22ª AULA

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•  Internalização celular das LDLs. 
•  Ciclo da Transferrina - importância fisiológica. 
•  Transcitose membranar - exemplo 
•  Hipercolesterolemia Familiar e receptor de LDL. 
Bibliografia 
•  Lodish H., Baltimore D., Berk A., Zipursky S.L., Matsudaira P., and Darnell J., Molecular 
Cell Biology (4ª Ed.), Freeman W.H. and Company (USA) (2000). 
•  Voet D. and Voet J.G. (2011) Biochemistry, 4 th edition, Wiley. Inc. 
Receptor-Mediated Endocytosis 
Important in the internalization of potentially or toxic extracellular proteins, 
such as proteases (destroyed afterwards in lysossomes) 
Liver contains a potent surface receptor– 
ASIALOGLYCOPROTEIN receptor– that binds 
galactose terminal glycoproteins 
Receptor-Mediated Endocytosis and Exocytosis 
Ligand binds to membrane receptor.
Clathrin- 
coated pit
Extracellular fluid
Intracellular fluid
To lysosome or 
Golgi complex
Receptor-ligand migrates to 
clathrin-coated pit.
Endocytosis 
Vesicle loses 
clathrin coat.
Ligands go to lysosomes 
or Golgi for processing.
Transport vesicle 
with receptors moves 
to the cell membrane.
Transport vesicle 
and cell membrane 
fuse (membrane 
recycling).
Exocytosis
Clathrin
Endosome
Receptors 
and ligands 
separate.
Receptor
4
1
2
3
4
9
8
7
6
5
Receptor-Mediated Endocytosis and Exocytosis 
Ligand binds to membrane receptor.
 Extracellular fluid
Intracellular fluid
Receptor
1
Receptor-Mediated Endocytosis �
and Exocytosis
Ligand binds to membrane receptor.
Clathrin- 
coated pit
Extracellular fluid
Intracellular fluid
Receptor-ligand migrates to 
clathrin-coated pit.
Clathrin
Receptor
1
2
Figure 5-24, steps 1–2 
Receptor-Mediated Endocytosis �
and Exocytosis
Figure 5-24, steps 1–3 
Ligand binds to membrane receptor.
Clathrin- 
coated pit
Extracellular fluid
Intracellular fluid
Receptor-ligand migrates to 
clathrin-coated pit.
Endocytosis 
Clathrin
Receptor
1
2
3
Receptor-Mediated Endocytosis and Exocytosis 
Ligand binds to membrane receptor.
Clathrin- 
coated pit
Extracellular fluid
Intracellular fluid
Receptor-ligand migrates to 
clathrin-coated pit.
Endocytosis 
Vesicle loses 
clathrin coat.
Clathrin
Receptor
4
1
2
3
4
Receptor-Mediated Endocytosis and Exocytosis 
Ligand binds to membrane receptor.
Clathrin- 
coated pit
Extracellular fluid
Intracellular fluid
Receptor-ligand migrates to 
clathrin-coated pit.
Endocytosis 
Vesicle loses 
clathrin coat.
Clathrin
Endosome
Receptors 
and ligands 
separate.
Receptor
4
1
2
3
4
5
Figure 5-24, steps 1–5 
Receptor-Mediated Endocytosis and Exocytosis 
Ligand binds to membrane receptor.
Clathrin- 
coated pit
Extracellular fluid
Intracellular fluid
To lysosome or 
Golgi complex
Receptor-ligand migrates to 
clathrin-coated pit.
Endocytosis 
Vesicle loses 
clathrin coat.
Ligands go to lysosomes 
or Golgi for processing.
Clathrin
Endosome
Receptors 
and ligands 
separate.
Receptor
4
1
2
3
4
6
5
Figure 5-24, steps 1–6 
Receptor-Mediated Endocytosis and Exocytosis 
Ligand binds to membrane receptor.
Clathrin- 
coated pit
Extracellular fluid
Intracellular fluid
To lysosome or 
Golgi complex
Receptor-ligand migrates to 
clathrin-coated pit.
Endocytosis 
Vesicle loses 
clathrin coat.
Ligands go to lysosomes 
or Golgi for processing.
Transport vesicle 
with receptors moves 
to the cell membrane.
Clathrin
Endosome
Receptors 
and ligands 
separate.
Receptor
4
1
2
3
4
7
6
5
Figure 5-24, steps 1–7 
Receptor-Mediated Endocytosis and Exocytosis 
Ligand binds to membrane receptor.
Clathrin- 
coated pit
Extracellular fluid
Intracellular fluid
To lysosome or 
Golgi complex
Receptor-ligand migrates to 
clathrin-coated pit.
Endocytosis 
Vesicle loses 
clathrin coat.
Ligands go to lysosomes 
or Golgi for processing.
Transport vesicle 
with receptors moves 
to the cell membrane.
Transport vesicle 
and cell membrane 
fuse (membrane 
recycling).
Clathrin
Endosome
Receptors 
and ligands 
separate.
Receptor
4
1
2
3
4
8
7
6
5
Receptor-Mediated Endocytosis and Exocytosis 
Exocytosis is the opposite of endocytosis 
Ligand binds to membrane receptor.
Clathrin- 
coated pit
Extracellular fluid
Intracellular fluid
To lysosome or 
Golgi complex
Receptor-ligand migrates to 
clathrin-coated pit.
Endocytosis 
Vesicle loses 
clathrin coat.
Ligands go to lysosomes 
or Golgi for processing.
Transport vesicle 
with receptors moves 
to the cell membrane.
