acid

Amino acids are amphoteric compounds; that is, they can react with either acids or bases. When an amino acid

is placed in a basic solution, it forms a negatively charged ion that will be attracted toward a positively charged

electrode. In acid solution, the amino acid forms a positively charged ion that will be attracted toward a negatively

charged electrode.

OH-

RCHCOOH RCHCOO- RCHCOO-

NH3+ H+ NH3

+ NH2positively charged ion zwitterion negatively charged ion

(in acid solution) (in basic solution)

Since amino acids are amphoteric, proteins, which are made up of amino acids, are also amphoteric. This

amphoteric nature of proteins accounts for their ability to act as buffers in the blood; they can react with either acids or

bases to prevent an excess of either.

At a certain pH, amino acids will not migrate toward either the positive or the negative electrode. At this pH, amino acids

will be neutral; there will be an equal number of + and ions. This point is called the isoelectric point.

Proteins, which are composed of amino acids, also have an isoelectric point, which is different for each protein. At its

isoelectric point, a protein has a minimum solubility, a minimum viscosity, and also a minimum osmotic pressure. At a

pH above the isoelectric point, a protein has more negative than positive charges. At a pH below isoelectric point, a

protein has more positive than negative charges.

Peptide BondsWhen two amino acids are joined together by the peptide bond the result is adipeptide.

O

CH3CHCOH + HNHCH2COOHNH2 alanine glycine

amine part of glycine rxts with acid part of alanine

O peptide bond

CH3CHCNHCH2COOH + H2O

NH2alanylglycine (ala-gly)

or

acid part of glycine react with the amine part of the alanine

O peptide bond

NH2CH2CNHCHCOOH + H2O

CH3glycylalanine (gly-ala)

When three amino acids combine, the product is called a dipeptide. When many amino acids join together, the product is

called apolypeptide.


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Each polypeptide can have any number of any one or different types of amino acids which can be present in any sequence.

The individual amino acid of a peptide is called the amino acid residue. Each polypeptide has one free carboxylic acid (

COOH) at one end which is called the C-terminal and a free amino group at the other end called N-terminal.

For just two amino acids, glycine and alanine, two different combinations have already been indicatedglycylalanine

and alanylglycine, in which the first member of each group acts as the one furnishing theOH from the acid group.

For three different amino acidssuch as glycine, alanine and valinethere are 6 possible combinations ( or tripeptide

linkages).1. glycylalanylvaline (gly-ala-val) 4. alanylvalylglycine (ala-val-gly)

2. glycylvalylalanine (gly-val-ala) 5. valylglycylalanine (val-gly-ala)

3. alanylglycylvaline (ala-gly-val) 6. valylalanylglycine (val-ala-gly)

Example: A dipeptide composed of glycine, alanine and serine.

H O CH2OH

CH2 N C CH

H2N C CH N COHO CH3 H O

gly ala ser

N-terminal C-terminal

CH3H O HC CH3

CH2 N C CH

H2N C CH N COH

O CH3 H O

gly ala valSTRUCTURE OF PROTEINS1. Primary structurerefers to the number and sequence of the A.A in the protein. (Peptide bonds)

Example is the 1o structure ofinsulin. A slight change in the amino acid sequence, such as the replacement of a singleamino acid with another,can change the entire protein. Example of this is the proteinhemoglobin that is composed of

146 amino acids in proper sequence to form the chain. Amino acid #6 is glutamic acid. If it is changed to Valine, a new

type of protein, Hemoglobin S, is produced and causes the genetic disease sickle cell anemia.

2. Secondary structurerefers to the regular recurring arrangement of the amino acid chain.

(a) -helix, occurs when the amino acids form a coil, or spiral. The coil consists of loops of amino acids held together by

H-bonds (between theH of the NH2 of one AA and the O of the C=O of the acid part of another AA.

Each turn of the helix contains an average of 3.6 amino acids. Such a structure is both flexible and elastic. Example of

this helical structure is hair and wool.

*When AA are coiled, they can form either a right- or left-handed spiral. Since AA in proteins are all of theL-configuration, the coils always are right-handed.

