Transcript AP151 ENZYMES

Proteins
In Ch 2
Protein Function
• Regulation
– act as chemical signals, e.g., non-steroidal hormones
• Enzymes
– promotion and regulation of chemical reactions
• Transport
– across the plasma membrane and through blood
• Contraction/movement
– cell motility and muscle contraction
• Energy source
– By breaking them down/catabolism
• Structural
• Immune function
– e.g., antibodies
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Basic Protein Structure
• Large molecules made of amino acids
– 20 different amino acids (aa)
• Different aa sequences  different proteins
– joined by peptide bonds (covalent bonds)
a
b
Amino Acid Structure
– Amino acids=amino group (NH2) + carboxyl group (COOH) +
functional group (R)
• Different A.A. = different functional group.
Its amino world without chemistry
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Example Amino Acids
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Linking Amino Acids:
covalent bonds called peptide bonds
What kind of rxn is this?
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Polypeptides v. Proteins
• <100 amino acids= polypeptide
• >100 amino acids =protein
Protein Structure: 1° structure
• Primary structure = amino acid sequence
– Covalent bonds
– Strength?
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Protein Structure: 2° structure
• Secondary structure = bent or twisted amino acid chains (beginning of
shape)
• due to bonding among a.a./between adjacent a.a. (between H of amino group and O’s)
• Hydrogen Bonds….Strength?
Beta pleated sheet
Alpha helix
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Protein Structure: 3° structure
• Tertiary structure = distinct 3-D shape (conformation)
• Due to bending and folding of a.a. chains (of the 2°
structure)
– Hydrogen and other weak bonds between functional groups
– Strength?
– Denatured (broken) by heat, pH
Protein Structure: 4° structure
• Quaternary structure = multiple polypeptide chains
are joined
• Not applicable to all proteins
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“composite molecules”
• proteins are combines with other molecules
– Glycoproteins = protein + carbohydrate
– Lipoproteins = protein + lipid
– Others…..
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Enzyme: Chapter 4
• Main Topics
– Enzymes and Activation Energy
– Characteristics of Enzymes
– Factors that influence enzyme activity/function
– Metabolic/Biochemical pathways
4-2
Generic Reactions
• Substrate(s)/reactant(s) = starting material
• Product = ending material(s)/product made
example:
A + B  AB
Requires activation energy
• Activation Energy:
4-4
Background
• activation energy
– cons
– pros
Enzyme Introduction
• proteins* that:
– allow chemical rxns to occur at need rates
– allow chemical rxns to be regulated
• By their present, absence, or amount
– Almost all rxns (processes) in the body occur
because of enzymes
*some enzymes are made of RNA, but we don’t go there
Enzymes continued
Few
substrates
react
high
Many
substrates
react
low
4-5
Significance of Enzymes
• Allow chemical rxns to occur at needed
rates under conditions that are compatible
with life (i.e., that the human body can
survive).
• Allow chemical rxns to be regulated so
specific processes can occur when and
where they are needed.
Mechanism of Enzyme Action
• Substrates fit into active sites
• Enzyme-substrate complex
formed
• Reaction occurs
• Products leave enzyme
• Enzyme is unaltered
– Enzyme is not used up/consumed
4-7
Enzyme Characteristics
• PROTEINS
• CATALYST: increase rates of chemical rxns without being altered or
“used up” (100’s-1000’s rxns/second!!!)
– lowering activation energy
– few enzyme; lots of reaction
• SPECIFICITY: binds only w/ specific substrate
–
(reality a very narrow range of molecules, important clinically/pharmaceutically)
– due to shape of binding site/active site (CONFORMATION)
– due to 3 structure—WEAK BONDS/H-BONDS
– structural differences not involving active site do not effect the rxn or
specificity
– isozymes different versions of enzyme (vary by 1-a few amino acids) in
different tissues that catalyze the same substrate/rxn
• “leak” into blood after tissue damage, but don’t act due to wrong conditions/lack
of substrates but can be measured/detected and be diagnostic
• Enzymes can be regulated—turned on/off
• Often mediate reversible reactions
– not always, sometimes different enzymes are needed for the synthesis
and decomposition of a substrate product pair.