Transport vesicle 
and cell membrane 
fuse (membrane 
recycling).
Exocytosis
Clathrin
Endosome
Receptors 
and ligands 
separate.
Receptor
4
1
2
3
4
9
8
7
6
5
Receptor-mediated Endocytosis (LDL Receptor) 
Lysosomes are like the stomach of 
the cell. They are organelles 
surrounded by a single membrane 
and filled with enzymes called acid 
hydrolases that digest (degrade) a 
variety of macromolecules. A 
vacuolar H+ ATPase pumps protons 
into the lysosome causing the pH to 
be ~5. 
All major classes of macromolecules 
are degraded in lysosomes 
Lysosomal glycoproteins are unusually 
highly glycosylated 
40 hydrolytic enzymes 
pH is one of the mechanisms for protecting contents in the cytosol 
APOTRANSFERRINA – libertada no 
plasma sanguíneo e transportada ao 
fígado e intestino onde liga Ferro 
Que propriedades do complexo Receptor-
Ligando causam a retenção do Ferro na 
célula e a secreção da apotransferrina? 
TRANSFERRINA - glicoproteína do plasma 
Transporta Fe para todos os tecidos a partir 
do FÍGADO (principal local de 
armazenamento no corpo) e do INTESTINO 
(local de absorção do Fe) 
Após endocitose, a transferrina e os seus 2 átomos de Fe ligados têm destinos diferentes 
Ferro Transferrina 
Transcytosis provides a way to deliver 
proteins across an epithelium. 
Transport of antibodies in 
milk across the gut 
epithelium of baby rats. 
Acidic pH of the gut favor 
association of antibody with 
Fc receptor whereas the 
neutral pH of the extracellular 
fluid favors dissociation. 
Transcytosis: a combination of 
endocytosis and exocytosis 
Transcytosis: a combination of endocytosis and exocytosis 
Colesterol é insolúvel nos fluidos do 
corpo – transportado por um 
transportador (LDL - lipoproteína) 
A captação celular de LDL é um dos 
exemplos mais bem compreendidos de 
endocitode mediada por receptores. As 
LDLs são complexos lipo-proteicos que 
transportam ésteres de colesterol no 
plasma. LDL fornece normalmente 
colesterol às células. 
Cortex adrenal muito rico em receptores de LDL 
Endocitose mediada por 
receptores das LDLs 
Colesterol 
LDL Receptor 
•  Binds to ApoB-100. 
•  Found on cell surface of many cell types 
•  Mediates delivery of cholestrol by inducing endocytosis 
and fusion with lysosomes. 
•  Lysosomal lipases and proteases degrade the LDL. 
Cholesterol then incorporates into cell membranes or is 
stored as cholesterol-esters. 
•  Receptor-mediated endocytosis enables a cell 
to acquire bulk quantities of specific materials that may 
be in low concentrations in the environment. 
–  Human cells use this process to absorb cholesterol. 
–  Cholesterol travels in the blood in low-density 
lipoproteins (LDL), complexes of protein and lipid. 
–  These lipoproteins bind to LDL receptors and enter 
the cell by endocytosis. 
–  In familial hypercholesterolemia,an inherited disease, 
the LDL receptors are defective, leading to an 
accumulation of LDL and cholesterol in the blood. 
–  This contributes to early atherosclerosis. 
Lipoproteins 
  Transport water insoluble TAG (triacilgliceróis), cholesterol and 
cholesterol-esters throughout circulatory system 
  Hydrophobic core containing TAG and cholesterol-esters 
  Hydrophillic surface made of proteins (apoproteins) and phospholipids 
Synthesis of intracelular cholesteryl ester 
  Cholesterol transported in bloodstream 
mostly as low-density lipoprotein, LDL.
  1500 cholesterols
  800 phospholipids
  1 protein
  Receptor-mediated endocytosis first elucidated in 
patients with familial hypercholesterolemia 
  lack LDL receptors on cell surface 
  very high blood levels of cholesterol 
Lipoprotein 
Complexes 
•  Cholesterol, cholesterol esters and triacylglycerol are transported in 
the blood as constituents of several different lipoprotein complexes.
•  Low density lipoprotein (LDL) is shown.
•  The density of lipoprotein complexes increases with the amount of 
protein they contain.
•  There are more than ten proteins, which vary among the complexes. 
Captação e degradação das LDL / Regulação da 
produção dos recetores de LDL 
Captação e degradação das LDL / Regulação da produção dos 
recetores de LDL 
AcilCoA:colesterol- aciltransferase 
Normally, the 
receptors associate 
with adaptin. 
Some individuals have 
defects in the 
cytoplasmic domain 
recognized by adaptin. 
Genetic defects results in elevated blood levels of LDL: 
•  absence of LDL receptor 
•  defective LDL-binding site in the LDL receptor 
Absence of LDL Receptor Leads to 
Hypercholesteremia and Atherosclerosis 
  Persons lacking the LDL receptor suffer from familial 
hypercholestermia 
  Result of a mutation in a single autosomal gene 
  Total plasma cholesterol and LDL levels are elevated. 
  Homozygous individuals have cholesterol levels of 680 
mg/dL. Heterozygous individuals = 300 mg/dL. Healthy 
person = <200 mg/dL 
  Most homozygous individuals die of cardiovascular 
disease in childhood 
Hypercholesterolemia Familial 
Aterosclerose 
Obstrução das artérias dependente de colesterol, caracteriza-
se pela deposição progressiva de lípidos, células e material 
da matriz extracelular na camada interna da artéria