(b) -pleated sheet (also called the pleated sheet) consists of parallel strands of polypeptides held together by hydrogen

bonds. Such a structure is flexible but not elastic. Example of this is one found in silk. It is strong, but resistant to

stretching. This type of structure is less common than the helix.

3. Tertiary structurerefers to the specific folding and bending of the coils into specific layers or fibers. The

tertiary structure gives proteins their specific biologic activity. They are stabilized by several types of bonds:

(a) salt bridges formed between +ly and ly charged groups within the protein moleculeexamples are the carboxyl


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and amino side chains found in glutamic acid, lysine, arginine and aspartic acid

(b) H-bonds can form between different segments of the coil.

(c) Disulfide bonds can form between cysteine groups in different parts of the coil.

(d) Hydrophobic bonds can be formed. Generally, non-polar amino acids are folded on the inside of the protein.

(e) Hydrophilic or polar group interactions between polar amino acids generally formed on the outside

4. Quaternary structuresome proteins have this structure, which occurs when 2 or more protein units, each with its own 10,

20, and 3

0structure, combine to form a more complex unit. Example is hemoglobin. It consists of 2 identical chains and 2

identical chains. Each chain enfolds a heme, which is an iron-containing group.

NITROGEN IN PROTEINThe average %N present in protein is 16%; that is, about

1/6 of protein is N. Because protein is the major food that contains N, the chemist can determine the amount of protein present

in a food substance by determining the amount of N present.

The amount of protein in the food can therefore be calculated by multiplying the weight of N by 6 and converting this to a

percentage of the total.

For example, suppose that a 100-g sample of food yielded 4 g of N on chemical analysis. Since the amount of total nitrogen

in protein is one sixth of the total amount of

protein present, the amount of protein present is 6 x 4 g or 24 g. Then the percentage of protein present in the original 100-g

sample is 24%.Melamine, C3H6N6a white, crystalline compound that is slightly soluble in water, melts @ 3540C and is a cyclic trimer of

cyanamide; used to make melamine resins and in tanning of leather. Also known as cyanurotriamide, cyanurotriamine and

cyanuramide; 1,3,5-Triazine-2,4,6-triamine.

NH2

N N

H2N N NH2CLASSIFICATION OF PROTEINS1. Simple proteinson hydrolysis, simple proteins yield only amino or derivatives of amino acids.

2. Conjugated proteinson hydrolysis, conjugated proteins yield amino acids + some other type of cpd.

Conjugated protein consists of a simple protein combined with a non-protein compound.

3. Derived proteinsare produced by the action of chemical, enzymatic and physical forces on the other

two classes of protein. Derived proteins include proteoses, peptones, polypeptides, tripeptides, and

dipeptides. They also can be hydrolyzed to amino acids.

CLASSIFFICATION ACCDG. TO SOLUBILITYProteins are classified accdg to their soly in various solvents and whether they are coagulated by heat.

SIMPLE PROTEINSType of Protein

Solubility

Coagulated by

heat Examples

Albumins Soluble in water and salt solutions yes Egg albumin; serumalbumin;

lactalbumin

Globulins Slightly soluble in H2O;

Soluble in salt solns

yes Serum globulin;

Lactoglobulin;

Vegetable globulin

Albuminoids Insol in all neutral solvents and in dilute

acid and alkali

no Keratin in hair, nails,

feathers; collagen

Histones Soluble in all salt solns;

insol in very dilute NH4OH

no Nucleohistone in thymus gland;

globin in hemoglobin

CONJUGATED PROTEINS

Type Prosthetic group (Non- Protein

portion of the combination)

Examples

Nucleoproteins Nucleic acids Chromosomes


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Glycoproteins Carbohydrates Mucin in saliva

Phosphoproteins Phosphate Casein in milk

Chromoproteins Chromophore group

(color-producing grp)

Hemoglobin, hemo-cyanin;

flavoproteins

cytochrome

Lipoproteins Lipids Fibrin in blood

Metalloproteins Metals Ceruloplasmin (with Cu)

Siderophilin (with Fe)

in blood plasma

Nucleoproteins are proteins combined with nucleic acids.

Glycoproteins are proteins containing CHO in varying amounts. Glucose is not found in glycoproteins, except for collagen.

