Proteins

Building blocks, structure and function

Learning outcomes• The basic structure of an amino acid

(structure of specific amino acids is not required).

• The formation of polypeptides and proteins (as amino acid monomers linked by peptide bonds in condensation reactions)

• The significance of the proteins primary structure in determining its 3-d structure and properties (globular and fibrous proteins and types of bond involved in 3-d structure)

Homework – learning objectives

• Compare and contrast the structures and functions of globular and fibrous proteins. Use examples of different types of proteins to illustrate your answer.

• Minimum 500 words

• Reference your work. Include at least three different references, at least one must be a book or journal.

ProteinsChemical nature• Contain the elements C, H, O, N (S, P sometimes)

• Monomer: Amino Acid• They all have the same basic structure• Apart from a variable group ‘R’ JOBS of Proteins• Structural components• Membrane carriers• Enzymes• Hormones• Antibodies

Enzymes – amylase, sucrase, pectinase Hormones – insulin, glucagon Oxygen transporters - haemoglobin Cell membrane transport – sodium channels Cell surface receptors – beta receptors Structural – hair, skin Cytoskeleton – spindle fibres cell division Antibodies - immunoglobulins Pigments - melanin

1000s different proteins different functions

All proteins are made from amino acids

Acid or carboxyl group Amino Group

The simplest Amino Acid

Joining amino acids togehter

2 amino acids molecules bond together

via a

condensation reaction

to form

a peptide bond

to make

a dipeptide

A Condensation Reaction

© Pearson Education Ltd 2008This document may have been altered from the original

The same condensation reaction occurs over and over again to join many amino

acids together to make a polypeptide

Breaking a peptide bond - Hydrolysis

Proteins from amino acids – Protein Synthesis

• Site of manufacture: Ribosome

• Uses m-RNA to put amino acids in the right order

• A specific polypeptide is made

PRIMARY Structure

The sequence of an amino acids linked together in a polypeptide chain

val

lys

phe

gly ar

g

cys

val

gly

PRIMARY STRUCTURE sequence of amino acids in the

polypeptide chain

Held together by STRONG peptide bonds

Definition of secondary structure

• A regular repeating pattern of shape in a polypeptide chain, for example an alpha- helix or beta pleated sheet

SECONDARY STRUCTUREPolypeptide chain folds forming

α-HELIX or β-SHEETStructure held by HYDROGEN

BONDS

α-HELIX

β-SHEET

Hydrogen bonds

Polypeptide Chain

β-SHEETα-HELIX

How is the shape maintained?

• By hydrogen bonds between different amino acids in chain.

• Although weak there are many of them

• So great stability given

Tertiary Structure

• The overall 3-D shape of a protein molecule.

What holds the shape in place?

..Different types of bond and interaction.

• Disulphide bridges• Ionic bonds• Hydrogen bonds• Hydophobic and

hydrophillic interactions

What is the role of the tertiary structure?

Vital to the protein’s function

• Many molecules must have a specific shape in order to do a job:

• Examples –

hormones to fit into the receptor site on a membrane

an enzymes’s active site must have a complementary shape to its substrate

Types of ProteinThe 3-D shape of proteins fall into two main

categories

• Globular – e.g. haemoglobin

compact and globe shaped

water soluble

• Fibrous- e.g. collagen

regular repeating sequences, fibres, insoluble

P103 OCR text book has a good table

What happens when there is a change of a single amino acid in

the sequence its primary structure?

The places in which the different types of bonds can form are determined by the amino acid sequence.

Change this and there is a change on the tertiary structure – its 3-D shape

And therefore the proteins ability to carry out is function

Quaternary Structure

• Where protein structure consists of more than one polypeptide chain – the overall 3-D structure of the molecule.

Haemoglobin• Job – to transport oxygen around body

• Globular protein

• Soluble in water

• Wide range of amino acids in primary structure

• Contains prosthetic group (a non-protein part) called haem

• Much of moleclue would into an alpha-helix

Collagen• Job – provide mechanical strength

arteries, bones tendons and cartilage

• Fibrous protein

• Insoluble

• 35% of amino acids are glycine

• No prosthetic group

Collagen