4. proteins
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Transcript of 4. proteins
Proteins are polymers of amino acids
They make up more than 50% dry mass of cells
Proteins contain C,H,O, and N. Some contain S
Proteins may form complexes with other substances
Diversity of functions
Proteins are diverse. Dome of their functions include:
Cell membranes
Antibodies
Haemoglobin
Keratin in hair
Collagen in bone and connective tissue
This is the amino groupIt contains nitrogen and is basicNitrogen bonds to 2 hydrogen atoms and the central carbon
This is the residual groupEach amino acid has a different R groupIt will determine the type of amino acid
Types of amino acidsThe R group gives amino acids different properties
There are 20 essential amino acids used to make proteins
The smallest amino acid is glycine where R = H
Some other R groupsThe amino acid is named according to the R group [in red] Each name has a abbreviation [in blue]
This is the a central carbonIf the R group is not H then this is an asymmetrical carbon
All amino acids can exist as optical isomers but in nature only one form is foundThis is the L -isomer
Amino acids form Zwitterions
The molecule has an overall neutral chargeThis only occurs at a particular pHThis is the isoelectric pointThe R group will change the isoelectric point for different amino acids
Or:
If the amino acid is placed in a more acidic (more H+) solution:
Protons are accepted here
Because protons are removed from solution the solution becomes less acidic again
The amino acid has acted as a buffer
To recapAmino acids form zwitterions – with both basic and acidic propertiesAt a certain pH – the isoelectric point - the ions formed are neutralThey are amphotericThey are able to donate or accept hydrogen ions to keep the pH the sameSo they act as buffers
Soluble proteins in cells and in the blood are important as buffers
This is a dipeptide molecule
Held by a peptide bond
Water is formed this is condensation
A peptide bond can be broken by hydrolysis or addition of water
In this way long chains of amino acids can be formed = polypeptidesNotice that there is still an amino end and a carboxylic end
The order of the amino acids in the chain is determined by the DNA sequence of the gene coding for the protein
This is the Primary Protein structure
The primary sequence is folded into a highly specific 3D structure which is held together by various bonds
Bonds involved in maintaining the shape of proteins
H bonds
Ionic bonds
Disulfide bond/ bridges
Hydrophobic interactions
1. H Bond [hydrogen bonds]
These are weak attractions between an electronegative oxygen in a carboxylic group and an electropositive H on OH or NH groupsThe large number of these bonds make them significant even though they are weak
2. Ionic bonds
These only form at the right pH
An electron is donated or accepted between ionized amine and carboxylic group.
A relatively weak bond broken by a change in pH
Ionic bonds may also form between residual groups
4. Hydrophobic interactions
Hydrophobic R groups in the polypeptide chain are shielded by Hydrophilic ones in an aqueous solution
There are 4 levels of Protein structure
Primary SecondaryTertiaryQuaternary
All proteins show this
Seen in most proteins
Seen in globular proteins
Seen in some proteins
Primary structure
This is the sequence of amino acids It is determined by the DNA codeThe amino acids are held together by peptide bonds
All proteins will have primary structure
Secondary structure
The polypeptide chain is twisted into
∝-helixOr a
β- pleated
sheet
Held together by H-bonds
β-Pleated sheets are formed by hydrogen
bonds between parallel chains of polypeptides or a single chain folded back on itself
Fibrous Proteins
e.g. collagenA tough protein used as connective tissueIt is an insoluble, fibrous proteinsCollagen and has a high tensile strength
Collagen is made from 3 α-helix molecules twisted like
a rope
The most common amino acid is glycine which is small because R=HThis allows the molecule to twist tightly
The collagen triple helices are bundled together into fibrilsThese form Collagen fibres
Notice how the joins of the fibrils are staggered to prevent lines of weakness forming
Fibrils
Fibre
e.g. elastinThis protein is found in connective tissue for instance in alveoliIt can be stretched and will recoil to the original shape
Tertiary Structure: globular proteins
This structure may involve any of these :
Hydrogen bonds
Disulfide bridges
Ionic bonds
Hydrophobic interactions
There may be sections with secondary structure
The proteins have an overall 3-D globular shape which is highly specific
because it is determined by the bonds forming between specific amino acids in the primary sequence
Globular proteins have specific shapes and are soluble
Enzymes are globular proteinsThey have a specific active site
The hydrophilic exterior R groups makes the molecule soluble in water
Quarternary Structure
e.g. Haemoglobin: made of 4 polypeptides
These are formed from 2 or more polypeptide chainsHeld together by hydrophobic interactions
Conjugated proteinsthese contain a non-protein group
For example the haem group in haemoglobinThere is one haem group in each of the 4 polypeptides