Cell Structure & Function Chapter 12

Endoplasmic Reticulum

Protein Synthesis, sorting

Golgi Complex

protein processing, and sorting

Endosomes

Carry and sort material brought into the cell

Lysosomes

Digest ingested material and unneeded cellular components

Peroxisomes

Lipid metabolism and scavenging of reactive oxygen species

Rough ER

• Characterized by ribosomes on the cytosolic side of the membrane

• Large flattened sheets

Smooth ER

• Lacks ribosomes and has other roles in the cell involving the processing and storage of nonproteins

• Tubular membranes

Differences between ER

Rough ER membrane - large flattened sheet

Smooth ER membrane - tubular structures

Transitional elements (ER exit sites) of the rough ER are an exception as they resemble the smooth ER. 

The lumenal spaces of rough and smooth ER are continuous. 

Cells involved in synthesis of secretory proteins have prominent rough E R networks.

Cells producing steroid hormones tend to have extensive networks of smooth E R.

Rough ER Function

• Membrane protein synthesis through cotranslational insertion into the ER through a pore complex as they are synthesized.

• The initial steps of addition of carbohydrates to glycoproteins

• Folding of polypeptides

• Recognition and removal of misfolded proteins

• Assembly of multimeric proteins

Smooth ER Function

• Drug detoxification - store enzymes for drug hydroxylation

• Certain carbohydrate metabolism - glycogen breakdown

• Calcium Storage - muscle contraction

• Steroid Biosynthesis - steroid hormone synthesis

Drug Detoxification (Smooth ER)

• involves hydroxylation

• Adding hydroxyl groups increases their solubility making them easier to excrete

• Catalyzed by a member of the cytochrome P-450 family of proteins (monooxygenases)

Hydroxylation Occurs via Electron Transport

• Electrons transferred to a heme group in cytochrome P-450

• Electron donated to O₂, one oxygen atom forms H₂O and the other is added to the substrate as a hydroxyl group. 

• R is drug being hydroxylated

Pharmacogenetics

• Investigates how inherited differences in genes, like P-450, can lead to differential responses to drugs and medications.

Carbohydrate Metabolism (Smooth ER)

Smooth ER in liver cells breakdown the stored glycogen; it contains glucose 6 phosphatase, an enzyme unique to smooth ER

Glycogen Metabolism

Glycogen is not stored in the ER, rather, G6P is transported into the ER lumen where G6Pase is bound

Calcium Storage  (Smooth ER)

• Sarcoplasmic reticulum (calcium storage) of muscle cells is an example of Smooth ER

• ER lumen contains high concentrations of calcium binding proteins

• Controlling muscle contraction by regulated calcium release.

Steroid Biosynthesis (Smooth ER)

• Synthesis of steroids and cholesterol

• Utilizes enzymes such as HMG-CoA reductase, the committed step in cholesterol biosynthesis.

• Found in smooth ER of liver cells and is targeted by cholesterol-lowering drugs called statins.

Biosynthesis of Membranes

• Mitochondria synthesize phosphatidylethanolamine.

• Peroxisomes synthesize cholesterol. 

  • From precursors from ER

• Chloroplasts contain enzymes for chloroplast-specific lipids.

• ER primary source of membrane lipids.

Membrane Lipid Biosynthesis in the ER

• Fatty acids synthesized in the cytosol are inserted into the cytosolic leaflet of the ER membrane.

• Phospholipid translocators (flippases) transfer specific phospholipids to the lumenal leaflet, generating membrane asymmetry.

• Distinct composition of cytosolic and lumenal monolayers established in the E R is transferred to other cellular membranes.

• Phospholipid exchange proteins - convey specific phospholipids to chloroplasts, mitochondria, or peroxisomes 

Sorting Signals

• A linear sequence of amino acids (signal sequence)

  • protein translocation into organelles

• Signal Patch - specific three dimensional arrangement of amino acids

  • Nuclear and vesicular transport

Intracellular Sorting of Proteins

Tags determine the fate or proteins after translation.

ER Signaling Sequences

• Proteins destined for secretion or membranes possess an N-terminal ER signal sequence (15-30 amino acids) with three domains: a positively charged N-terminal region, a hydrophobic core, and a polar region near the cleavage site.

Signal hypothesis

For polypeptides destined for the E R, the N-terminus contains an E R signal sequence that directs ribosome-mRNA-polypeptide complexes to the surface of the rough ER.

Signal Recognition Particle (SRP)

• Contact of the ribosome with the E R is mediated by SRP

• SRP recognizes and binds to the E R signal sequence and then binds to the E R membrane.

SRP Mechanism

1. mRNA binds to a free ribosome. The polypeptide is synthesized until the E R signal sequence has been formed

2. SRP binds the ER signal sequence and blocks translation

3. The SRP binds the ribosome to a translocon in the ER membrane.

Translocon

• Protein Complex made of:

  • SRP receptor which SPR binds

  • Ribosome Receptor - holds ribosome in place

  • Pore protein - Forms a channel for the growing polypeptide to enter the ER lumen

  • Signal peptidase -