Glycoproteins are present in most organisms, including animals, plants, bacteria, viruses and fungi. Human cell membranes are

about 5% carbohydrate, which is present as glycoproteins and glycolipids. Glycophorin is a glycoprotein found in membranes of

human erythrocytes. Heparin, which inhibits clotting of blood, is also a glycoprotein.

Functions of glycoproteins:

1. in cell membranes

2. structural proteins (collagen)

3. lubricants (mucin and mucous membranes)

4. transportation molecules for vitamins, lipids, minerals and trace elements5. immunoglobulins as interferon

6. hormones such as thyrotropin (TSH)

7. enzymes such as hydrolases and nucleases

8. hormone receptor sites

9. for specification of human blood types

LIPOPROTEINSLipoproteins are proteins containing lipids; they are present in cell membranes.

Lipids such as cholesterol and triglycerides are not soluble in water and so must be complexed to a water-solube carrier

protein, which is called lipoprotein.

Plasma lipoproteins consist of a neutral lipid core of triglyceride and cholesterol, protein and phospholipid. The relative

proportions of nonpolar lipid, protein, and polar lipid determine the density, size and charge of the resulting lipoproteins. The

density of lipoproteins has been used to classify them:

1. Chylomicronsproduced in the intestinal mucosa and are used to transport dietary lipids into the blood plasma via the

thoracic lymph duct. They are responsible for the creamed-tomato-soup appearance of blood following a meal containing

fats.

2. Very low density lipoproteins (VLDL) transport glycerides synthesized by the liver to other parts of the body.

3. Intermediate low density lipoproteins (IDL)from the breakdown of VLDL

4. Low density lipoproteins (LDL)end products of breakdown of VLDL; they provide cholesterol for cellular needs. LDL is

thought to promote coronary heart disease by first penetrating the coronary artery wall and then depositing cholesterol to

formatherosclerotic plaque.

5. High density lipoproteins (HDL)involved in the catabolism of other lipoproteins. They incorporate the cholesterol andphospholipid released by a lipoprotein. HDLs may also remove excess cholesterol from peripheral tissues.

6. Lipoprotein(a), similar to LDL; The striking similarity of lipoprotein(a) to human plasminogen has stimulated intense

studies as to a possible link between atherosclerosis and thrombosis. Primarily, results have indicated that lipoprotein(a) is

an independent risk factor (similar to total cholesterol) for coronary heart disease.

Elevated LDL levels have been associated with an increased risk of developing coronary artery disease, whereas elevated HDL

levels appear to reduce the risk. Women have higher HDL levels than men (55 vs. 45 mg/100 mL), and this may account for

womens lower rate of heart disease. Aerobic exercise increases HDL levels (marathon runners average 65 mg/100 mL).

CLASSIFICATION ACCORDING TO FUNCTION

Type of Protein Example Use1. Structural Collagen In connective tissues

Keratin In hair and nails

2. Contractile Myosin, actin In muscle contraction

3. Storage Ferritin In storage of iron needed to make hemoglobin

4. Transport Hemoglobin In carrying oxygen

Serum albumin In carrying fatty acids

5. Hormones Insulin In metabolism of CHO

6. Enzymes Pepsin In digestion of protein

7. Protective Gamma globulin In antibody formation

Fibrinogen In blood clotting

8. Toxins Venoms Poisons


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FUNCTIONS OF PROTEINSAmong all biomolecules, proteins have the most variety of functions. Proteins provide:

1. Structure, protection and support; Examples are collagen (connective tissues)

fibroin (silk protein)

elastin (blood vessels)

2. Defense and protection against chemical and mechanical injury.

Keratin in the skin provide protection.

Fibrinogen and thrombin induce blood clotting.Antibodies or immune-globulin protects man from infection.

3. Regulation through the body hormones. Examples are insulin, glucagon, growth and sex hormones.

4. Catalysis through enzymes. Enzymes are proteins that direct and regulate the speed of biochemical

reactions. Examples are amylase, oxidase, phosphatase, etc.

5. Transport as carrier of ions or molecules across cell membranes or between cells. Other transport

proteins are hemoglobin, carrying O2 to the tissues and lipoproteins, LDL and HDL, that transport lipids

from the liver and intestines to other organs.

