Bevington 4 cell turnover

Question 1

Protein degradation by

Answer 1

proteases

Question 2

RNA degradation by

Answer 2

RNAses

Question 3

DNA turnover (repair) by

Answer 3

Excision nucleases etc

Question 4

Lipid degradation (lipolysis) by

Answer 4

Lipases etc

Question 5

Autophagy-

Answer 5

• “self-eating”
  Membrane vesicles fuse & accumulate (nucleation & extension) autophagosome forms to engulf a piece of cytoplasm (including organelles).
  An organelle bounded by a double membrane forms transiently only for autophagy
  -Mitochondria & endoplasmic reticulum may be source of autophagosome’s double membrane

Question 6

Lysosomes

Answer 6

part of endosomal compartment of animal cells, degrading intracellular components & material endocytosed from outside.

Question 7

Cell proteins constantly recycled -in organelles

Answer 7

(e.g. lysosomal & autophagosomal proteases max at low pH)

Question 8

Cell proteins constantly recycled - in the cytosol

Answer 8

a) Calcium-activated cytosolic proteases (Calpains), neutral pH
b) Ubiquitin-proteasome pathway (UPP) – major contributor to total cellular protein turnover

Question 9

Ubiquitin:

Answer 9

Ubiquitin small protein (76 AA residues) covalently attached to proteins targeted for degradation (e.g. Mis-folded proteins)

Question 10

Proteasome

Answer 10

huge macro-molecular cylindrical complex in cytosol, contains “unfoldases” & proteases => degrades polyubiquitinated proteins

Question 11

poly-ubiquitin chain is needed as marker for

Answer 11

proteasomal degradation, UPP 3 = sets of enzymes: E1, E2 and E3

Question 12

WHY TURNOVER?

Answer 12

1) To control the time course of gene expression
Proteins and the mRNAs encoding them may need to be turned “on” and “off” in the cell
2) Proteins may “mis-fold”
3) Cumulative damage to the molecules

Question 13

Reactive oxygen species (ROS):

Answer 13

cause damage to mis-fold proteins, DNA, Oxidised lipids

• “bi-product” of electron transport & oxidative phosphorylation in mitochondria
• high energy radiation (e.g. ultra-violet)
• inflammatory cells of immune system e.g. neutrophils

Question 14

Ribonucleases (RNases):

Answer 14

enzymes that degrade mRNA

Question 15

Lipid Peroxidation

Answer 15

Some Oxidised molecules not degraded & replaced

⇨ oxidised lipids in atherosclerosis=> heart disease

Question 16

ROS add

Answer 16

OH grp s