Biochem enzymes

Question 1

What are enzymes and what is there function?

Answer 1

Biological catalysts and they speed up the rate at which the point of equilibrium is reached- do NOT change the point of equilibrium

Question 2

What are enzymes made from?

Answer 2

Mostly protein (ribozymes are made from RNA)

Question 3

What characterises enzymes?

Answer 3

1) Efficient: work at 37 degrees in solution near neutral pH and increase reaction rate by 10^20
2) Specific: each enzyme has a limited range of substrates and some can distinguish stereoisomers
3) Potent: Each enzyme can convert many substrate molecules to product per second

Question 4

What is the transition state and how do enzymes effect it?

Answer 4

Transition state is the reaction intermediate species with the greatest free energy.
Enzymes bind and stabilise the transition state

Question 5

Enzyme substrate complexes are usually unstable. T or F?

Answer 5

True

Question 6

What are the symptoms of glycogen storage disease?

Answer 6

Hypoglycaemia, liver swelling, skin and mouth ulcers, bacterial /fungal infection

Question 7

What is glycogen storage disease?

Answer 7

Enzyme deficiency meaning glycogen cannot enter the transition phosphorylated state and cannot be transformed back into glucose. (12 different defects in glucose or glycogen metabolising enzymes)

Question 8

What are Co-enzymes and co-factors?

Answer 8

Small molecules required by enzymes to catalyse a reaction.
Co-enzymes =organic molecules
Co-factors = inorganic metal ions

Question 9

What is a apoenzyme and a holoenzyme?

Answer 9

Apoenzyme = enzyme without a co factor
Holoenzyme = enzyme with a cofactor

Question 10

How do cofactors work with enzymes?

Answer 10

Form a metal coordination centre within the enzyme and the enzyme is referred to as a metalloprotein. (Zn, Fe, Cu) Involved in redox reactions

Question 11

How do coenzymes work with enzymes?

Answer 11

Associate with the enzyme transiently and change charge or structure during the reaction but are regenerated. Some are involved in redox reactions (NAD+ or FAD) others in group transfer processes (CoA or ATP)

Question 12

What is the name given to tightly bound coenzymes?

Answer 12

Prosthetic groups eg haem in haemoglobin or cytochromes in oxidative phosphorylation

Question 13

Where are many coenzymes derived from?

Answer 13

Vitamins eg NAD+ and FAD

Question 14

Why are the symptoms of vitamin deficiencies related to enzyme activity?

Answer 14

Most vitamins function as coenzymes

Question 15

What is the lock and key model?

Answer 15

Active site of unbound enzyme is complementart to the shape of the substrate

Question 16

What is the induced fit model?

Answer 16

Binding of the substrate induces a conformational change in the enzyme => complementary fit and formation of enzyme substrate complex

Question 17

What are isoenzymes?

Answer 17

Isoforms of enzymes that catalyse the same reaction but have different properties and structure eg isoforms of haemoglobin, adult and foetal

Question 18

When are different isoforms synthesised?

Answer 18

Different stages of foetal and embryonic development

Question 19

Different isoforms can be present in different tissues and different cellular locations T or F?

Answer 19

True

Question 20

There are 2 isoforms of lactate dehydrogenase. Where are they found and why is this useful?

Answer 20

1) Heart- premotes aerobic metabolism. Lactate –> pyruvate
2) Muscle- promotes anaerobic metabolism. Pyruvate –> lactate
This is useful as relative amounts can be useful as diagnostic markers

Question 21

Lactate Dehydrogenase: What can be inferred from lots of M type being produced in the heart?

Answer 21

Must be a hypoxic environment as M subunit is produced in the presence of hypoxic sensitive transcriptional factors

Question 22

Creatinine Kinase (isoenzyme) is a dimeric protein in the blood which binds to muscle sarcomeres. What type of creatinine kinase is produced normally in the muscle, brain and heart?

Answer 22

Muscle produces M form (MM)
Brain produces B form (BB)
Heart produces M and B to form a heterodimer (MB)

Question 23

Creatinine Kinase: what does presence of BB in the blood suggest?

Answer 23

Stroke or brain tumour

Question 24

Creatinine Kinase: what does presence of MB in the blood suggest?

Answer 24

Heart attack

Question 25

What does a kinase enzyme do?

Answer 25

Add a phosphate group

Question 26

What does a phosphatase enzyme do?

Answer 26

Remove a phosphate group

Question 27

How is enzymatic activity regulated reversibly?

