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Chapter 3Chapter 3Proteins and Enzymes (Chapter 7)
Protein Structure – determined by Protein Structure – determined by
foldingfolding• Can be globular (spherical) or fibrous
(long fibers)• Proteins fold because of interactions
between R groupso The structure directly determines its function
Four levels of protein Four levels of protein structurestructure
• 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
Amino acidsubunits
+H3N Amino end
25
20
15
10
5
1
Primary Structure
Secondary – coils or Secondary – coils or folds in backbonefolds in backbone
• H bonds between the backbone (not the R groups)
• helix – a coil
• pleated sheet – folded structure
Tertiary - interactions between R Tertiary - interactions between R
groupsgroups• Hydrophobic
interactions – nonpolar amino acids end up clustered in centero Van der Waals
• Hydrogen Bonds – polar amino acids H bond
Polypeptidebackbone
Hydrophobicinteractions andvan der Waalsinteractions
Hydrogenbond
Tertiary - interactions between R Tertiary - interactions between R
groupsgroups• Ionic Bonds –
positively and negatively charged aas bond
• Disulfide bridges – covalent bonds of two cysteines (–SH)
Polypeptidebackbone
Disulfide bridge
Ionic bond
Quaternary – more Quaternary – more than 1 polypeptide than 1 polypeptide
subunitsubunit• Polypeptides
with tertiary structure are aggregatedo Example -
hemoglobin
β Chains
HemeIron
α Chains
Hemoglobin
Denaturation - Denaturation - unravelingunraveling
• Caused by:o High or low pHo Increased salt concentrationo High temps
EnzymesEnzymes
• catalytic proteins• speed up metabolic
reactions by lowering energy barriers
• not consumed or changed
EnzymesEnzymes• 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 (EActivation Energy (EAA) )
• needed to start a chemical reactiono heat from the
surroundings
Enzymes Lower the EEnzymes Lower the EAA BarrierBarrier
Enzyme ActivityEnzyme Activity
• Lower an EA barrier by:o Orienting substrates
correctlyo Straining substrate bondso Providing a favorable
microenvironmento Covalently bonding to the
substrate
• Affected by:o temperature o pHo chemicals that
specifically influence the enzyme
Enzyme ActivityEnzyme Activity
Enzymes and Enzymes and BiotechnologyBiotechnology
• 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 lactose-free products
Enzymes and Enzymes and BiotechnologyBiotechnology
EnzymeEnzyme InhibitorsInhibitorsCompetitive inhibitors
• Bind to the active site
• Compete with the substrate
Competitive Example:Competitive Example:• Ethylene glycol:
antifreezeo Ethanol is used to treat
ethylene glycol poisoning
o Reversible inhibition
Ethylene glycol Ethylene glycol Products formed Products formed
ethanol ethanol (inhibitor)(inhibitor)
Cause irreversible damage to the Cause irreversible damage to the kidneyskidneys
Cause irreversible damage to the Cause irreversible damage to the kidneyskidneys
Competitive Example:Competitive Example:• Sulfanilamide: antibiotic
o similar structure to para-aminobenzoic acid (PABA), (pathway for folic acid)
o IRREVERSIBLE COMPETITIVE INHIBITION
PABAPABA Folic acidFolic acid
Sulphanilamide (inhibitor)Sulphanilamide (inhibitor) NO Folic acid formation after inhibitionNO Folic acid formation after inhibition
EnzymeEnzyme InhibitorsInhibitorsNoncompetitive inhibitors
• Bind to another part of an enzyme
• Cause the enzyme (active site) to change shape
Noncompetitive Noncompetitive Example:Example:
Morphine• Binds to a site other
than the active site of the enzyme Nitric oxide synthase
• The enzyme stays inhibited
Arginine Arginine Nitric Nitric oxideoxide
CitrullineCitrullineNitric oxide synthaseNitric oxide synthase
Opioids ( morphine) in
hibit NOS
Noncompetitive Noncompetitive Example:Example:
Cyanide (CN-)• Attaches to the –SH
group of an enzyme CYTOCHROME C OXIDASE
• results in the inhibition of cellular respiration!!!
OXYGEN OXYGEN
WATERWATER
Cytochrome c oxidaseCytochrome c oxidase
CNCN- - Cya
nide
Cyanid
e
FeedbackFeedback InhibitionInhibition• The end product of a
metabolic pathway shuts down the pathway
• Prevents wasting resources