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Proteins. Hannah Barreca Daria Lukasz Ian Reucroft Roshelle Belfer Stephanie Puthumana. B2.1 – Draw the general formula of 2-amino acids. Amino Acids contain a central carbon chain with An amino group NH 2 A carboxyl group COOH A hydrogen Side chain. faculty.irsc.edu. thefullwiki.org. - PowerPoint PPT Presentation
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Hannah BarrecaDaria LukaszIan Reucroft
Roshelle BelferStephanie Puthumana
Amino Acids contain a central carbon chain with◦ An amino group NH2
◦ A carboxyl group COOH◦ A hydrogen◦ Side chain
faculty.irsc.eduthefullwiki.org
Crystalline solids Very high melting point in excess of 200°C Very soluble in water, not soluble in organic solutions Exist as ionic species
Contain amine (-NH2) and carboxylic acid (-COOH) functional group
The amine is a base, the acid is an acid◦ This means that protons (H+) move from the carboxyl group to
the amine group◦ This forms a zwitterion
Zwitterions have net charge of zero◦ Are both positively and negatively charged◦ This is the form of amino acids even when solid◦When dissolved, zwitterion ions are formed
If OH¯ ions added, pH rises◦ NH3
+ loses H+ ◦ Amino acid is negative
Lower pH by adding acid◦ -COO¯ gains H+ ◦ Amino acid is positive
Thus amino acids can be separated by electrophoresis◦ Positive acids travel to cathode◦ Negative acids travel to anode
If positive amino acid has alkali added to it◦ Ion has two acidic hydrogens◦ -COOH hydrogen is more acidic and is removed to make water◦We have zwitterion again!◦ Net charge of zero is restored◦ Acid wouldn’t go anywhere in electrophoresis
pH which results in lack of movement is isoelectric point◦ Varies from amino acid to amino acid◦ Not necessarily at pH 7, more often pH 6◦ pHI
Buffer action◦ - COO¯ is proton acceptor: acts as base◦ NH3
+ is proton donor: acts as acid◦ Amino acids thus are amphiprotic and act as buffers,
depending on environment
http://www.ncbi.nlm.nih.gov/books/NBK28418
/
Amino acids are linked with amide groups, known as peptide bonds◦ Atoms in the amide group are linked with covalent bonds(Don’t draw peptide bonds as dotted lines, like in bio)
Peptide bonds form during a condensation reaction between the carboxyl group of one amino acid and the amino group of another amino acid◦Water is released as a product of this reaction
Condensation reaction between glycine and alanine
Image from: http://dl.clackamas.edu/ch106-08/images/68025.jpg
Primary:◦ Amino acids link together to form polypeptide chain in a
condensation reaction◦ The order in which amino acids are linked is determined by the
genes. Ex: Met is always first.◦ Peptide bonds link amino acids
Covalent bond intramolecular
Secondary: ◦ The polypeptide chain is coiled into
either an alpha-helix or beta-pleated sheet
◦ Hydrogen bonds define the structure Alpha-helix: bonding causes the
polypeptide to twist into a helix Bonds are within a molecule, so could
be considered intra-molecular (or intra-chain) forces
Beta-pleated sheets: bonding enables the polypeptide to fold back and forth upon itself like a pleated sheet Bonds are between chains, so could be
called inter-chain forces
Tertiary:◦ Alpha-helices and beta-sheets are folded into compact
globule◦ The folding is driven by non-specific hydrophobic interactions
Between non-polar side chains Ex: Between 2 alkyl side chains in valine. These are attracted by van der
Waals forces and create non-polar regions on the interior of the protein◦ But the structure is stable only when protein parts are locked
into place by specific tertiary interactions between the side groups (R groups) of each amino acid
Tertiary:◦ Specific interactions between R groups:
Disulfide bonding Covalent bonds strongest interactions Ex: Form between S atoms in cysteine
Hydrogen bonding Form between polar side chains Ex: hydrogen bonding is between the R group in serine and aspartic acid
Ionic bonding Form between charged side chains Ex: (CH2)4NH3
+ in lysine and CH2COO- in aspartic acid
Quaternary:◦ Multiple polypeptide chains join together◦ Subunits are formed◦ The quaternary structure is stabilized by same
non-covalent interactions as tertiary structure Hydrophobic interactions Disulfide bonding Hydrogen bonding Ionic bonding
◦ Examples: In hemoglobin, each of 4 subunits have heme group containing iron Collagen has triple helix structure (3 chains of DNA) consisting of
many subunits
Type Example Function
Structural Collagen Connective tissue in skin/tendons
Myosin Controls contraction of muscles
Enzyme Lactase Breaks lactose into glucose and galactose through hydrolosis
Hormone Insulin Stores glucose as glycogen
Immunoprotein Antibodies Protect body from foreign substances (e.g. pathogens)
Transport Hemoglobin Carries oxygen from lung to cells
Energy source Casein Protein in milk
http://themedicalbiochemistrypage.org/protein-structure.html
http://www.chemguide.co.uk/organicprops/aminoacids/proteinstruct.html
Amino Acids. (n.d.). Lecture 27. Retrieved August 24, 2011, from http://butane.chem.uiuc.edu/cyerkes/
the acid base behaviour of amino acids. (n.d.).chemguide: helping you to understand Chemistry. Retrieved August 24, 2011, from http://www.chemguide.co.uk/org