HUMAN BIOCHEMISTRY

-fatin-hanis-bella

B1.1: CALCULATE THE ENERGY VALUE OF A FOOD FROM ENTHALPHY OF COMBUSTION

DATA

∆H = mc∆TKeys: m = massc = specific heat of water (4.18 Jg-1C°-1)∆T = change in temperature

Example:1.00 g cereal raises the temperature of 400 cm3

water in an insulated food calorimeter from23.7 C° to 33.4 C°. Calculate the energy valueper gram of the cereal, assuming the heatcapacity of the calorimeter is negligible andgiven the specific heat of water = 4.18 Jg-1C°-1

(answer: 16.2 kJ per gram of cereal)

B2.1: DRAW THE GENERAL FORMULA OF 2-AMINO ACIDS

• Central carbon of amino acids is made up of hydrogen (H), amino group (-NH), carboxylic acid group (-COOH) and side chain (R)

B2.2: DESCRIBE THE CHARACTERISTIC PROPERTIES OF 2-AMINO ACIDS

• Amino acids are:– Colorless– Crystalline solids– Exist as zwitterions (dipolar ions)– Relatively high melting points– More soluble in water than organic solvents– Are amphoterics (capable of behaving as acids or

bases)

H2NCHRCOOH + H2O H3N+CHRCOOH + OH-

At lower pH, H+ added reacts with OH- and theforward reaction is favoured to replace some ofthe OH- used up. Thus, in an acidic solution, the-NH2 group is protonated.

H2NCHRCOOH + H2O H2NCHRCOO- + H3O+

At higher pH, the base added react with H3O+

and the forward reaction is favoured to replacesome of the H3O+ used up. Thus, in an alkaline

solution, the carboxylic acid group donates aproton and converted to the carboxylate ion.

When a strong acid is added to an aqueous solution of amino acid, the zwitterions accepts the proton thus

minimizing the effect of the acid added.

If a strong base is added, the zwitterion donates H+ to neutralize the base to

form water.

B 2.3 : DESCRIBE THE CONDENSATION

REACTION OF

2 - AMINO ACID TO FORM POLYPEPTIDES

Amino acid contain both an amine functional group and a carboxylic acid function group. When they are both attached to the same carbon atom, they known as 2-amino acid or α–amino acid

diagram of 2-amino acid or α–amino acid :

H H₂N C COOH

R-2-amino acid-

There are about twenty 2-amino acid that occur naturally.

All the twenty 2-amino acid are the basic “building block” of protein in our body.

It consist of long polypeptides which formed by condensation reaction between amino acid.

All amino acid have both a carboxyl group and an amino group which they able to undergo condensation reaction to form substituted amides.

Adjacent amino and carboxylic acid functional group are joined together to form peptide linkage.

This process call as condensation reaction as it involve the formation

of water molecule.

The reaction between two amino acid result in the formation of a

dipeptide and water.

Definition of dipeptide : a product which is a substituted amide made

up of two amino acid joined by a peptide bond or peptide linkage.

The two amino acid residues are joined by a strong carbon - nitrogen

bond.

The process of condensation polymerization is repeated until

long chain of amino acids is formed that is known as

polypeptides.

One example of alanine and glycine, can form two dipeptides:

NOTE: Alanine : have CH₃ on carbon molecule. Glycine : just have H on the carbon molecule.

B 2.4 : DESCRIBE AND EXPLAIN THE STRUCTURE

OF PROTEIN

PRIMARY STRUCTURE : Definition : Linear sequences of amino acids in

polypeptides chain.Primary structure of protein is determined by the

sequence of bases in gene that codes for protein. If there is changing if a single amino acid due to a

mutation in DNA of its gene will alter its properties drastically. Example: sickle anaemia cell.

Each protein has its own unique primary sequence if amino acid.

Primary structure of protein gives the protein ability to carry out its characteristic function.

Eg : a tripeptide containing the amino acid : lysine, glycine and leucine lys-gly-leu

Secondary structure is the chain of amino acid that folds itself due to the intramolecular hydrogen bond.

