Topic 4: Enzymes Flashcards Preview

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Flashcards in Topic 4: Enzymes Deck (47):
1

Enzymes definition

Organic catalysts and protein molecules that speed up chemical reactions in cells by reducing the activation energy
not used in the reaction

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Why are enzymes necessary

reaction rate of chemical reaction usually increases with temperature
this cannot occur in living cells as proteins become denatured
therefore every step in a biochemical pathway is controlled by a specific enzyme
enzymes increase the rate of chemical reactions which would otherwise occur too slowly to sustain life

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Enzyme structure

globular protein (spherical)

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Different types of enzymes in animal cells

4000

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Enzyme characteristics

tertiary or quaternary proteins
substrate specific (enzyme-substrate specificity)
can be reused, therefore only required in small quantities (intracellular is reused, extracellular cannot be)
do not act as reactants or products
do not change the direction of a reaction
not permanently denatured at low temperatures (but speeds are slow)
stereospecific- reaction produces a single product

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Ribozyme

non protein enzyme that acts on ribosome

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Enzymes are water soluble

Surface of enzyme contain polar amino acids, whereas interior contain nonpolar amino acids
Polar amino acids on surface can dissolve in polar water molecules

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Reversible reaction

When one product accumulates, the reaction can be reversed to produce the former substrate instead

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Monomeric enzymes

made out of a single polypeptide chain
lysozyme, trypsin, papain
tertiary structure of protein

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Oligomers

made out of two or more polypeptide chains
lactose synthase
quaternary structure of protein

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Lactate dehydrogenase

Oligomeric enzyme LDH
has multiple binding sites
occurs in different regions of the body, with each region having a unique conformation of different subunits
formed form the combination of two subunits, H and M (muscle and heart) into a structure of four varying combinations of subunits

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4H LDH

Heart

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2H2M LDH

Lungs

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1H3M LDH

Kidneys

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4M LDH

Muscle and liver

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Enzyme lowering the activation energy

Enzyme distorts the bonds into their transition state form, thereby deducing the amount of energy required to complete the transition

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Activation energy

The energy that must be overcome in order for a chemical reaction to occur

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Action of an enzyme

Enables the reactions to occur at lower temperatures, reducing the amount of activation energy required for the reaction to occur

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Synthesising reactions

Anabolic
Endergonic
Photosynthesis, protein synthesis
net gain in energy

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Breakdown reactions

Catabolic
Exergonic
Cellular respiration
energy produced as a result of the reaction is greater than the activation energy
net loss in energy

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Free energy

the amount of available energy from a reaction
A reaction will proceed spontaneously if its products contain less free energy than the reactants.

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Intercellular enzymes

react within the cell
e.g. glycolysis in the mitochondria, photosynthetic pathway
can be reused

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Extracellular enzymes

react outside the cell
e.g. digestive enzymes are manufactured in cells but work extracellularly
cannot be reused

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Catalyst

Substance than increases the rate of a chemical reaction by reducing the activation energy

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How enzymes are specific in their action

The polypeptide chains are folded in such a way as to form one or more pockets or grooves on the surface, creating a specialised region (active site) into which one type of substrate molecule can fit
Enzymes have varying degrees of specificity

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Proenzymes

AKA zymogens, precursors
Become active only in response to specific signals
e.g. pepsinogen, prothrombin

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Method to name enzymes

Attaching the suffix 'ase' to the name of the substrate on which it acts

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Substrate

compound acted on by an enzyme

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Catalase

One of the fastest enzymes
In the liver
Decomposition of H2O2
2H202 -> 2H20 + O2
Substrate is hydrogen peroxide

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Acetylcholinesterase

catalyses the breakdown of the neurotransmitter acetylcholine at several types of synapses

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Active site

precise place on the surface of an enzyme to which the substrate molecules become attached
shape of the active site ensures that substrate with complementary shape will combine- lock and key hypothesis
Have substrate binding amino acids and catalytic amino acids

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Substrate-binding amino acids

R groups form charged, or uncharged, hydrophilic or hydrophobic surfaces in the active site, which bind precisely with particular parts of the substrate molecules

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Catalytic amino acids

Involved with making or breaking of covalent bonds in the substrate molecule
decrease activation energy

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Substrate binding

Substrate molecules bind to the enzyme active site via non covalent interactions
electrostatic interactions
hydrogen bonding
van der waals interactions
hydrophobic interactions

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Electrostatic interactions (active site)

amino acids with a charge will attract an oppositely charged substrate

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Hydrogen bonding (active site)

hydrogen bond forms with substrate typically between the amide and carboxyl groups of the amino acid

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Van der waals interactions (active site)

polar uncharged amino acids

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Hydrophobic interactions (active site)

amino acids with hydrophobic side chains repel water

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Coenzymes

Some enzymes are not active alone but require the presence of these relatively small organic molecules
may be independent molecule, or may actually be attached to its enzyme
Contain some member of vitamin B complex
When the enzyme needs to get activated, coenzyme forms with apoenzyme (protein) part to form holoenzyme
the two parts then seperate after the reaction
e.g. NAD, NADH, FAD

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Cofactor

Particular ions required by some enzymes for their activity
binds to active site, then separates after reaction

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Prosthetic group

Ion bound permanently to the enzyme, of which it is an integral part

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Efficiency of an enzyme

measured by determining how fast the substrate disappears, and how fast the product is formed

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Metabolic pathways

Enzymatic pathways that form as a result of the common occurrence of a series of dependent chemical reactions

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Feedback inhibition

If the product of a series of enzymatic reactions begins to accumulate within the cell, it may specifically inhibit the action of the first enzyme involved in its synthesis
The further production of the enzyme is halted
Last product binds to the first enzyme at allosteric site to change its shape

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Rate of reaction depends on several factors

Temperature
pH value
Enzyme concentration
Substrate concentration
Product concentration
Inhibitors

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Psychrophiles

bacteria living in very low temperatures

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Thermophiles

bacteria living in very high temperatures