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Unformatted text preview: Lecture 2 Proteins, polypeptides, and amino acids • Proteins - functions and synthesis • Polypeptides • Amino acids • Ionisation and pI • Primary Structure • Peptide bonds Signalling between tissues and cells Control proteins Biosynthesis of proteins (translation) What do proteins do? Just about everything!!!!
Enzymes Receptors Physiological functions Breakdown of nutrients (fuels) RNA transcription Structural proteins Proteins are the working molecules of life. Transport across membranes
DNA replication Biochemical functions
Biosynthesis of cellular components Carrier molecules
Cell structure (shape) and ﬂexibility Movement Protein synthesis
Transcription DNA mRNA Translation mRNA polypeptide Campbell Fig 17.26 Polypeptides are linear polymers of amino acids. 1 There are 20 amino acids used to build proteins. amino group 333 NH2 H C COOH R
-carbon carboxyl group The general structure of a polypeptide Campbell Fig 5.18 Proteins fold into complex shapes
1 Proteins are linear polymers of amino acids. 333 333 The biologically functional forms of proteins are folded into specific 3-dimensional structures. 1 Myoglobin Staphlococcal nuclease Triose phosphate isomerase Pyruvate kinase Amino Acids are Stereoisomers (optical mirror images)
All amino acids (except glycine) have four different groups attached to the -carbon. Only L-amino acids are found in proteins. H R R H C C NH3+ COO- COO- NH3+ L isomer D isomer The properties of the 20 amino acids are a function of their side-chains (R-groups).
Side chains vary in: Size. Shape. Charge. Hydrogen bonding capacity. Chemical reactivity. Hydrophobic Learn these Phe Pro Gly Asp Lys Ala Ser Cys Hydrophilic Campbell Fig 5.17 See also E&E p49-51 [36-7] Amino Acids are Dipolar Ions at Physiological pH
NH2 HC R COOH
• uncharged NH3+ H C COOR • in aqueous solution at pH 7 “zwitterions” The carboxyl group has lost a proton. The amino group has gained a proton. Charge on Amino Acid Changes With pH
–NH3+ and –COO- groups change their ionisation state with changes in pH. NH3+ H C COOH
Predominant structure at acid pH H+ NH3+ HC COOR H+ NH2 H C COOR R H+ pKa ~ 2.5 Predominant structure at neutral pH H+ pKa ~ 9.5 Predominant structure at alkaline pH pKa = the pH at which a group is 50% dissociated If the pH is less than the pKa value the proton is on. If the pH is greater than the pKa value the proton is off. Titration of Alanine
CH3 HC NH3+ C OH O +1 pKa 1 2.35 CH3 HC NH3+ C OO0 pKa 2 9.87 CH3 HC NH2 C OO -1 pH Moles OH- Added Ionisation of Amino Acids Some amino acids also have titrateable side chains. Ex. The titration of glutamate (glutamic acid) Titration of Glutamate
(An acidic amino acid)
O C OpKa2 (CH ) 22 HC NH3+ 4.07 C O-1 O O C OH (CH2)2 HC NH3+ C OH +1 O O C OH 1 pKa (CH2)2 HC NH3+ 2.1 C O0O O C O3 pKa (CH2)2 HC NH2 9.47 C O-2 O pH Moles OH- added pI
Isoelectric point: the pH at which a molecule carries no NET charge pI= 1 (pKi + pKj) 2 ...
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