Two type of secondary structure :α-helix : the protein is twist in a spiralling manner like

coiled spring.β pleated sheet : give a sheet- like structure.

Example of :α-helix : keratin that found in nail and hairβ pleated sheet : silk fibre

SECONDARY STRUCTURE :

Tertiary structure is the overall folding of chain by interaction between distant amino acid and gives protein its three-dimensional shape of single protein.

(when the secondary structure is folded further to form 3D shape of globule)

Tertiary sturucture of protein is folded each other because of they have attraction between amino acid in polypetides

The interaction that involve : Hydrogen bond Van Der Waal’s between non polar side group Ionic attraction between polar groups Disulphide bridge

Example of tertiary structure of protein : Immunoglobin Antibody Enzyme Insulin (Protein based hormone)

TERTIARY STRUCTURE :

Quaternary structure : separate polypeptides chains can interact together to give a more complex structure.

Eg : Haemoglobin has quaternary structure that include 4 protein chain ( two α– chains and two β - chains) which is grouped together around four haem groups

QUARTERNARY STRUCTURE :

B 2.5 Explain how proteins can be analyzed by chromatography and electrophoresis.

Chromatography.•Primary structure of protein can be identified by chromatography paper or electrophoresis.•However, protein must be hydrolyzed first by hydrochloric acid to successively release amino acids.

Procedure :1. A small spot of an unknown amino acid sample is placed in a solvent (eluent) placed

in developing tank with cover.2. Eluent rises up the paper due to capillary action.3. When it meets the sample spot, the different amino acid partition themselves

between the eluent and the paper to some extents4. The move up at different rates.5. When the eluent almost reach the top, the paper is removed from the tank, dried

up, and sprayed with organic dyes (ninhydrin) to develop chromatogram by coloring acids.

6. Compare the position of all spots.

Rf value = =

x y

Alternative ways :If sample of known amino acid is not available, Rf value (retention factor) can be measured and compared with known values as each amino acid had a different Rf value.Sometimes, we may found that two acids have the same Rf value using the same solvent, but using a different solvent may result different Rf value. Chromatogram can be turned through 90o and run again using the second solvent.

Electrophoresis

• Structure of amino acid changes at different value of pH.• At low pH (acidic medium), amine group will be

protonated.• At high pH (alkaline medium), the carboxylic group will

lose a proton.• This is why amino cid can act as buffers.• If H is added, they are removed as –NH and if OH is added,

-COOH will lose a proton to remove OH- ions in water.• For each amino acid, there is a unique pH value named

isoelectric point where the acid exists as zwitterions.

Procedures :

• Electrophoresis is carried out in polyacrylamide gel as medium. Hence, the process is called PAGE (polyacrylamide gel electrophoresis)

• The sample is placed in the centre of the gel and potential different is applied across it.

• Depending on the pH of amino acid, the sample will move at different rates toward the negative and positive electrodes.

• At isolelectric point, a particular amino acid will stop moving as its charge is balanced already.

• When separation is completed, the acids can be sprayed with ninhydrin and identified by comparing distances they have travelled with standard samples or from comparison of their isoelectric point.

B2.6 List the major functions of proteins in the body.

1. Structural• Protein such as collagen (found under the skin) and keratin (found in hair and nails) provide structure and

strength.

2. Biological catalysts• Enzymes act as biological catalyst that speed up the reaction in human body by providing alternate

pathways for the reaction. It lowers the activation energy of the reaction.3. Hormones• Insulin is essential for human body.

4. Antibodies• Types of protein produced result to presence of any foreign materials in human body. It provide immunity

against the diseases. For example, immunoproteins.

5. Transport• Haemoglobin in red blood cells carry oxygen from lungs to the cell. It also carries carbon dioxide from

cells to lung to be excreted out.

6. Energy.• Protein can also be a source of energy for human. Instead of carbohydrates and fats, protein can also be

metabolized to provide energy under certain condition.