• Names usually end in “ase” and based on substrate or reaction type
Factors that Influence Activity
• Concentration of substrates
– Saturation
• Concentration of enzyme
• Presence/absence of cofactor:
• Presence/absence of Coenzymes
• TEMPERATURE:
• pH:
• Chemical Activation/Deactivation of enzyme
Substrate Concentration
• More substrate = more/faster reaction
• To a point  saturation/saturated
• Saturation
– plateaus because all enzymes are filled / “occupied” or “busy” with substrates
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Enzyme Concentration
• Related to encounter rate between enzyme and substrate and the
number of enzymes “available” to substrate(s)
• Plateaus encountered when enzymes are saturated
– specific processes can be selectively regulated by altering the amount
of enzyme available/active
reaction rate
high enzyme concentration
saturation
medium enzyme concentration
saturation
low enzyme concentration
saturation
substrate
concentration
Cofactors
• cofactor: ion or molecule (often inorganic, e.g., mineral) that must
simultaneously bind the enzyme (at active site or elsewhere) for the
substrate to bind (and react)
–
E.g., Ca+2, Mg+2, Mn+2, Cu+2, Zn+2, & selenium
– No cofactor no enzyme function  no reaction
– Fewer cofactor  less functional enzyme  less reaction
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Coenzymes
• Coenzymes:
a. non-protein, organic molecules that deliver or “escort”
molecules necessary for the rxn to enzymes
b. Or, a non-protein molecule needed for enzyme to have its
functional shape
– No cofactor no enzyme function  no reaction
– Fewer cofactor  less functional enzyme  less reaction
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Temperature
• Every enzyme has a temperature range in which it works
– Optimal temp = works best
• Typically near normal physiological values
• activity decreases with deviations both above an below optimum
– progressively slows with decreases in temp
– eventually denatures at high temp causing rxn to stop
THIS DIAGRAM IS JUST A REPRESENTATIVE EXAMPLE!
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pH
• optimum pH = where enzyme works best
– (usually reflective of the pH of tissue where it functions)
– Different enzymes can have different optimal pH
• deviations from optimum  decrease activity
• extreme deviations  denature enzymes  activity stops
4-11
Examples of pH optima
4-12
Enzyme Activation & inhibition
• Enzymes can be turned on (activated) and turned off
(deactivated) by a wide variety of means that usually involve
changing the shape of or blocking the active site
Example:
– Phosphorylation  turns on
– Dephosphorylation  turns off
substrate
Dephosphorylated:
Substrate can’t fit
Phosphorylated:
substrate can fit
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Enzyme Activation & inhibition cont.
• Inhibition or activation by other molecules
substrate
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CLINICAL AP--Isozymes
• dead and damaged cells (compartments
containing enzymes specific to that
tissue—isozymes) leak enzymes into other
compartments such as plasma where they
lack much activity because of the general
absence of their specific substrates.
measuring the amount of these “leaked”
enzymes can be diagnostic.
Reaction Rates and Enzyme Turnover
• Reaction rates are related to enzyme concentration (number of
enzymes)
• Enzymes do not last forever, they breakdown and need to be
replaced  enzyme turnover
• If new enzyme production < enzymes lost  reaction rates go down
• If new enzyme production > enzymes lost  reaction rates go up.
• Reaction rate = quantity broken down v. quantity synthesized
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Reversible Reactions
• Some enzymatic reactions are reversible
– Direction: higher concentration to lower
concentration
– e.g. carbonic anhydrase catalyzes
» H2O + CO2  H2CO3
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Pathways and their behavior
•
•
•
•
•
simple/linear pathways
inhibitions effects
branching pathways
inhibition of branch point
inhibition of one branch increases product
of the other. could be good or bad, natural
or pathologic (diseases of defiency or
excess)
Metabolic Pathways
•sequences of reactions
– Initial substrate  intermediates final product
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End product inhibition:
• End/final product inhibits the first (or earlier) enzyme
• Slows or stops production of final product
• Increasing end product trigger reduced production of end
product
End-Product Inhibition and branched pathways
• A product causes inhibition (of branch point).
– Reduces final product
– Pushes rxn down other pathway
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Inborn Errors of Metabolism
• inherited genetic defects
– Dysfunctional enzyme
• Can cause:
– ↑ intermediates
– ↓ products
– ↑ alternate products
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