PROTEINS_Biochemistry.doc

7/28/2019 PROTEINS_Biochemistry.doc

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PROTEINS

Proteins are present in every living cell.

Their very name, derived from the Greek word proteios, which means holding first

place, signifies the importance of these substances.

Another aspect of proteins is their importance in our nourishment. proteins are needed forgrowth and maintenance of body tissue.

Some common foods with high (over 10%) protein content are fish, beans, nuts, cheese,

eggs, poultry, and meat.

These foods tend to be scarce and relatively expensive. Therefore, proteins are the classof foods that is least available to the undernourished people of the world.

A. THE STRUCTUREFUNCTION CONNECTION

Proteins function as structural materials and as enzymes (catalysts) that regulate the countless

chemical reactions taking place in every living organism, including the reactions involved in thedecomposition and synthesis of proteins.

All proteins are polymeric substances that yield amino acids on hydrolysis.

a. simple proteins - those that yield only amino acids when hydrolyzed

b. conjugated proteins - those that yield amino acids and one or more additionalproducts

There are approximately 200 different known amino acids in nature.

Some are found in only one particular species of plant or animal, others in only a few life-forms.

But 20 of these amino acids are found in almost all proteins.

B. THE NATURE OF AMINO ACIDS

Each amino acid has at least two functional groups: an amino group ( -NH2) and a carboxyl

group (-COOH).

alpha ()amino acids -the amino acids found in proteins because the amino group isattached to the first or () carbon atom adjacent to the carboxyl group.

The beta () position is the next adjacent carbon The gamma () position the next, and so on.

The following formula represents an -amino acid:

Amino acids as a whole are represented by this general formula:

Amino acids are divided into FOUR GROUPS based on the characteristics of the aminoacid side chains.


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1. Nonpolar amino acids: The side chain for each of these amino acids is either aliphatic oraromatic in nature and is hydrophobic. These amino acids dontinteract with water well and are commonly found buried in the

middle of a protein structure.

Ex. Alanine, Isoleucine, Leucine, Methionine, Phenylalanine, Proline, Tryptophan, Valine

2. Polar, uncharged amino acids: These groups are attracted to water and are, thus, hydrophilic.

Ex. Asparagine, Cysteine, Glutamine, Glycine, Serine, Threonine, Tyrosine

3.Acidic amino acids: Each of these amino acids has a side chain that contains a carboxylic acid.Acidic amino acids are also known as negatively charged amino acids.

Ex. Aspartic acid, Glutamic acid

4. Basic amino acids: The side chain of each of these amino acids contains a nitrogen that canact as a base. Basic amino acids are also known as positively charged

amino acids.

Ex. Arginine, Histidine, Lysine

Perhaps the most important role played by amino acids is as the building blocks for proteins.

However, selected amino acids also have physiological importance on their own. Manyneurotransmitters are amino acids or their derivatives.

a. Glycine and glutamic acids - function as chemical messengers between nerve cells insome organisms.

b. Tyrosine - is converted to the very important neurotransmitter dopamine.

A deficiency of this amino acid derivative causes Parkinsons disease, which can berelieved by another compound formed from tyrosine, L-dopa.

Tyrosine is also the parent compound for the flight-or-fight hormone epinephrine

(adrenaline) and the metabolic hormone thyroxine.

c. Histidine - is converted in the body to histamine.

Histamine causes the stomach lining to secrete HCl but is probably best knownfor causing many of the symptoms associated with tissue inflammation and colds;

this is the reason antihistamines are such popular and important over-the-countermedications.

C. ESSENTIAL AMINO ACIDS

During digestion, protein is broken down into its constituent amino acids, which supply much ofthe bodys need for amino acids.

Ten of the amino acids are ESSENTIAL AMINO ACIDS because they are essential to thenormal functioning of the human body.

Since the body is not capable of synthesizing them, they must be supplied in our diets if

we are to enjoy normal health. Some animals require other amino acids in addition to those listed for humans.

On a nutritional basis, proteins are classified as complete or incomplete.

a. Complete protein - supplies all the essential amino acids

b. Incomplete protein - is deficient in one or more essential amino acids. Many

proteins, especially those from vegetable sources, are incomplete.

D. D-AMINO ACIDS AND L-AMINO ACIDS


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L-amino acids - have the amine group on the left side of the -carbon.Only L-amino acids occur in proteins

D-amino acids have the amine group on the right side of the -carbon.D-amino acids occur in nature

E. AMPHOTERISM

Amino acids are amphoteric(or amphiprotic); that is, they can react either as an acid or

as a base.

F. FORMATION OF POLYPEPTIDES

Dipeptide- two amino acid units

Tripeptide: If three amino acid residues are included in a molecule

Tetrapeptide - if four

Pentapeptide - if five and so on.

Peptides containing up to about 4050 amino acid units in a chain are called POLYPEPTIDES.

G. PROTEIN STRUCTURE

Commonly, the termprotein refers to a peptide that contains 50 or more amino acids.

In general, for a protein molecule to serve a specific biological function, it must have a closely

defined overall shape. Chemists typically describe large proteins on several levels:

a. The PRIMARY STRUCTURE of a protein is the amino acid sequence of that protein.

b. The SECONDARY STRUCTURE of a protein can be characterized as a regular, three-dimensional structure held together by hydrogen bonds between the components of the

peptide linkages (the C=O and the H-N ).

