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![Page 1: Proteins, Enzymes and Nucleic Acids The DNA/RNA non-specific Serratia nuclease prefers double-stranded A-form nucleic acids as substrates Gregor Meiss,](https://reader036.fdocuments.in/reader036/viewer/2022062309/5697bfc11a28abf838ca429d/html5/thumbnails/1.jpg)
Proteins, Enzymes and Nucleic Acids
The DNA/RNA non-specific Serratia nuclease prefers double-stranded A-form nucleic acids as substratesGregor Meiss, et al. 1999. JMB 288: 377–390
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Proteins
Multipurposemolecules
Schematic representation of secondary structures:
Image from National Energy Research Scientific Computing Center
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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 protein hormones) • receptors
– defense (antibodies) – movement (actin & myosin)– storage (bean seed proteins)
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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
hemoglobin
H2O
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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
H C—OH
||O
R
|—C— |
H
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Effect of different R groups:Nonpolar amino acids
Why are these nonpolar & hydrophobic?
nonpolar & hydrophobic
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Effect of different R groups:Polar amino acids
polar or charged & hydrophilic
Why are these polar & hydrophillic?
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Building proteins
• Peptide bonds– covalent bond between NH2 (amine) of one
amino acid & COOH (carboxyl) of another
peptidebond
dehydration synthesisH2O
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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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Sickle cell anemia
I’mhydrophilic!
But I’mhydrophobic!
Just 1out of 146amino acids!
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Figure 5.21
PrimaryStructure
Secondaryand TertiaryStructures
QuaternaryStructure Function Red Blood
Cell Shape
subunit
subunit
Exposedhydrophobicregion
Molecules do notassociate with oneanother; each carriesoxygen.
Molecules crystallizeinto a fiber; capacityto carry oxygen isreduced.
Sickle-cellhemoglobin
Normalhemoglobin
10 m
10 m
Sic
kle-
cell
hem
og
lob
inN
orm
al h
emo
glo
bin
1
23
456
7
1
23
456
7
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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 oxygen and hydrogen atoms within the backbone.
– forms sections of 3-D structure• -helix• -pleated sheet
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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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Noncovalent Interactions between Proteins and
Other Molecules
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Figure 5.19
Antibody protein Protein from flu virus
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Quaternary (4°) structure• More than one polypeptide chain bonded together
– only then does polypeptide become functional protein
collagen = skin & tendons hemoglobin
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Chaperonin proteins • Guide protein folding
– provide shelter for folding polypeptides– keep the new protein segregated from cytoplasmic
influences
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Enzymes • Biological catalysts
– proteins (& RNA) – facilitate chemical reactions
• increase rate of reaction without being consumed• reduce activation energy• don’t change free energy (G) released or required
– required for most biological reactions– highly specific
• thousands of different enzymes in cells
– control reactionsof life
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Figure 8.13
Course ofreactionwithoutenzyme
EA
withoutenzyme EA with
enzymeis lower
Course ofreactionwith enzyme
Reactants
Products
G is unaffectedby enzyme
Progress of the reaction
Fre
e en
erg
y
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Induced fit model
– 3-D structure of enzyme fits substrate– substrate binding cause enzyme to change
shape leading to a tighter fit • “conformational change”• bring chemical groups in position to catalyze
reaction
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How does it work?
• Variety of mechanisms to lower activation energy & speed up reaction– synthesis
• active site orients substrates in correct position for reaction
– enzyme brings substrate closer together
– digestion• active site binds substrate & puts stress on
bonds that must be broken, making it easier to separate molecules
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Enzymes and temperature• Different enzymes function in different
organisms in different environments
37°Ctemperature
reac
tion
rate
70°C
human enzymehot springbacteria enzyme
(158°F)
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Rat
e o
f re
acti
on
0 1 2 3 4 5 6 7 8 9 10pH
(b) Optimal pH for two enzymes
Optimal pH for pepsin(stomachenzyme)
Optimal pH for trypsin(intestinal
enzyme)
Two enzymes function in the same organism, but in different environments
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Compounds which regulate enzymes
• Inhibitors– molecules that reduce enzyme activity– competitive inhibition– noncompetitive inhibition– irreversible inhibition– feedback inhibition
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Irreversible Inhibition
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Reversible Inhibition
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Reversible Inhibition
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Regulatory site (one of four)
(a) Allosteric activators and inhibitors
Allosteric enzymewith four subunits
Active site(one of four)
Active formActivator
Stabilized active form
Oscillation
Nonfunctionalactive site Inactive form
InhibitorStabilized inactive form
Allosteric Regulation of Enzyme Activity
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Active siteavailable
Isoleucineused up bycell
Feedbackinhibition
Active site ofenzyme 1 isno longer ableto catalyze theconversionof threonine tointermediate A;pathway isswitched off.
Isoleucinebinds toallostericsite.
Initial substrate(threonine)Threoninein active site
Enzyme 1(threoninedeaminase)
Intermediate A
Intermediate B
Intermediate C
Intermediate D
Enzyme 2
Enzyme 3
Enzyme 4
Enzyme 5
End product(isoleucine)
Feedback Inhibition of Metabolic Pathways
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Nucleic AcidsInformationstorage
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proteins
DNA
Nucleic Acids• Function:
– genetic material• stores information
– genes– blueprint for building proteins
» DNA RNA proteins
• transfers information– blueprint for new cells– blueprint for next generation
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Nucleotides
• 3 parts – nitrogen base (C-N ring)– pentose sugar (5C)
• ribose in RNA• deoxyribose in DNA
– phosphate (PO4) group
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Types of nucleotides• 2 types of nucleotides
– different nitrogen bases – purines
• double ring N base • adenine (A)• guanine (G)
– pyrimidines• single ring N base • cytosine (C)• thymine (T)• uracil (U)
Purine = AGPure silver!
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Nucleic polymer• Backbone
– sugar to PO4 bond
– phosphodiester bond• new base added to sugar of
previous base• polymer grows in one direction
– N bases hang off the sugar-phosphate backbone
Dangling bases?Why is this important?
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Pairing of nucleotides• Nucleotides bond between
DNA strands– H bonds– purine :: pyrimidine– A :: T
• 2 H bonds
– G :: C• 3 H bonds
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Copying DNA• Replication
– 2 strands of DNA helix are complementary
• have one, can build other
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When does a cell copy DNA?• When in the life of a cell does DNA have
to be copied?– cell reproduction
• mitosis
– gamete production• meiosis
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Nucleic Acids Are Informational Macromolecules
• Genome—complete set of DNA in a living organism
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An interesting note…• ATP
Adenosine triphosphate
++
modified nucleotide adenine (AMP) + Pi + Pi
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RNA
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Ghosts of Lectures Past
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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