CLASSIFICATION ACCORDING TO SHAPE

1. Globular proteinsconsists of polypeptides folded into the shape of a ball. They have a length-to-width ratioof less than 10. They are soluble in water or form colloidal dispersions and have an active function. Examples are

hemoglobin, albumin, and the globulins.

2. Fibrous proteinsconsist of parallel polypeptide chains that are coiled and stretch out. They have a length-to-width ratio

greater than 10. Fibrous proteins are insoluble in water. Examples include collagen,fibrin, and myosin.

PROPERTIES OF PROTEINS1. Colloidal NatureProteins form colloidal dispersions in water. Being colloidal, proteins will pass through a filter paper but

not through a membrane. This inability of proteins to pass through a membrane is of great importance in the body. Proteins

present in the bloodstream cannot pass through the capillaries and should remain in the bloodstream. Since proteins cannot

pass through membranes, there should be no protein material present in the urine. The presence of protein in the urine

indicates damage to the membrane in thekidneyspossibly nephritis.

NEPHRITIS OR NEPHROSIS IS A DISEASE OF THE KIDNEYS IN WHICH THE FILTRATION PROCESS PERMITS PROTEIN

MOLECULES FROM THE BLOOD TO BE DISCHARGED IN THE URINE. THIS DEPLETION OF PROTEINS IN THE BLOOD INTER-

FERES WITH THE PASSING OF FLUID FROM NORMAL TISSUE AND CAUSES SWELLING THROUGHOUT THE BODY,

PARTICULARLY AROUND THE EYES, HANDS AND FEET. WHEN THERE ARE NO OTHER COMPLICATIONS, THE CONDITION IS

NOT LIFE THREATENING AND CAN EASILY BE TREATED.

2. Denaturation of protein refers to the unfolding and rearrangement of the secondary and tertiary structures of a protein

without breaking the peptide bonds. A protein that is denatured loses its biologic activity. When the conditions for

denaturation are mild, the protein can be restored back to its original conformation by carefully reversing the conditions that

caused the denaturation. This is called reversible denaturation. If the conditions that caused the denaturation

are drastic, the process is irreversible.

Denaturation agentsPhysical agents

(a) Heat: Water is necessary for denaturation and coagulation by heat. The steps in heat coagulation are: 1st

denaturation;

2nd

flocculation; 3rd

coagulation.

(b) Light: Clouding of the lens of the eye in old-age cataract is probably due to denaturating of the globulins present in the

lens. Glass workers are particularly subject to cataract presumably as a result of infrared rays emanating from the molten

glass. Change of fibrinogen to fibrin in blood clotting may be partly due to light.

(c) Surface action

(d) High pressure

(e) Mechanical agitation

Chemical agents

(a) Organic solvents (alcohol, acetone, ether)

(b) Acids and alkalies

(c) Salts of heavy metals

(d) Enzymes

(e) Detergents

(f) Urea and guanidine


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Alteration of Properties due to Denaturation

I. Physical (a) Increase in the viscosity of the soln

(b) Increase in the rate of diffusion

(c) Cannot be crystallized

(d) Decreased in solubility

II. ChemicalSome groups in the denatured protein molecules become exposed and readily detectable.

Among these groups are the sulfhydryl (SH), disulfide (SS) and the phenolic group. Theyare shielded in the native state. Denaturation decreases the solubility of proteins.

III. Biological

(a) Increased digestibility by proteolytic enzymes has been found in certain denatured proteins.

(b) Enzymatic or hormonal activity is usually destroyed.

(c) Modification of the specific immunological properties.

3. Chemical Agents or Conditions That Cause

Protein Denaturation

(a) Alcohol coagulates or precipitates all types of protein except prolamines. Alcohol is used as a disinfectant

because of its ability to coagulate the protein present in bacteria. Alcohol denatures protein by forming hyd-

rogen bonds that compete with the naturally occurring H-bonds in the protein. Such a pxs is not reversible.

As an antiseptic, alcohol, specifically ethanol works best at a concentration of70 % in water. Pure alcohol coagulates

protein on contact. If pure alcohol is pouredover a germ, it would penetrate the cell wall of that organism in all directions,

coagulating the protein just inside the cell wall. This ring of coagulated protein would then prevent the alcohol from

penetrating farther into the cell, and no more coagulation would take place. At this time the cell would become

dormant, but not dead. Under proper conditions, the organism could again begin to function.