Secondary structures provide strength and stability for proteins.

c. The TERTIARY STRUCTURE of a protein refers to the distinctive and characteristicconformation, or shape, of a protein molecule.

d. The QUATERNARY STRUCTURE refers to the shape of the entire complex molecule and is

determined by the way in which the subunits are held together by noncovalentbondsthatis, by hydrogen bonding, ionic bonding, and so on.

G. PROTEIN FUNCTIONS

Proteins are the machinery that makes life possible

1. Structural support: Cells in multicell organisms need to adhere to each other and separate

themselves from their environment. Structural proteins have the strengthnecessary for this function.

For example, this class of proteins is found in connective tissue and skin.

2. Storage: Cells must store nutrients when they are in abundance for times of scarcity. Storageproteins serve as containers for a variety of chemicals needed for life.

For example, the liver sequesters iron in a storage protein after an iron-rich meal inorder to avoid anemia.

3. Transport: Even the simplest cells need to move chemicals from one place to another forexample, across the cell membrane.


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Plants and animals must move chemicals not only within cells but also between cells.

For example, the oxygen you breathe is moved from the lungs to the tissues by thetransport protein hemoglobin.

4. Defense: Defensive proteins kill or repel other organisms.

Immunoglobulins (antibodies) protect us from bacterial invaders.

Many snakes and spiders have a competitive advantage because of poisonous defensiveproteins in their venom.

5. Motion/movement: Muscle protein is one example of a protein with a movement function.

In general, animals are absolutely dependent on muscles for survival.

In fact, chemicals are even moved within cells using motion proteins.

6. Regulation: Regulatory proteins have the responsibility for coordinating lifes many processes.

Insulin, for example, is one of the protein hormones that control blood-glucose levels.

They also coordinate intracellular metabolism.

After a meal, the livers metabolic machinery often shifts to storing glucose in the

polysaccharide glycogen.

Glucose transport and the glycogen synthesis reactions are activated by regulatory

proteins while simultaneously they restrict the use of glucose for other purposes.

7. Catalysis: Catalysis increases the rate of a chemical reaction.

Metabolic reactions are much too slow without a catalyst to sustain life, so essentially

every metabolic reaction requires an enzyme.

H. SOME EXAMPLES OF PROTEINS AND THEIR STRUCTURES

1. Fibrous Proteins

Almost all structural proteins are fibrous proteins.

As their name suggests, these proteins have a fiber-like or elongated shape.

They have highly developed secondary structures from which they derive the strength needed

for a support or structural role.

Found in such diverse structures as hair, horns, fingernails, and hooves.

The most abundant protein in the animal kingdom, COLLAGEN, is a fibrous protein.

Collagen toughens the cartilage found in between bones and provides the flexible material

that forms skin and blood vessels.

Ligaments and tendons owe their unstretchable resilience to collagen

2. Globular Proteins

have a characteristic compact, roughly spherical shape.

Because this structure is frequently very complex, these proteins are capable of very

complex functions.

a. MyoglobinA Storage Protein Myoglobin is one of the simplest of the globular proteins.

It functions as a storage protein, binding oxygen for muscle tissue.

b. HemoglobinA Transport Protein Hemoglobin is found in the red blood cell and transportsoxygen.


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c. Fatty Acid Binding ProteinA Transport Protein. This protein transports fatty acids throughthe blood stream

d. FerritinA Storage Protein Some storage proteins also are formed into simple sacks.

Ferritin is a storage protein in the liver that stores iron to guard against anemia in times

of scarcity

e. MyosinA Protein for MovementMyosin is an important protein in muscle tissue.

It converts chemical energy into physical motion.

f. Human Growth HormoneA Regulatory Protein Human growth hormone is a regulatoryprotein responsible for coordinating overall growth.

g. Immunoglobulin GA Defense Protein. These proteins bind molecules that are foreign tothe body, as a defense against disease

I. TESTS FOR PROTEINS AND AMINO ACIDS

Many tests have been devised to detect and distinguish among amino acids, peptides, and

proteins. Some examples are:

a. Xanthoproteic Test

Proteins containing a benzene ringfor example, the amino acids phenylalanine,

tryptophan, and tyrosinereact with concentrated nitric acid to give yellow products.

Nitric acid on skin produces a positive xanthoproteic test, as skin proteins are modified by

this reaction.

b. Biuret Test

A violet color is produced when dilute copper(II) sulfate is added to an alkaline solution of

a peptide or protein.

At least two peptide bonds must be present, as the color changes only when peptide

bonds can surround the Cu ion.

Thus, amino acids and dipeptides do not give a positive biuret test.

c. Lowry Assay

A dark violet-blue color is produced by a combination of the biuret reaction and thereduction of molybdates/ tungstates by tyrosine and tryptophan amino acids.

This combination of two reactions makes the Lowry assay much more sensitive than the

biuret test.

d. Bradford Assay

The most sensitive common test for proteins.

A dark blue color develops as the protein binds to a specific dye, Coomassie Brilliant Blue.

e. Ninhydrin Test

Triketohydrindene hydrate, generally known as ninhydrin, is an extremely sensitive

reagent for amino acids

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PROTEINS_Biochemistry.doc