Answer 27

Phosphorylation reactions converting the enzyme to the active or inactive form

Question 28

How is enzyme activity regulated irreversibly?

Answer 28

Irreverable covelent modification => activation of enzymes | or partial proteolysis

Question 29

What is a zymogen?

Answer 29

Inactive precursor of an enzyme which can be converted to an active enzyme by cleavage of a covelent bond

Question 30

Trypsinogen and chymotrypsinogen are zymogens produced in the pancreas. Where are they converted to active enzymes?

Answer 30

Small intestine where enteropeptidase cleaves trypsinogen to form acitve trypsin which cleaves chymotrypsinogen to chymotrypsin

Question 31

How does the substrate concentration [S] effect the rate of the catalysed reaction V. What is V? What is Vmax? What is Km?

Answer 31

V = moles of substrate converted to product per second Vmax is where there is no limiting substrate Km is the substrate concentration when the rate is 50% of Vmax.

Question 32

What is the michaelis-menton constant and what does it explain? PLEASE SEE SHEET

Answer 32

Km= (K-1 +K2) / K1 | The relationship between Vmax and Km

Question 33

The michaelis-menton model. What is K1? What is K-1? What is K2?

Answer 33

``` K1= forwards RATE CONSTANT for enzyme association with the substrate K-1= the backwards rate constant for enzyme dissociation with the substrate K2= forwards rate constant for enzyme conversion of a substrate to product ```

Question 34

How can you measure Vmax and Km? | NB: not accurate as the velocity will never reach Vmax.

Answer 34

1) Measure the initial rate of reaction at a known substrate concentration 2) Repeat at increasing substrate concentrations 3) Plot initial reaction rates as functions of substrate concentration 4) At infinite substrate concentration the initial rate approaches V max 5) Km is the [S] at 50% of V max

Question 35

What is the michaelis menton equation?

Answer 35

V = Vmax[S] / (Km + [S]). If you take the recepricle you get a straight line and accurately determine Vmax and Km. Describes the rate of catalysis as a function of substrate concentration.

Question 36

On a lineweaver-burk plot, where is Vmax and Km found?

Answer 36

``` Vmax = intersection with Y axis Km = intersection with the X axis ```

Question 37

What is implied by a low and high Km?

Answer 37

Low Km = enzyme only need a little substrate to work at half maximal velocity (efficient) High Km = enzyme needs lots of substrate to work at half maximal velocity (less efficient but more sensitive and responsive)

Question 38

What is MODY (maturity onset diabetes of the young)?

Answer 38

Loss of glucokinase activity => loss of insulin mediated glucose homeostasis.

Question 39

Hexokinase (RBC) and Glucokinase (Liver and Pancreas) both catalyse the same reaction: Glucose + ATP --> ADP + Glucose-6-phosphate. Which has a lower Km and why?

Answer 39

Hexokinase has a low Km so energy production is maintained even if glucose levels fall Glucokinase has a high Km meaning if glucose levels fall less glucose will be used by the liver and converted into glycogen (enables glucose sensing). Glucose abundance in liver is regulated by insulin and only excess glucose metabolised

Question 40

Proline hydroxilases regulate hypoxia inducable transcriptional factors. Does it have a high or low Km for oxygen?

Answer 40

High Km which allows it to be sensitive to levels of hypoxia over physiological ranges of oxygen

Question 41

What is orthosteric inhibition?

Answer 41

Inhibition at the same site eg competitive

Question 42

What is allosteric inhibition?

Answer 42

Inhibition at a site other than the active site inducing a conformational change

Question 43

Can irreversible inhibition be competitive?

Answer 43

No

Question 44

What happens to Vmax and Km in competitive inhibition?

Answer 44

``` Vmax = same Km = varies ```

Question 45

Explain methanol poisoning by competitive inhibition?

Answer 45

1) Methanol is the substrate for alcohol dehydrogenase 2) Causes severe tissue damage and blindness by conversion to formaldehyde and drives metabolic acidosis 3) Alcohol dehydrogenase Km for ethanol is 20x greater than for methanol 4) Treat patients with 40% ethanol, dialysis and ventilation

Question 46

What happens to Vmax and Km in non competitive inhibition?

Answer 46

``` Vmax = lower Km = same ```

Question 47

Do allosteric enzymes follow Michaelis menton kinetics?

Answer 47

No- increasing the substrate concentration results in a sigmoid curve eg binding of oxygen to haemoglobin

Question 48

What is the most common mechanism of feedback inhibition?

Answer 48

Allosteric control using end products to inhibit the upstream rate