The Complex Proteome

Question 1

Locations of Protein Synth. Processes

Eukaryotic

Answer 1

• RNA processing, transcription occur in nucleus
  * Nuclear envelope allows for more intricate control in cellular processes
• Translation occurs in cytosol by free or bound ribosomes

Question 2

Factors Affecting Composition of Proteome

Answer 2

• Organisms developmental stage
• Response to internal and external signals

Question 3

How Cells Detect Changes in Environment

Answer 3

Changes = Stimuli resulting in important cellular responses

Question 4

Absorption Pathways of Glucose in Bloodstream and Following Sensory Response

(What is insulin?)

Answer 4

ABSORPTION PATHWAYS into bloodstream:

• epithelial surfaces in mouth
• microvilli cells of small intestines (which are connected to blood vessels)

IN THE BLOOD:

• Sensory responses in specialized Beta islet cells of pancreas detect increase of blood glucose
• Pancreas increases synthesis and secretion of insulin
• Insulin: Effector protein that communicates and produce a response w/ target cells leading to a decrease in blood glucose (increases cell uptake of glucose)

Question 5

Regulation of Protein Biosynthesis

Insulin example

Answer 5

• Occurs at transcriptional and translational levels
• Physiological events control increase/decrease in transcription/translation of the protein

Insulin Example:

• Increased blood-glucose leads to increased transcription and translation of insulin (+ glucose transporter protein)
• Dense RER found within beta cells of pancreas (lots of ribosomes!)

Question 6

Insulin Structure

Answer 6

• translated ppt is 110 amino acids long (preproinsulin)
• functional (secreted/mature) insulin is 51 amino acids long

Dorothy Hodgkin’s X-Ray crystallography found that functional insulin is made of 2 chains:

• α-chain: 21 amino acids
• β-chain: 30 amino acids

^ This is achieved by post-translational modifications to preproinsulin and proinsulin

Question 7

Post-Translational Modifications of Insulin

proteolytic cleavage?

Answer 7

PREPROINSULIN:

• preproinsulin has N-terminal
• N-terminal interacts w/ SRP to facilitate translocation into ER lumen

PROINSULIN:

• Signal sequence cleaved from preproinsulin, creating proinsulin (through a process called proteolytic cleavage)
• Proinsulin folds with the help of chaperone proteins
• Formation of three disulfide bonds

MATURE INSULIN:

• Transported to golgi for further folding
• Release of C chain (by proteolytic cleavage)
• Forms mature insulin dimer (containing both A & B chains)
• N-terminal and C-terminal amino acid residues are now able to bind to insulin receptors

Question 8

Post-Translational Modifications

Purpose and Examples

Answer 8

Purpose is to increase functional diversity of proteum.

• Cleavage
• Folding
• Disulfide bridge formation
• Covalent attachment of other molecules
  
  • Phosphorylation: Attachment of phosphate group
  • Methylation: Attachment of methyl group
  • Acetylation: Attachment of acetyl group
• Degradation of proteins

Question 9

Phosphorylation

Answer 9

Post-translational modification p.t. the covalent attachment of a phosphate group to serine, threonine, or tyrosine in a protein

• Facilitated by enzymes called kinases
• Reversible

Question 10

Methylation

Answer 10

Post-translational modification p.t. the covalent attachment of a methyl group

Question 11

Acetylation

Answer 11

Post-translational modification p.t. the covalent attachment of a acetyl group to a specific amino acid residue in a protein

Question 12

Receptor Proteins

Answer 12

• Recieve and interpret information from signalling molecules
• Binds to signal molecule and produce a response within cell

Question 13

Insulin Receptor + How is it activated?

Answer 13

• Belongs to receptor kinases family
• Increases cell uptake of glucose from bloodstream when activated (bound to insulin)
• Monomeric when inactive

ACTIVATION:

• Binding to insulin causes receptor monomers to pair up at receptor tails (cytoplasmically situated) by phosphate group (dimerization by phosphorylation) – dimeric when active!
• Phosphate group provide binding sites for intracellular signalling proteins that activate glucose transport proteins

Question 14

How does the binding of insulin to receptor kinase effect the cell? How is signal amplification carried out?

Initiation, maintenance, termination

Answer 14

• Extracellular signal (insulin) binds to receptor kinases causing series of transducer and amplifier proteins to activate
• Leads to INITIATION of intracellular response: activation of glucose transporter proteins at cell surface
• Increases absorption of glucose in cell
• Initiation and MAINTENANCE of signal regulated by positive feedback loops
• Negative feedback loops lead to intracellular signal TERMINATION
• Double Negative feedback leads to inhibition of an inhibitor

Question 15

Double Negative Feedback Loops

Answer 15

• Inhibitor of the signal can be inhibited
• Allows for fine control in cell in response to extracellular signal

Question 16

Alternative Splicing

Answer 16

Same primary transcript spliced in different ways to produce mature mRNA isoforms that lead to production of different but related proteins.

• Enables 1 pre-mRNA molecule to spliced at different junctions
• Results in different mature mRNA molecules that contain DIFFERENT COMBINATIONS of transcribed exons
• Some exons may be removed (like introns) leading to many isoforms of mature mRNA from same pre-mRNA
• Occurs as spliceosome may recognize exon to be an intron in varying transcripts
• Helps in the regulation of gene expression

Question 17

What Happens to Glucose in the Bloodstream?

Answer 17

• Glucose detected by beta islet cells and insulin is secreted
• Receptor kinases detect insulin and trigger cytoplasmic signalling proteins to induce glucose transport proteins
• Increased cell uptake of glucose
• Adipose tissues take up glucose and fatty acids and store excess as triglycerides
• Liver and muscle cells take up glucose and store excess as glycogen

Question 18

Alternative Splicing of Receptor Kinase Gene

Answer 18

• Gene has 22 exons
• Exon 11 spliced along w introns in skeletal muscle receptors
• Exon 11 is present in liver receptors
• Skeletal muscle receptors have higher insulin affinity than liver resulting in more uptake of glucose to meet high energy needs

Question 19

Signal Termination of Insulin

Answer 19

• Blood glucose levels return to normal
• Negative feedback loop where drop in blood glucose detected by pancreatic cells
• Results in decrease of secretion of insulin
• Limits further response in entire system

Question 20

Translational Errors underlying Diabetes

Answer 20

• Mistake in alternative splicing / post translational modifications of receptor kinase / insulin protein
• Inability to activate glucose transport proteins
• Decreased cell glucose uptake
• Hyperglycemia –> diabetes