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AP Biology
Proteins
© 2014 Pearson Education, Inc.
3.5 What Are Proteins?
– Proteins are molecules composed of chains of amino acids
– Proteins have a variety of functions
– Enzymes are proteins that promote specific chemical reactions
– Structural proteins (e.g., elastin) provide support
© 2014 Pearson Education, Inc.
Table 3-3
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Proteins Most structurally & functionally diverse group Function: involved in almost everything
enzymes (pepsin, DNA polymerase) structure (keratin, collagen) carriers & transport (hemoglobin, aquaporin) cell communication
signals (insulin & other hormones) receptors
defense (antibodies) movement (actin & myosin) storage (bean seed proteins)
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play a role in cell membrane function
receptor proteins: recognize and bind to substances wanting
to enter membrane
transport proteins: help transport substances across cell
membrane
build
muscles
© 2014 Pearson Education, Inc.
Figure 3-17 Structural proteins
SilkHorn Hair
© 2014 Pearson Education, Inc.
3.5 What Are Proteins?
Proteins are molecules composed of chains of amino acids (continued)
– Proteins are polymers of amino acids joined by peptide bonds
– All amino acids have a similar structure
– All contain amino and carboxyl groups
– All have a variable “R” group
– Some R groups are hydrophobic
– Some are hydrophilic
– Cysteine R groups can form disulfide bonds
AP Biology
Amino acids Structure
central carbon amino group carboxyl group (acid) R group (side chain)
variable group different for each amino acid confers unique chemical
properties to each amino acid like 20 different letters of an
alphabet can make many words (proteins)
—N—H
HC—OH
||O
R
|—C—
|
H
Oh, I get it!amino = NH2 acid = COOH
© 2014 Pearson Education, Inc.
Figure 3-18 Amino acid structure
hydrogen
aminogroup
variablegroup (R)
carboxylicacid group
AP Biology
3 - Proteins (Honors)
contain C, H, O, and N (nitrogen)
amino acids (monomer) bonded together
© 2014 Pearson Education, Inc.
3.5 What Are Proteins?
Amino acids are joined by dehydration synthesis
– An amino group reacts with a carboxyl group, and water is lost
– The covalent bond resulting after the water is lost is a peptide bond, and the resulting chain of two amino acids is called a peptide
– Long chains of amino acids are known as polypeptides, or just proteins
© 2014 Pearson Education, Inc.
Figure 3-20 Protein synthesis
amino acid
aminogroup
amino acid peptide water
aminogroup
peptidebond
carboxylicacid group
dehydrationsynthesis
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Building Proteins
dehydration synthesis / condensation rxn:
http://www.wisc-online.com/objects/index_tj.asp?objID=AP13304
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Proteins Structure
monomer = amino acids 20 different amino acids
polymer = polypeptide protein can be one or more polypeptide
chains folded & bonded together large & complex molecules complex 3-D shape
Rubisco
hemoglobin
growthhormones
H2O
© 2014 Pearson Education, Inc.
Figure 3-19 Amino acid diversity
glutamic acid (glu) aspartic acid (asp)
phenylalanine (phe) leucine (leu)
Hydrophilic functional groups
cysteine (cys)
Hydrophobic functional groups Sulfur-containing functional group
© 2014 Pearson Education, Inc.
3.5 What Are Proteins?
A protein can have as many as four levels of structure
– Primary structure is the sequence of amino acids linked together in a protein
– Secondary structure is a helix, or a pleated sheet
– Tertiary structure refers to complex foldings of the protein chain held together by disulfide bridges, hydrophobic/hydrophilic interactions, and other bonds
– Quaternary structure occurs where multiple protein chains are linked together
© 2014 Pearson Education, Inc. Animation: Protein Structure
© 2014 Pearson Education, Inc.
Figure 3-21 The four levels of protein structure
hydrogenbond
Tertiary structure:Folding of the helix resultsfrom hydrogen bonds with surrounding water moleculesand disulfide bridges betweencysteine amino acids
Quaternary structure:Individual polypeptides arelinked to one another byhydrogen bonds or disulfidebridges
Secondary structure:Usually maintained byhydrogen bonds, whichshape this helix
Primary structure:The sequence of aminoacids linked by peptidebonds
heme group
helixpro
lys
val
ala
lys
his
lys
val
gly
lys
leu
© 2014 Pearson Education, Inc.
Figure 3-22 The pleated sheet and the structure of silk protein
hydrogenbond
Pleated sheet Structure of silk
stack of pleated sheets disorderedsegment
strandof silk
AP Biology
Building proteins Peptide bonds
covalent bond between NH2 (amine) of one amino acid & COOH (carboxyl) of another
C–N bond
peptidebond
dehydration synthesisH2O
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Building proteins Polypeptide chains have direction
N-terminus = NH2 end C-terminus = COOH end repeated sequence (N-C-C) is the
polypeptide backbone can only grow in one direction
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Protein structure & function
hemoglobin
Function depends on structure 3-D structure
twisted, folded, coiled into unique shape
collagen
pepsin
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Primary (1°) structure Order of amino acids in chain
amino acid sequence determined by gene (DNA)
slight change in amino acid sequence can affect protein’s structure & its function even just one amino acid change
can make all the difference!
lysozyme: enzyme in tears & mucus that kills bacteria
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Secondary (2°) structure “Local folding”
folding along short sections of polypeptide interactions between
adjacent amino acids H bonds
weak bonds between R groups
forms sections of 3-D structure -helix -pleated sheet
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Secondary (2°) structure
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Sulfur containing amino acids Form disulfide bridges
covalent cross links betweens sulfhydryls stabilizes 3-D structure
You wonderedwhy permssmell like
rotten eggs?
H-S – S-HH-S – S-H
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Tertiary (3°) structure “Whole molecule folding”
interactions between distant amino acids hydrophobic interactions
cytoplasm is water-based
nonpolar amino acids cluster away from water
H bonds & ionic bonds disulfide bridges
covalent bonds between sulfurs in sulfhydryls (S–H)
anchors 3-D shape
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Quaternary (4°) structure More than one polypeptide chain bonded
together only then does polypeptide become
functional protein hydrophobic interactions
collagen = skin & tendons hemoglobin
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Protein structure (review)
amino acid sequence
peptide bonds
1°
determinedby DNA R groups
H bonds
R groupshydrophobic interactions
disulfide bridges(H & ionic bonds)
3°multiple
polypeptideshydrophobic interactions
4°
2°
© 2014 Pearson Education, Inc.
3.5 What Are Proteins?
The functions of proteins are related to their three-dimensional structures
– Precise positioning of amino acid R groups leads to bonds that determine secondary and tertiary structure
– Disruption of secondary and tertiary bonds leads to denatured proteins and loss of function
AP Biology
Protein denaturation Unfolding a protein
conditions that disrupt H bonds, ionic bonds, disulfide bridges temperature pH salinity
alter 2° & 3° structure alter 3-D shape
destroys functionality some proteins can return to their functional shape
after denaturation, many cannot
In Biology,size doesn’t matter,
SHAPE matters!
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Denatured protein
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Sickle cell anemia
I’mhydrophilic!
But I’mhydrophobic!
Just 1out of 146
amino acids!
