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Proteins
The 411
• Proteins account for more than 50% of the dry mass of most cells
• Protein functions include structural support, storage, transport, cellular communications, movement, and defense against foreign substances
• Most important, protein enzymes function as catalysts in cells, regulating metabolism by selectively accelerating chemical reactions without being consumed.
Amino Acids
• Small – molecules (20 variants)– 1 Amino group– 1 Carboxyl Group– 1 “R” Group– Amino acids differ in their properties due
to differing side chains, called R groups
• Joined by Peptide Bonds to form Polypeptides– A protein consists of one or more
polypeptides– Each polypeptide has a unique linear
sequence of amino acids
Fig. 5-UN1
Aminogroup
Carboxylgroup
Alpha carbon
DNA T A C C G C T C C G C C G T C G A C A A T A C C A C T
mRNA ____________________________________________________________________________
tRNA ____________________________________________________________________________
AA ____________________________________________________________________________
How to Make Proteins!
Fig. 5-17a
Nonpolar
Glycine (Gly or G)
Alanine (Ala or A)
Valine (Val or V)
Leucine (Leu or L)
Isoleucine (Ile or I)
Methionine (Met or M)
Phenylalanine (Phe or F)
Tryptophan (Trp or W)
Proline (Pro or P)
What makes all Amino Acids different?
Fig. 5-17b
Polar
Asparagine (Asn or N)
Glutamine (Gln or Q)
Serine (Ser or S)
Threonine (Thr or T)
Cysteine (Cys or C)
Tyrosine (Tyr or Y)
Fig. 5-17c
Acidic
Arginine (Arg or R)
Histidine (His or H)
Aspartic acid (Asp or D)
Glutamic acid (Glu or E)
Lysine (Lys or K)
Basic
Electricallycharged
Peptidebond
Fig. 5-18
Amino end(N-terminus)
Peptidebond
Side chains
Backbone
Carboxyl end(C-terminus)
(a)
(b)
Polypeptide Formation
Structure
GrooveGroove
Levels of Structure
• The primary structure – Unique sequence of amino acids
• Secondary structure– Found in most proteins, consists of coils and folds
in the polypeptide chain due to Hydrogen Bonding
• Tertiary structure– Determined by interactions among various side
chains (R groups)– Fully Folded
• Quaternary structure– Results when a protein consists of multiple
polypeptide chains
Amino acidsubunits
+H3N Amino end
25
20
15
10
5
1
Primary Structure Primary
• Primary structure, the sequence of amino acids in a protein, is like the order of letters in a long word
• Primary structure is determined by inherited genetic information
Fig. 5-21c
Secondary Structure
Beta pleated sheet
Examples ofamino acidsubunits
alpha helix
• The coils and folds of secondary structure result from hydrogen bonds between repeating constituents of the polypeptide backbone
• Typical secondary structures are a coil called an helix and a folded structure called a pleated sheet
Fig. 5-21f
Polypeptidebackbone
Hydrophobicinteractions andvan der Waalsinteractions
Disulfide bridge
Ionic bond
Hydrogenbond
Tertiary
Quaternary
• Quaternary structure results when two or more polypeptide chains form one macromolecule
• Collagen is a fibrous protein consisting of three polypeptides coiled like a rope
• Hemoglobin is a globular protein consisting of four polypeptides: two alpha and two beta chains
Fig. 5-21g
Polypeptidechain
Chains
HemeIron
Chains
CollagenHemoglobin
Fig. 5-22
Primarystructure
Secondaryand tertiarystructures
Quaternarystructure
Normalhemoglobin(top view)
Primarystructure
Secondaryand tertiarystructures
Quaternarystructure
Function Function
subunit
Molecules donot associatewith oneanother; eachcarries oxygen.
Red bloodcell shape
Normal red bloodcells are full ofindividualhemoglobinmoledules, eachcarrying oxygen.
10 µm
Normal hemoglobin
1 2 3 4 5 6 7Val His Leu Thr Pro Glu Glu
Red bloodcell shape
subunit
Exposedhydrophobicregion
Sickle-cellhemoglobin
Moleculesinteract withone another andcrystallize intoa fiber; capacityto carry oxygenis greatly reduced.
Fibers of abnormalhemoglobin deformred blood cell intosickle shape.
10 µm
Sickle-cell hemoglobin
GluProThrLeuHisVal Val
1 2 3 4 5 6 7