Topic Outline 3. Proteins

a. Definitionb. Classificationc. Formation of peptide bondd. Test for proteinse. Principle source and dietary importance of proteins

4. Fatsa. Definitionb. Classificationc. Test for fats d. Principle source and dietary importance of fats

3. PROTEIN (meaning 'foremost or first')

• (a) Definition: Proteins are complex organic compounds always containing the elements carbon, hydrogen, oxygen and nitrogen and in some cases sulphur.

• 170 aa known, 20 occur commonly.

• Proteins as diverse as:

– enzymes, hormones , mushroom poisons,

antigens, antibodies , nutritive milk proteins

(globular proteins)

– feathers, spider webs, lens protein of the

eye, turtle shell, nail, silk thread (fibrous

proteins).

Basic units/building blocks are amino

acids.

Amino AcidsGeneral Formula

H

C

R

H

N

H

AMINO GP(BASIC)

C

O

OH

CARBOXYL GP(ACIDIC)

R = residual group

(b) Classification of proteins

Criteria used in classification:1) Structure• Fibrous (eg. Keratin, collagen, silk) • Globular (eg. Enzymes, plasma

proteins some hormones)• Intermediate (eg. Fibrinogen)

(NB: Differences between fibrous and globular proteins.)

(b) Classification of proteins

Criteria used in classification:2) Composition Simple, Conjugated

3. Functions of Proteins

1) Nutrition -- enzymes

2) Respiration & Transport -- Haemoglobin, myoglobin

3) Growth -- hormones

4) Excretion -- urease

5) Support & movement -- muscles

7) Sensitivity & coordination -- hormones

8) Reproduction -- hormones

d. Formation of peptide bonds

H2O

Structure = dipeptide Process = Condensation

HH

N C C

R1H O

OH HH

N C C

R2H O

OH

carboxyl endamino end

HH

N C C

R1H O

HH

N C C

R2 O

OHPeptide bond(covalent bond)

(d)Test for Proteins

Biuret’s test1) Add to the test soln an equal amount of 10% NaOH. (test works in an alkaline medium)

2) Add 0.5% copper sulphate solution dropwise, shaking after each drop.3) A purple/violet colour indicates the presence of proteinCare is needed since addition of excess CuSO4 results in the disappearance of the purple colouration (complex formation)

Basis of Test:

• a test for the peptide linkage• N atoms in the peptide chain forms

a purple complex with Cu2+

• works in alkaline medium• purple complex is the Biuret cpd

which is derived from urea which also contains the -CONH- gp giving a +ve result

C

O

N

H

Biuret Test for Protein Biuret Test for Protein

The peptide bond peptide bond of a protein is sufficiently reactive to change the Biuret reagent from blue to purple.

This test is based on the interaction between the copper ions in the Biuret reagent and the amino groups in the peptide bonds. Positive test

with biuret's reagent

Original blue colour

Tube 1: Water (control)Tube 2: Albumin (protein)Tube 3: Starch                          

1 2 3

e.      Principle source and dietary importance of proteins

• http://www.nutristart.com/protein.html - detailed descriptions of the dietary source and importance of proteins

• http://www.nutramed.com/nutrition/proteins.htm• http://www.americanheart.org/presenter.jhtml?identifi

er=11234 - an account of why high-protein diet is not recommended

Importance of proteins• Protein is important for everyone, regardless of age

or activity level. Protein is best known for its ability to build and maintain lean body mass. It also maintains strong hair, skin, and teeth. But it doesn’t stop there – protein is also vitally important in maintaining blood, organs, tendons, and in the production and smooth functioning of hormones, enzymes, immune cells and brain neurotransmitters. In short, every tissue and cell in our body relies on protein for survival.

Source of proteins

• Conventional protein sources include meat, fish, poultry, eggs, milk and cheese. Beans, legumes and soybeans (i.e. soymilk and tofu) are fairly high in protein.

Essential vs. Non-essential AA

• Essentials - those that cannot be synthesized by body source: from food

e.g. Leucine, Lysine

• Non-essentials - can be synthesized by the human body (usually using “backbone” from other aa or sugars )

e.g. glutamine, proline

How much proteins do I need?

• In order to maintain healthy muscles and to encourage fat loss, we need to eat high quality protein 4 to 6 times per day. The servings should be evenly divided throughout the day, and roughly 2 to 3 hours apart. For women, servings of protein should be between 15 to 25 grams, and men 30 to 40 grams. (The only exception is immediately after a work out when our bodies can absorb 25 per cent of our total daily protein intake).

(I) What are lipids?

• Diverse group defined by their solubility rather than by their structures.

