MGD

Question 1

How is a zymogen converted to its active form?

Answer 1

Cleavage

Question 2

What is a zymogen?

Answer 2

An inactive precursor of a proteolytic enzyme

Question 3

Describe how a protein can be retained in the ER.

Answer 3

Protein contains a sequence of aa at its C-terminus: KDEL-lys-asp-glu-leu. The protein is folded as delivered in vesicles to Golgi where KDEL receptor present in cis-Golgi. Protein binds to receptor as low pH in Golgi and is returned to the ER where higher pH causes protein to be released from receptor and receptor returned to Golgi. Signal is retained to stop protein from returning to Golgi and staying there.

Question 4

Describe how a protein is transported to the ER if destined for secretion.

Answer 4

Protein synthesised on ribosome from mRNA. Protein synthesised with a hydrophobic signal sequence at its N-terminus. This is recognised by a GTP-bound signal recognition particle which binds to the protein during its translation and halts translation. The SRP guides the ribosome synthesising the protein to the ER where it binds to an SRP receptor on the cytosolic face. SRP then detaches and protein remains unfolded as synthesised through ER membrane as translation continues. Protein fed through ER via a pore in membrane-peptide translocation complex. Signal then cleaved by a signal peptidase. Ribosome dissociates. Energy required in form of GTP attached to SRP, which is hydrolysed.

Question 5

Define polyploidy and give the most common cause of polyploidy.

Answer 5

an abnormal number of chromosomes which is a multiple of the haploid number e.g. 69 or 92.
Polyspermy.

Question 6

How may a protein be modified in the ER?

Answer 6

Signal cleavage, disulphide bond formation-via protein disulphide isomerase, this increases protein stability, and N-linked glycosylation- oligosaccharide- protein transferase, carbs added via an N-glycosyl link to the amide N of the amino side chains of Asn-asparagine.

Question 7

How may a protein be modified in the Golgi?

Answer 7

O-linked glycosylation-glycosyl transferase-carbs added via hydroxyl side chains of Serine-threonine residures via glycosidic link, trimming + modification of N-linked oligosaccharides via hydrolases, and further proteolytic processing.

Question 8

What is constitutive secretion and give an example of a protein secreted in this way.

Answer 8

Continuous process of secretion, proteins packaged into vesicles and exocytosed e.g. serum albumin, collagen.

Question 9

What is regulated secretion and give an example.

Answer 9

Secretions in response to a part. stimulus/signal e.g. hormone, proteins released when needed. e.g. insulin- released in response to high blood sugar and neurotransmitters on AP being received, digestive enzymes-selective secretion at apical membrane.

Question 10

Describe the inheritance of mitochondria.

Answer 10

Maternal lineage

Question 11

Define an autosome

Answer 11

a chromosome other than a sex chromosome

Question 12

Non-disjunction can occur with autosomes aswell as sex chromosomes. Which 3 trisomies are capable of surviving to birth?

Answer 12

Trisomy 13, 18 and 21

Question 13

Anagram for collagen synthesis

Answer 13

CHAADT ATECLA

Question 14

Describe collagen synthesis

Answer 14

Tropocollagen subunits synthesised as preprocollagen- with hydrophobic signal sequence.
-Cleavage of signal sequence as proteins enters ER lumen. Procollagen formed with N and C terminal peptides to prevent collagen fibre formation inside cell-would cause cell lysis.
-Hydroxylation of proline + lysine residues.
-Addition of N-linked oligosaccharides.
-Addition of galactose to hydroxylysine residues.
-Disulphide bonds formed as chains aligned
-Triple helical procollagen formed
Golgi:
-Addition of glucose compltetes O-linked oligosaccharides.
-Transport vesicle
-Exocytosis of procollagen
-Cleavage of N and C-terminal peptides EC by procollagen peptidases.
-Lateral assoc. of collagen molecules, covalent cross-linking as lysine residues form aldehyde derivatives via lysyl oxidase, and these spontaneously form aldol cross-links within and between triple-helical molecules.
-Aggregation of collagen fibrils forms collagen fibres.

Question 15

Why does Vit C deficiency cause scurvy?

