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Metabolism & Nutrition > Enzymes > Flashcards

Enzymes Flashcards

1
Q

What is induced fit?

A

Selectivity of the enzymes for their substrates.

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2
Q

What are coenzymes & Cofactors?

What is the difference between them?

A

Non protein components essential to enzyme activity. Coenzymes are organic molecules that bind loosly to the active site. Cofactors do not bind the enzyme.

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3
Q

What is Km ?

A

Concentration of substrate when the enzyme rate is half of Vmax.

Describes the affinity of the enzyme for the substrate.

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4
Q

What do Oxidases do

A

Oxidases oxidize substrates
Means take away an H+
The substrate is thus oxidized and the enzyme, reduced.

OIL; RIG

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5
Q

What do esterases do ?

A

Cleave ester bonds by adding water.

Sub-class of hydrolases

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6
Q

What do dehydrogenases do?

A

Take away hydrogen

E.g pyruvate dehydrogenase

Type of oxidoreductase

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7
Q

Role of oxido-reductases?

A

Transfer hydrogen or electrons, or use molecular oxygen.

Dehydrogenases, peroxidases, oxygenases, electron carriers NADH & FADH2

Hydroxylases are the most common mono-oxygenases.

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8
Q

What do hydrolases do?

A

Cleave bonds by addition of water.

Esterases, peptidases, proteases, glycosidases, lipases, phospholipases

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9
Q

What do transferases do ?

A

Transfer a group (other than hydrogen or oxygen) from one molecule to another.

Kinases are transferases.

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10
Q

What do isomerases do?

A

Change positions of isomers i.e. switch groups around.

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11
Q

What do lyases do?

A

Cleave a group off a molecule, leaving a double bond i.e. molecular bonds are made without involving hydrolysis. Formation of double bonds by removal of water.

Dehydratases, enolases and aldolases are lyases.

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12
Q

What do ligases do?

A

Form covalent bonds between molecules, hydrolyzing a high-energy bond in the process.

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13
Q

What is the difference between apoenzyme & holoenzyme?

A

Apoenzyme : non-functional enzyme when not bound to its cofactor.
Holoenzyme is the functional version when the cofactor is present.

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14
Q

Cofactors that are tightly bound to the enzyme are called ?

A

Prosthetic groups.

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15
Q

Cofactors that are complexe organic molecules are called ?

A

Coenzymes.

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16
Q

What are isoenzymes?

A

Different versions of the same enzyme that catalyse the same rx, but differ in their structure or composition.

17
Q

What are zymogens and give 2 examples.

A

An inactive precuror of an enzyme. It is activated when cleaved, to release the active site.

Trypsin, chemotrypsin.

18
Q

What is covalent catalysis and give an exemple.

A

A covalent intermediate forms between the enzyme and the substrate.
For instance serine proteases and the catalytic triad.
Catalytic triad : serine, histidine and glutamic acid
Active site of the protease contains an AA serine whose side chain hydroxyl gr form covalent bond with carbonyl carbon of peptide bond, causing its hydrolysis. For serine to catalyze this reaction, it requires the help of his and glutamic acid.

Trypsin, chemotrypsin and elases are serine proteases that use triads

19
Q

Lead affects which enzyme involved in the synthesis of heme ?

A

ALA-dehydratase.

20
Q

When the reaction rate is independent of substrate concentration, this is known as ?

A

zero order rate.

21
Q

What is first orde rate?

A

At low substrate concentrations, every substrate molecule is bound to enzyme molecule and thus the reaction rate is proportional to the substrate concentration.

22
Q

What are coupled (redox) reactions ?

A

The product of one reaction becomes the substrate for another.

E.g. glycolysis.

23
Q

The Y intercept on a Lineweaver & Burk curve represents ?

A

1/ Vmax

24
Q

The X intercept on a Lineweaver & Burk curve represents?

A

-1/Km

25
Q

What are the 3 classes of reversible inhibitors?

A
  1. Competitive inhibitors (active site)
  2. Noncompetitive inhibitors (allosteric binding)
  3. Uncompetitive inhibitors (rare; binds ES complex)
26
Q

Competitive inh. impact on Vmax and Km ?

A

Vmax unchanged
Km increased

Reversed by î [substrate] which outcompetes the inh.

27
Q

What type of inhibitors ?
* Viagra (Sildenafil) & PDE5 enzyme
* Malonate in succinate dehydrogenase

A

Competitive inhibitors.

28
Q

How do non-competitive inhibitors affect Vmax & Km?

Overall activity is decreased.

A

Km is unaltered
Vmax is decreased

Inh. cannot be reversed by î substrate concentration, as the inhibiter does not bind the active site. It is allosterically bound which prevents enzyme-substrate products to form as efficiently even though the substrate can bind the enzyme.

29
Q

Uncompetitive inhibitors bind only to enzyme-substrate complexes. What is the impact on Km & Vmax ?

A

Both are decreased.

No products can be made at all. Inhibition cannot be reversed by increasing [substrate].

30
Q

What kind of inhibitor is lithium (trx of manic depression)

A

Uncompetitive inh.

31
Q

The michaelis-menten plot for an irreversible inh. looks like the plot of what type of reversible inh?

A

Non competitive

Same Km, Lower Vmax.

32
Q

Example of irriversible inhibitor.

A

Cyanid
causes covalent modification of enzyme structure. It binds covalently to the mitochondrial cytochrome oxidase (complex 4) which inhibits all the rx associated with electron transport chain.

33
Q

irriversible inhibitors are usually poisons that are unsuitable for therapeutic purposes. Name 2 exceptions.

A

Aspirin
Disulfiram

Aspirin irriversibly inhibits cyclo-oxygenase (COX) enzyme by binding serine in the active site. This enzyme is responsible for initiating or amplifying inflammatory responses & creates mediators of sensations, including pain. Ibuprofen binds the same structure but in a reversible way.

34
Q

Ex of enzyme that is allosterically regulated.

A

PFK-1 (rate limiting enzyme of glycolysis).
Allosterically regulated by ATP, citrate, Fructose 2,6 bis-phosphate, etc.

PFK-1 is a regulatory protein that displays cooperativity - it is multimeric (binds many different susbtrates).

35
Q

Usually, covalent modifications are irriversible except ?

A

Phosphorylation by kinase.

36
Q

What kind of enzyme removes a phosphate?

A

Phosphatase

37
Q

What are the usual sites for phosphate addition ?

A

Threonine
Serine
Tyrosine

R GROUPS WITH HYDROXYL RESIDUES (COO-)

38
Q

Phosphorylation of glycogen phosphorylase ……

A

activates glycogen utilization

Glucagon causes that.

39
Q

Phosphorylation of glycogen synthase….

A

inhibits glycogen synthesis

Metabolism & Nutrition (9 decks)

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