Download Chapter 8

Chapter 3
Proteins and Enzymes (Chapter 7)
Protein Structure – determined by
folding
• Can be globular (spherical) or fibrous (long
fibers)
• Proteins fold because of interactions
between R groups
o The structure directly determines its function
Four levels of protein
structure
• Primary – sequence of amino
acids
• Secondary – coils or folds in
backbone
• Tertiary – interactions between R
groups
• Quaternary – more than 1
polypeptide subunit
Fig. 5-21a
Primary Structure
1
5
H3N
Amino end
+
10
Amino acid
subunits
15
20
25
Secondary – coils or folds
in backbone
• H bonds between the
backbone (not the R
groups)
•  helix – a coil
•  pleated sheet –
folded structure
Tertiary - interactions between R
groups
• Hydrophobic
interactions –
nonpolar amino acids
end up clustered in
center
o Van der Waals
• Hydrogen Bonds –
polar amino acids H
bond
Hydrophobic
interactions and
van der Waals
interactions
Hydrogen
bond
Polypeptide
backbone
Tertiary - interactions between R
groups
• Ionic Bonds – positively
and negatively
charged aas bond
• Disulfide bridges –
covalent bonds of two
cysteines (–SH)
Polypeptide
backbone
Disulfide bridge
Ionic bond
Quaternary – more than 1
polypeptide subunit
β Chains
• Polypeptides
with tertiary
structure are
aggregated
o Example hemoglobin
Iron
Heme
α Chains
Hemoglobin
Denaturation - unraveling
• Caused by:
o High or low pH
o Increased salt concentration
o High temps
Enzymes
• catalytic proteins
• speed up metabolic
reactions by
lowering energy
barriers
• not consumed or
changed
Enzymes
• substrate – the reactant
that an enzyme acts on
• enzyme-substrate complex:
the enzyme bound to its
substrate
• active site – the region on
the enzyme where the
substrate binds
• induced fit – the way the
substrate fits into the active
site (the enzyme changes
shape slightly)
Activation Energy (EA)
• needed to start
a chemical
reaction
o heat from the
surroundings
Enzymes Lower the EA Barrier
Enzyme Activity
• Lower an EA barrier
by:
o Orienting substrates
correctly
o Straining substrate bonds
o Providing a favorable
microenvironment
o Covalently bonding to the
substrate
Enzyme Activity
• Affected by:
o temperature
o pH
o chemicals that
specifically
influence the
enzyme
Enzymes and Biotechnology
• Human babies are born with lactase, an enzyme that
breaks down lactose found in milk
• Naturally, as humans age they become increasingly
lactose intolerant as they stop producing lactase
• Some cultures have higher percentages of people who
are lactose intolerant (Asia), some lower percentages
(Europe)
• Lactase is produced on large scales to create lactosefree products
Enzymes and Biotechnology
Enzyme Inhibitors
Competitive inhibitors
• Bind to the
active site
• Compete
with the
substrate
Competitive Example:
• Ethylene glycol: antifreeze
o Ethanol is used to treat
ethylene glycol poisoning
o Reversible inhibition
Ethylene
glycol
ethanol
(inhibitor)
Products formed
Cause irreversible damage to the
kidneys
Competitive Example:
• Sulfanilamide: antibiotic
o similar structure to paraaminobenzoic acid
(PABA), (pathway for
folic acid)
o IRREVERSIBLE
COMPETITIVE INHIBITION
PABA
Sulphanilamide (inhibitor)
Folic acid
NO Folic acid formation after inhibition
Enzyme Inhibitors
Noncompetitive inhibitors
• Bind to another part
of an enzyme
• Cause the enzyme
(active site) to
change shape
Noncompetitive Example:
Morphine
• Binds to a site other
than the active site of
the enzyme Nitric oxide
synthase
Arginine
• The enzyme stays
inhibited
Nitric oxide synthase
Citrulline
Nitric
oxide
Noncompetitive Example:
Cyanide (CN-)
• Attaches to the –SH
group of an enzyme
CYTOCHROME C
OXIDASE
• results in the inhibition
of cellular respiration!!!
Cytochrome c oxidase
OXYGEN
WATER
Feedback Inhibition
• The end product of a
metabolic pathway shuts
down the pathway
• Prevents wasting
resources