However, if only 70% is poured over a germ, the diluted alcohol also coagulates the protein but at a slower rate, so that it

penetrates all the way through the cell before coagulation can block it. Then all the cells are coagulated, and the organismdies.

(b) Salts of Heavy Metals

Heavy metal salts, such as mercuric chloride or silver nitrate (lunar caustic), precipitate protein.

These denature protein irreversibly by disrupting the salt bridges and the disulfide bonds present in the protein. They are

very poisonous if taken internally because they coagulate and destroy proteins present in the body.

Antidotes for metallic poisons: egg white, milk and other liquid protein. The heavy metal salts react with the egg white and

precipitate out. The egg white colloid has a charge opposite to that of the heavy metal ion and so attracts it. The ppt thus

formed must be removed from the stomach by an emetic or the stomach will digest the egg white and return the

poisonous material to the system. An emetic is a medicine used to induce vomiting. Or it can be removed by a stomachtube to prevent the digestion ofprotein and the liberation, re-solution and absorptionof the poisonous metal.

AgNO3 is used in cauteries. It precipitates the proteins of as silver salts. To cauterize is to destroy dead or abnormal

tissue by applying a caustic, intenseheat or cold. Dilute silver nitrate solution is used as a disinfectant in the eyes of

newborn infants.

(c) Heat. Gentle heating causes reversible denaturation of protein, whereas vigorous heating denatures protein irreversibly by

disrupting several types of bonds. Egg white, a substance containing a high percentage of protein, coagulates on heating.

Heat coagulates and destroys protein present in bacteria. Hence, sterilization of instruments and clothing for use in

operating rooms requires the use of high temperatures. The presence of protein in the urine can be determined by heating a

sample of urine, which will cause the coagulation of any protein material that is present.

denature protein irreversibly by disrupting salt bridges and H-bonds.

Tannic acid has been used extensively in the treatment of burns. When this substance is applied to a burn area, it causes

the protein to precipitate as a tough covering, thus reducing the amount of water loss from the area. It also reduces

exposure to air. Newer drugs have taken the place of tannic acid for burns, but an old-fashioned remedy still in use for

emergencies involves the use of wet tea bags which contain tannic acid. Tannic acid is also used to relieve diarrhea.

Commercially, it is used to precipitate collagen in hides, thus yielding leather.

Picric acid is used in the treatment of burns because it produces an astringent effect on the tissue, diminishes secretion of

the mucous membranes and prevents absorption of toxins.


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(e) Radiation. Ultraviolet or x-rays can cause protein to coagulate. The radiation denatures irreversibly by disrupting the

H-bonds and the hydrophobic present in the protein. In the human body the skin absorbs and stops the UV rays from the

sun so they do not reach the inner cells. Proteins in cancer cells, which are rapidly dividing cells, are more susceptible to

radiation than those present in normal cells, so x-irradiation is used to destroy cancerous tissues.

(f) pH Changes in pH can disrupt H-bonds and salt bridges causing irreversible denaturation. Proteins are coagulated by such

strong acids as concd HCl, H2SO4, and HNO3. Casein is precipitated from milk as a curd when it comes in contact with the HClof the stomach. Hellers ring test is used to detect the presence ofalbumin urine. A layer of concd HNO3 is placed carefully

under a sample of urine in a test tube. If albumin is present, it will precipitate out as a white ring at the interface of the two

liquids. If acid or base remains in contact with protein for a long period of time, the peptide bonds will break.

(g) Oxidizing and reducing agents. OA such as bleach and nitric acid and RA such as sulfites and oxalates denature protein

irreversibly by disrupting disulfide bonds.

(h) Salting Out. Most proteins are insoluble in saturated salt solutions and precipitate out unchanged. To separate protein

from a mixture of other substances, the mixture is placed in a saturated salt solution such as NaCl, Na2SO4, or (NH4)2SO4.

The protein precipitates out and is removed by filtration. The protein can then be purified from the remaining salt by the

process of dialysis.

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Proteins Hand Out