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Biochem > Enzyme Regulation > Flashcards

Enzyme Regulation Flashcards

1
Q

Feedback inhibited enzymes

A

Excessive build up of end products inhibits enzyme activity

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2
Q

Covalently modified enzymes

A

Reversible attachment of small molecules that inhibit activity (phosphorylation, methylation and ribosylation)

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3
Q

Allosteric enzymes

A

Conformational changes by reversible non covalent binding of co-factors and regulatory sub-units

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4
Q

Zymogens and proenzymes

A

Become activated by proteolytic cleavage of inactive precursor molecules

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5
Q

Enzyme specificity- Absolue

A

Defined to one substrate in its one define stereoisomeric state

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6
Q

Enzymes specificity- Group

A

Enzymes can recognise compounds belonging to a group e.g phosphates and kinases

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7
Q

Enzyme specificity- Promiscuity

A

Enzymes that can act upon several substrates regulation is very important
Specific inhibition is hard

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8
Q

Allosteric Regulation

A

Causes changes (inhibitor or activator) to active site so enzyme can bind or not bind

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9
Q

Homotropic regulation

A

The substrate and regulator bind to the same site

regulatory site=active site

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10
Q

Heterotrophic regulation

A

The regulator binds to site other than the active site

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11
Q

Co-operative regulation

A

Binding of the first substrate molecule to the active site makes it easier for more substrate to bind

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12
Q

Test for co-operability

A

The hill plot can be used to estimate the degree of cooperation between subunits

Log(theta/1-theta) = nlog [S]- nlogKA

Theta = fraction of bound enzyme 
[S] = ligand concentration 
KA= [S] at 50% occupation
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13
Q

Aspartate Transcarbamoylase

A

Catalyses the first step in pyrimidines
Precisely regulated to produce the right amount of CTP
Negativity allosterically regulated by CTP
T state (less active)= favoured by CTP

R state (more active)= favoured by substrate binding

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14
Q

Apo-enzyme

A

Just protein

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15
Q

Holo-enzyme

A

Protein and co-factor or co-enzyme

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16
Q

Cofactors and Coenzymes

A

Many enzymes require prosthetic groups that are non amino acid in nature

17
Q

Where do the Co’s come from

A

Most cannot be synthesised

A precursor molecule must come from the diet

18
Q

Co-enzymes

A

Organic: Carry groups between enzymes

Most are vitamin derived

19
Q

Co-factors

A

Inorganic- Metal ions

Required for enzyme activity

20
Q

Co-enzymes and Co-substrates

A

Co-enzymes often donate chemical groups such as C, H and NH2
These are consumed by the reaction and are hence called Co-substrates

21
Q

Common co-enzymes

A

Biotin- carboxylation

Coenzyme A - Acyl transfers

FAD and NAD- Oxidation and Reduction

22
Q

Aspartate Transcarbamoylase continued

A

6 regulatory and 6 catalytic subunits
Catalytic sites show normal MM kinetics and are unresponsive to CTP

Regulatory subunits have no catalytic ability but binds to CTP

23
Q

CTP binding

A

Stabilises the T state
Inhibition changes the quaternary structure which makes is harder for substrate to bind
More substrate is required to achieve reaction rate

24
Q

ATP and aspartate carbamolylase

A

Positive allosteric effector

Competes with CTP for site (alleviating inhibition)

Makes substrate binding easier and shifts equilibrium to the R state

Two reasons:
Signals high energy for mRNA synthesis
Maintaining the balance or pyrimidine and purine rings

25
Q

Protein kinase A

A

Key name in the modulation of metabolic enzymes by covalent modification

Phosphorylates proteins
Each regulatory sub-unit (R) has a segment that is a pseudo-substrate for the catalytic Subunits
A rise in cAMP = binds to R and changes PKA shape = C subunits phosphorylates proteins

26
Q

Phosphorylation

A

Most common reversible modification

Adds two negative charges which alters electrostatic interactions

Hexo-kinase is an enzyme that catalyses phosphorylation

Reversed by phosphatases

27
Q

Glycogen Phosphorylase

A

Inter conversion between a and b forms of the enzymes adds or removes phosphate to the serine residue of to control its activity
Phosphatase and kinase respond to needs of cell

Phosphorylation at serine in response to epinephrine destabilises the N-terminal segment from acidic to basic environment
Allosterically activated by AMP(shows ATP levels are low) non covalently
Addition of glucose exposes the serine residue (phosphorylated) to phosphatase which converts the enzyme to inactive b-form

28
Q

Riboflavin cofactors

A

Riboflavin phosphate
Flavin adenine dinucleotide (FAD)

Involved in metabolism of carbs, fats and proteins.
Hydrogen carriers in respiratory chain

29
Q

Covalent regulation

A

Enzymes can be modified to change activity

Covalent regulation = reversible and sensitive
*can change from 0% to 100% activity

Phosphorylation (pyruvate dehydrogenase)

Adenylation (glutamate synthase)

30
Q

Activation by proteolysis

A

Most proteases are synthesised as larger molecules, during the activation phase the inhibitory segment is cleaved

Activation may be after the protease is delivered to the compartment within the cell

31
Q

Serine protease inhibitors

SERPINS

A

Proteins that block or enter protease active site to prevent substrate access

After the serpin is cleaved but prior to hydrolysis of of acyl-enzyme intermediate

The serpin undergoes S to R transition, causing the protease to go from the top to the bottom of the serpin the acyl-enzyme is hydrolysed really slowly and remains covalently attached and inhibited

32
Q

SERPIN cleavage

A

Since the serpin must be cleaved

It becomes consumed and is therefore

Irreversible enzyme inhibitor

33
Q

PEST proteins

A

Rich in
Pro (P)
Glu (U)
Ser (S)
Thr (T)
More rapidly degraded than other proteins
They have phosphorylation sites that allow them to be targeted by ubiquitin

34
Q

Accelerated degradation

A

Proteins are turned over at a rate indicated by their half life

E.g haemoglobin 1/2 life = 120 days
But if an artificial amino acid is incorporated it can be degraded in 10 minutes

35
Q

Degradation mechanisms

A

Non selective

  • active proteases
  • released in cells or present in lysosomes
  • lysosomes usually degrade membrane proteins

Selective

  • UPS (Ubiquitin proteome system)
  • ATP dependent system requiring proteins to be tagged with ubiquitin for delivery to proteasome
36
Q

Regulation of glycolysis

A

Controlled by:
Feedback inhibition
Allosteric regulation
Phosphorylation

Biochem (5 decks)

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