• Therefore, they differ widely in size, structure and functions.

• They are not soluble in water (inorganic solvent) but soluble in organic solvents (like benzene, ether, chloroform).

• Basically, lipids are esters of fatty acids and alcohols.

• They contain C, H and O.

• Examples are:

triglycerides,

phospholipids,

waxes and steroids.

(II) Components of lipids

(1) What are fatty acids?

C13H27COOH

Contain the acidic group (carboxyl group – COOH)General formula:

R-COOH(R= H, CH3, C2H5, etc)R = CnH2n+1

R is referred to as the ‘hydrocarbon’ tail -hydrophobic (therefore insoluble in water)

(2) Alcohols

• Organic compounds containing the hydroxyl group (-OH) as the functional group.

• General formula: R-OH where R is the alkyl group or a hydrogen atom

• E.g. Ethanol - C2H5 OH

Glycerol -

(b) Classification of lipids

• Based on different number and

composition of fatty acids and alcohol

and

• Whether they contain other elements

like sulphur, phosphorus, etc.

• Most common lipids• Called FATS if solid at 20oC• Called OILS if liquid at 20oC

Triglycerides

c. Formation of lipids• Organic compounds formed when an acid and

an alcohol undergo condensation reaction / or dehydration (where water is removed).

Alcohol + Acid --------------- ester + water Esterification

R-OH + HOOC-R1 --------- R-OOC-R1 + H-OH

•An ester bond (-COO-) is formed•Hydrolysis is the opposite of condensation, in which water is added to esters to regenerate the acid and alcohol.Ester ---------------------- Alcohol + Acid

Acid/ Alkali/ enzyme

R1

R2

R3

3 H2O

O ||H-O-C-R1

O ||H-O-C-R2

O ||H-O-C-R3

Triglycerides

• Structure : 1 glycerol + 3 fatty acids

(d) Test for lipids(1) Ethanol emulsion test

• Method: – Add 2cm3 of sample to ethanol.

– Shake vigorously.

– Add equal volume of cold water.

• Result: cloudy white emulsion observed

• Theory: – Ethanol emulsifies fats.

– When water is added, an emulsion of tiny lipid droplets reflect light to give an opalescent appearance.

The tube on the right contains oil and water.The oil can be seen floating on the water in the tube on the right.

The one on the left forms an emulsion. The oil is dispersed within the water.

Test for lipidsTest for lipids

Ethanol emulsion testEthanol emulsion test

Ethanol emulsion testEthanol emulsion test

In this test, the ethanol will first dissolve the lipids. Vigorous shaking is required to facilitate this process.

Cold water will cause the formation of an emulsion, of tiny ethanol-lipid droplets suspended in water. These reflect light and give a white opalescent appearance.

(2) Grease Spot test• Method:

– Put a small drop of sample n a piece of paper.

– Soak in water for 1-2 mins.– Dry over bunsen burner.– Hold paper up against light.– Result: If fats are present, spot remains

translucent when the surrounding is dry.

Note: this test is less reliable than the Ethanol Emulsion Test

Grease spot test (Left negative, right positive)

Sudan III TestSudan III Test

Fat globules are stained red and are less dense than water (red- stained layer of oil separates from colourless water below)

(Left negative, right positive)           

SUDAN III test 

Under the microscopeBelow left: oil and water X 40 - Note the large fat droplet/ globule on the upper, right half of the photograph.Below right: oil, water and detergent (emulsifier) X 40 - The large oil droplets have been broken up into smaller droplets after shaking.

                                                              

e. Principle source and dietary importance of fats• http://www.findarticles.com/p/articles/

mi_m0KFY/is_4_20/ai_98464597 - good and bad fats?

• http://www.healthgoods.com/Education/Nutrition_Information/Nutrition_and_Health/nutrition_guide_men.htm - overall recommended nutrition standards

Principal source of fats

   Fats and oils used in cooking, such as peanut oil, olive oil, corn oil etc.

   Salad dressings and spreads, such as butter, margarine, mayonnaise, peanut butter etc.

• Skin of meat

Importance of trigyclerides• Most efficient energy stores

compared to carbohydrates – due to higher proportion of H compared to O

• Good storage material – compact and insoluble

• Oxidation provides water

• Heat insulator and keeps marine animals buoyant

Lipids and diet• Non-essential FA CAN be synthesized by

body from carbohydrates

• Essential FA CANNOT be synthesized by body; obtained from diet

– Actual function not known -> important for proper structure and functioning of cells

– Shortage leads to retarded growth, kidney failure, reproductive deficiency

– E.g. linoleic acid and linolenic acid (unsaturated FA)

– Especially abundant in vegetable and seed oils