Transcript Proteins - Humble ISD

Hannah Barreca
Daria Lukasz
Ian Reucroft
Roshelle Belfer
Stephanie Puthumana
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Amino Acids contain a
central carbon chain
with
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An amino group NH2
A carboxyl group COOH
A hydrogen
Side chain
faculty.irsc.edu
thefullwiki.org
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Crystalline solids
Very high melting point in excess of 200°C
Very soluble in water, not soluble in organic solutions
Exist as ionic species
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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
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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
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If OH¯ ions added, pH rises
◦ NH3+ loses H+
◦ Amino acid is negative
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Lower pH by adding acid
◦ -COO¯ gains H+
◦ Amino acid is positive
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Thus amino acids can be separated by electrophoresis
◦ Positive acids travel to cathode
◦ Negative acids travel to anode
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If positive amino acid has alkali added to it
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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
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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
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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.ni
h.gov/books/NBK284
18/
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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)
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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
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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
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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 intrachain) 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
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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
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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
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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
Hormone
Insulin
Breaks lactose into glucose and
galactose through hydrolosis
Stores glucose as glycogen
Immunoprotein
Antibodies
Transport
Hemoglobin
Protect body from foreign substances
(e.g. pathogens)
Carries oxygen from lung to cells
Energy source
Casein
Protein in milk
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http://themedicalbiochemistrypage.org/proteinstructure.html
http://www.chemguide.co.uk/organicprops/aminoacid
s/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