Biological Molecules 2 2013.ppt

Biomolecules 2 Proteins Pork Chop Willie 1

Biological Molecules 2 Proteins

Learning outcomes

Describe the basic structure of an amino acid and the formation of polypeptides and proteins (as amino acid monomers linked by peptide bonds in condensation reactions)

Explain the significance of a protein’s primary structure in determining its three-dimensional structure and properties (globular and fibrous proteins and types of bonds involved in three dimensional structure).

Pork Chop Willie 2Biomolecules 2 Proteins

Proteins - Functions

Wide range of Antibodies Enzymes Many hormones

Various proteins make up muscles, ligaments, tendons, Hair Important components of cell membranes


Building blocks: Amino acids

All proteins composed of Amino acids 20 different amino acids Plants can make all 20 amino acids Animals can only make some, the rest

obtained in their diet (essential amino acids)



Amino Acids R groups can be any of 20

different forms giving 20 naturally occurring amino acids

Primary structure

Two amino acids join together in a condensation reaction to form a dipeptide

This process is repeated to form a polypeptide chain

A protein is made up of one or more polypeptide chain

The sequence of amino acids is known as the primary structure


Pork Chop Willie 10

Secondary structureα-helix

A chain of amino acids may twist to form an α-helix

Within the helix, hydrogen bonds form between C=O of the carboxylic acid and –NH2 of the amine group


Secondary Structureβ-pleated sheet

Several chains may link together, with hydrogen bonds holding the parallel chains in an arrangement known as β-pleated sheet


Secondary structure


Tertiary structure

Polypeptide chains often bend and fold to produce three-dimensional shape.

Chemical bonds and hydrophobic interactions between R group maintain this final tertiary structure of the proteins


Tertiary structure

An R group is polar when the sharing of electrons within it is not quite even

Polar R groups attract other polar molecules and are therefore hydrophilic (water attracting)

The non-polar groups are hydrophobic (water repelling)

Non-polar hydrophobic R groups are arranged to face inside the proteins excluding water from the centre of the molecule


Tertiary structure


Quaternary structure

A protein may be made of several polypeptide chains held together

e.g. haemoglobin, insulin Only proteins with several polypeptide

chains have quaternary structure


Haemoglobin

Haemoglobin consists of four polypeptide subunits two are called α-chains, the other two are called β- chains

Each unit is held in place by a number of bonds and interactions.

These interactions give the molecule a specific shape.

This shape is important for the molecule to carry out its function.


Haemoglobin

A specialised part of each polypeptide called a haem group contains an iron(Fe2+) ion.

Haem group gives haemoglobin its colour Each haem group binds with one oxygen

molecule Therefore one haemoglobin molecule

contains four oxygen molecules


Globular proteins

Folded into a compact spherical shape These proteins are soluble due to the

hydrophilic side chains that project from the outside of the molecule.

This make them important in metabolic reactions.

Enzymes are globular proteins


Globular Proteins

Enzymes three dimensional shape is crucial to their ability to form enzyme substrate complexes and catalyse reactions in cells.

Three dimensional shape is critical in their role of binding to substances. e.g proteins in membranes

Antibodies are also globular proteins and rely on shape to bind to microorganisms


Fibrous proteins

Do not fold in to ball shape but remain as long chains

Several chains can be cross-linked for additional strength.

These insoluble proteins are important structural molecules.


Fibrous Proteins

Keratin in Hair and Skin. Collagen in skin. Tendons Bones Cartilage Blood vessel walls


Fibrous Protein:Collagen


Collagen


Denaturation

If proteins are heated or the pH is changed they can loose their shape and unfold.

Such proteins are said to be denatured. In a cell denatured proteins are broken up

and the amino acids reused unless they can be refolded.

Protein aggregates are toxic and accumulations can causes diseases e.g. Altzheimer’s disease, Scrapie


Enzymes

Many proteins catalyse chemical reactions.

These proteins are known as enzymes.

All enzymes have a definite shape and have a part known as the active site where the substrate(s) bind.


Biological Molecules 2 2013.ppt

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