proteins major group of biological molecules. proteins monomers: amino acids ▫always contain an...

ProteinsMajor group of biological molecules

Proteins

•Monomers: amino acids▫Always contain an amino group and

carboxylic acid group•Polymers: peptides

▫Each different sequence of amino acids is a different peptide

•Polymerization Rxn: Condensation Reactions

•Examples: meat, skin, hair, nails, casein

Amino Acids

•The “R” can be an H or any hydrocarbon or organic functional group

•Amino acids are classified based on properties of R group

Amino Acid ClassificationsType of Amino Acid R group contains

Non-polar/hydrophobic Hydrocarbon

Polar but charges Alcohol, sulfhydryl (-SH), or amide (-CONH2)

Basic Amino (–NH2)

Acidic Carboxylic Acid (-COOH)

Zwitterions• Amino acids(AA) can become charged in two

places – thus making them dipolar ions

• Zwitterions result from internal acid/base reaction where the carboxylic acid group on the AA transfers an H to the amino group on the same AA

Amino Acids are Amphoteric

•AA contain both an acid and base group and are therefore amphoteric (able to act as either and acid or a base)

•As a result, amino acids are very good buffers – resist changes in pH

•pH changes just affect the charge of the AA

Isoelectric Point

•Isoelectric point is when the AA is neutral – it is a zwitterion

•The isoelectric point of each AA is different

•Changes in pH cause differing charges on the AA

Condensation to make Proteins

• The bond that results from the condensation reaction of AA to make a protein is called a peptide bond

• The carboxylic acid group on one AA reacts with the amino group on another AA

• Literally millions of possible AA sequences of varying lengths

4 Levels of Structure for Proteins• Primary-

▫amino acid sequence connected through peptide bonds to form a polypeptide

• Secondary- ▫ through hydrogen bonds between carboxylic

groups and amino groups• Tertiary-

▫ further twisting and folding of polypeptide on itself via interactions of R groups

• Quaternary-▫ Interactions between polypeptide chains

Primary Structure

•Dictates rest of protein structure

•Forms the covalent backbone of the molecule

•All polypeptide bonds

Secondary Structure

•Influenced by R groups

•All hydrogen bonding

•Types:▫α-helix▫β-pleated sheet

Alpha(α) Helix• looks like a spiral-

forms via hydrogen bonding between AA 4 units apart

• It’s flexible and elastic

• Folding of 1 polypeptide (intra-chain H bonds)

Beta (β)- pleated sheets• Hydrogen bonding

between polypeptides (inter-chain hydrogen bonds)

• Looks like waves stacked on each other

• Are inelastic and therefore well defined

• Aka Fibrous proteins

Tertiary Structure = conformation•Results from interactions of R groups•All with in the same polypeptide

•Globular conformations are very common and important▫Include all enzymes and hormones▫Water soluble because hydrophilic R

groups on outer surface and hydrophobic on interior

Stabilization of Globular Proteins•In order of strength:•Hydrophobic interactions

▫– between nonpolar side chains•Hydrogen bonding –

▫between polar side chains•Ionic Bonds –

▫between charged side chains•Disulfide bridges –

▫ covalent bonds between sulfur atoms in cysteine

Denaturing of Proteins

•Denaturing – when a protein loses its tertiary structure

•Results in protein being inactive/ losing its function

•Caused by:▫Temperature changes▫pH changes

Quaternary Structure

•Association between different polypeptides

•Ex: ▫Collagen – triple helix of chains▫Haemoglobin – 2 alpha and 2 beta chains

Hydrolysis of Proteins

•Proteins are analyzed by knowing its amino acid composition

•To do this, the peptide bonds in the polypeptide must be broken

•Are broken through hydrolysis reaction – adding water back onto the amino acids (OH onto carboxyl group and H onto amino end)

Separation of AA Sequence

•Chromatography▫Particularly useful with colorful substances▫http://www.youtube.com/watch?v=J8r8hN0

5xXk•Electrophoresis

▫Based on movement of charged particles in an electric field

▫http://video.mit.edu/watch/gel-electrophoresis-10949/

▫http://learn.genetics.utah.edu/content/labs/gel/

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