Proteins : Structure & Function

COURSE TITLE: BIOCHEMISTRY 1COURSE CODE: BCHT 201

PLACEMENT/YEAR/LEVEL: 2nd Year/Level 4, 1st Semester

M.F.Ullah, Ph.DShowket H.Bhat, PhD

WHAT IS A PROTEIN?Proteins are polymer (chain) made

up of smaller units called amino acid which are linked together in a specific sequence by peptide bonds

Formation of a Dipeptide

Dehydration synthesis

Amino Acid + Amino Acid --> Dipeptide

Amino Acid + Dipeptide --> Tripeptide

A.A. + A.A. + …..+ Tripeptide --> Polypeptide

AMINO ACID: Sequence

• Dipeptide – 2 amino acids• Tripeptide – 3 amino acids• Oligopeptides – 4-10 amino acids• Polypeptide – more than 10 amino acids• Proteins in the body and diet are long

polypeptides (100s of amino acids)

Peptide bond formation in details:

- Each polypeptide chain starts on the left side by free amino group of the first amino acid. It is

termed as amino terminus or N- terminus.

- Each polypeptide chain ends on the right side by free COOH group of the last amino acid. It is

termed as carboxy terminus or C-terminus.

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Peptides

• Amino acids linked by amide (peptide) bonds

Gly Lys Phe Arg Ser

H2N- -COOH

end Peptide bonds end

Glycine-lysine-phenylalanine-arginine-serine

Protein structure:

There are four levels of protein structure (primary, secondary, tertiary and quaternary)

1. Primary structure: • The primary structure of a protein is its unique sequence of amino

acids.

2- Secondary structure: Results from hydrogen bond

formation between hydrogen of –NH group of peptide bond and the carbonyl oxygen of another peptide bond. According to H-bonding there are two main forms of secondary structure:

α-helix: It is a spiral structure resulting from hydrogen bonding between one peptide bond and the fourth one β-sheets: is another form of secondary

structure in which two or more polypeptides (or segments of the same peptide chain) are linked together by hydrogen bond between H- of NH- of one chain and carbonyl oxygen of adjacent chain (or segment).

• 3. Tertiary structure is determined by a variety of interactions (bond formation) among R groups and between R groups and the polypeptide backbone.a. The weak interactions include: Hydrogen bonds among polar

side chains Ionic bonds between

charged R groups ( basic and acidic amino acids)

Disulphide bonds between cysteine residues of the polypeptide chain

Hydrophobic interactions among hydrophobic ( non polar) R groups.

• 4. Quaternary structure: results from the aggregation (combination) of two or more polypeptide subunits held together by non-covalent interaction like H-bonds, ionic or hydrophobic interactions.

• Examples on protein having quaternary structure:– Collagen is a fibrous protein of three polypeptides (trimeric) that are

supercoiled like a rope.•This provides the structural strength for their role in connective tissue.

– Hemoglobin is a globular protein with four polypeptide chains (tetrameric)

– Insulin : two polypeptide chains (dimeric)

Structural organization of protein

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Types of Proteins

Type Examples• Structural tendons, cartilage, hair, nails• Contractile muscles• Transport hemoglobin• Storage milk• Hormonal insulin, growth hormone• Enzyme catalyzes reactions in cells• Protection immune response

PROTEINS: Function

1. Structural Functions:• Collagen – is the most abundant protein

in mammals, and gives bone and skin their strength

• Keratin – provides structure to hair and nails

2. Functions as ENZYMES• Enzymes are proteins that catalyze chemical

reactions without being used up or destroyed in the process

• Used in – digestion, releasing of energy from nutrients for fuel, triggering reactions that build muscle and tissue

3. Functions as HORMONES• Hormones are chemical messengers that

are made on one part of the body, but act on cells in other parts of the body

• Example: Insulin & Glucagon (hormones that maintain blood glucose level).

4. IMMUNE FUNCTION• The Immune Response is a series of steps

your body takes to mount an attack against invaders (such as bacteria, viruses & parasites)

• Antibodies are blood proteins that attack and inactivate bacteria and viruses

• Once an antibody has been made for a certain invader, your body can more quickly respond (Immunization)

How Much Protein Do We Need?

Adults: 0.8 grams of protein per kilogram of body weight

per day

Endurance Athletes:1.2 to 1.4 g/kg/day

Heavy Weight Trainers:1.7 to 1.8 g/kg/day

Protein SourcesAlmonds (1 cup) 24 grams

Pinto Beans (1 cup) 15 grams

Cheese (1 oz.) 7 grams

Ham (3 oz.) 18 grams

1 Egg 6 grams

2% Milk (1 cup) 8 grams

Clams (3 oz.) 60 grams

Whole Wheat Bread 3 grams

Lean Hamburger 30 grams

Peanut Butter (1 T) 4 grams

Salmon (3 oz.) 20 grams

Tofu (4 oz.) 9 grams

Yogurt (8 oz.) 10 grams

White rice (1 cup) 4 grams

DENATURING of PROTEINS

• Acid, alkaline, heat, alcohol, and agitation can disrupt the chemical forces that stabilize proteins and can cause them to lose their biological structure & function (activity). This process is called as DENATURATION.

• Denaturing of proteins happens during food preparation (cooking, whipping, adding acids) or digestion (in the stomach with hydrochloric acid)

Denaturation

• The main function of red blood cell• Transport of O2 from lungs to tissue• Transport of CO2 from tissue to lungs

• Hemoglobin (Hb) is the iron-containing oxygen-transport metalloprotein in the red blood cells. Hemoglobin in the blood carries oxygen from the respiratory organs (lungs or gills) to the rest of the body (i.e. the tissues) where it releases the oxygen to burn nutrients to provide energy to power the functions of the organism, and it also collects the resultant carbon dioxide to bring it back to the respiratory organs to be released out of the body.

• Each red cell has 640 million molecules of Hb

Structure and function of hemoglobin

Hemoglobin has a quaternary structure made up of four subunits. Each subunit is composed of a protein chain tightly associated with a non-protein heme group.

A heme group consists of an iron (Fe) ion (charged atom) held in a heterocyclic ring, known as a porphyrin.

This porphyrin ring consists of four pyrrole molecules cyclically linked together (by methene bridges) with the iron ion bound in the center. The iron ion, which is the site of oxygen binding, coordinates with the four nitrogens in the center of the ring, which all lie in one plane.

The iron is bound strongly (covalently) to the globular protein and also allows reversible binding of oxygen with it.

In adult humans, the most common hemoglobin type is a tetramer (which contains 4 subunit proteins) called hemoglobin A, consisting of two α (141 amino acids) and two β (146 amino acids) subunits joined together in a quaternary structure by non-covalent interactions such as salt bridges, hydrogen bonds, and the hydrophobic effect. This is denoted as α2β2.

In human infants, the hemoglobin molecule is made up of 2 α chains and 2 γ chains (α2 γ 2)

called hemoglobin F. The gamma chains are gradually replaced by β chains as the infant grows.

Structure of Haemoglobin

Structure of Hemoglobin

Hemoglobin consists of four subunits, each of which has a binding site for oxygen. Therefore, four oxygen molecules can bind to one hemoglobin.