Answer 15

Vit. C, along with Fe2+, is required for the activity of the prolyl hydroxylase enzyme which catalyses the hydroxylation of proline residues in tropocollagen, forming hydroxyproline residues, which increases the number of inter-chain H bonds to increase stability. If enzyme unable to function, weak tropocollagen triple helices result. This can then result in bleeding from gums-collagenous ligaments attaching teeth to gums affected-loose teeth, nose and hair follicles.

Question 16

Describe the structure of collagen.

Answer 16

Basic unit=tropocollagen-protein comprising 3 polypeptides(alpha chains), with the primary sequence glycine-X-Y, repeat sequence, mostly proline and hydroxyproline in X and some Y positions. Each L handed alpha chain twisted together to form a R-handed triple helix with high tensile strength. Inter-chain H bonds formed with formation of hydroxyproline residues.

Question 17

Why is glycine the only aa able to be accommodated inside the collagen triple helix?

Answer 17

Only aa with a small enough side chain in helix.

Question 18

How are the peptides in collagen prevented from assuming a shape other than an alpha chain?

Answer 18

Proline residues which have the correct geometry for extended alpha chain conformation.

Question 19

Define Km.

Answer 19

The substrate concentration which gives 1/2 maximal velocity of an enzyme.

Question 20

What would a high Km mean?

Answer 20

Enzyme has a low affinity for the binding molecule as a high substrate concentration is required for the enzyme to reach 1/2 of its maximal velocity.

Question 21

Define Vmax.

Answer 21

The maximum rate of activity of an enzyme when all active sites are occupied by substrate.

Question 22

Define a buffer

Answer 22

mixture of weak acid and its conjugate base, which minimises changes in pH

Question 23

What is pH

Answer 23

a measure of the concentration of H+ ions in solution. Strong acids fully dissociate.

Question 24

What is pI

Answer 24

the pH at which a protein has no overall net charge, so exists as a zwitterion.

Question 25

Describe acidic proteins

Answer 25

Have a large proportion of -vly charged aa, and a low pH, so a low pH at which protein exists with no overall net charge, if pH>pI- then protein deprotonated. Protein requires many +ve H+ ions to become neutral.

Question 26

Describe basic proteins

Answer 26

Have a large proportion of +vly charged aa, high pI, few +ve H+ions required to become neutral.

Question 27

What is pK

Answer 27

measure of extent of acid dissociation. More dissociation= lower pKa.

Question 28

Define genotype

Answer 28

the genetic composition of an organism

Question 29

Define phenotype

Answer 29

the characterisitics of an organism as a result of its genotype and the effect of the environment.

Question 30

Describe the allosteric regulation of enzymes and give an activator and inhibitor of the enzyme phosphofructokinase, which catalyses a rate-limiting step in glycolysis.

Answer 30

Binding of effector molecules to enzyme at site other than AS, and binding makes it easier for other molecules to bind, resulting in +ve cooperativity, as enzyme has tense and relaxed state, conformational change between 2 occurs, so sigmoidal relationship between substrate concentration and rate of reaction.
Activator-ADP, fructose 2,6 bisphosphate
Inhibitor- ATP, H+, citrate

Question 31

What is complementation?

Answer 31

Where there are 2 different genes for a particular condition, so a child of 2 parents who both have the recessive condition, but each have different genes, can be born without the condition as both genes contribute to the phenotype.

Question 32

Describe the structure of haemoglobin.

Answer 32

Quaternary protein structure, comprising 2 alpha and 2 beta globin chains, so 4 chains in total, forming a tetramer. Each chain has a haem group which can bing 1 O2 molecule, and which contains 1 ferrous ion. Exhibits cooperative binding.

Question 33

Explain benefit of Bohr effect in exercise.

Answer 33

Increase lactic acid and H+ in exercise, bind to Hb to change its conformation to T state (low affinity), so in rapidly respiring skeletal muscle cells, oxygen is more readily released from haemoglobin as Hb has a lower affinity. In lungs, Hb=R state, so high affinity, so oxygen taken up more readily.

Question 34

Describe the difference between Marasmus and Kwashiorkor

Answer 34

Marsmus: protein and carbohydrate deficieny, so body goes into staration, so muscles breakdown, release protein into bloodstream, so oncotic pressure exerted by proteins maintained in blood, but kwashiorkor: just protien deficiency, so lack of albumin in bloodstream, reducing oncotic pressure, so fluid moves into tissues, causing tissue oedema and ascites.