Archives for category: Active Transport

2-  Transport across membranes: Similar to how transport functions via the nucleus, most proteins are transported across the biomembrane with the help of receptor proteins which form a complex with the import molecule.

Protein translocators located between the biomembrane (transmembrane protein or integrins) identifies the signal sequence by unfolding the protein and transports it across the membrane into the organelle or cell (similar to the way NPCs do).

Once the protein enters the organelle or cell, its signal sequence is cut off by a peptidase enzyme and the protein is folded into its final 3-D shape again with the help of helper molecules.

 

In the case of proteins intended for secretion out of the cell, as soon as their N-terminal contains the signal sequence that will indicate this, a signal recognition particle binds to it and forms a protein complex causing synthesis to slow down. The SRP-protein complex will travel and bind to a SRP-receptor on a rough endoplasmic reticulum (RER) membrane. Once the protein gets to the RER membrane, its synthesis will continue at the normal rate until it is complete. During translocation out of the cell, the signal sequence is cut off by a signal peptidase located in the RER membrane and released from the translocation channel and degraded to amino acids.

RER serves as an entering point for proteins that have to go to other organelles like the Golgi complex and lysosome. They can also ferry proteins out of the cell by transport vesicles formed from the budding off of their biomembrane.

 

There are three mechanisms in which newly synthesized proteins are inserted into the RER membrane (the mechanisms vary with the type of protein);

  • Type I; signal sequence on N-terminals enters first and continues to elongate until a hydrophobic stop-sequence is reached, and then inserted in the membrane and forms the anchor for that protein. Of course then signal sequence is cut by protease.
  • Type II; these proteins have rather long hydrophobic regions that will be anchored in the membrane with the C-terminal leading. Protein continues to be inserted until it reaches the hydrophobic stop-sequence, but the signal sequence is not cut.
  • Type III; same as type I, the only difference is that the signal sequence is not cut.

 

And according to the number of times that protein passes through the membrane, the proteins of the ER membrane can be divided into single pass transmembrane protein, double pass transmembrane protein, and multi-pass transmembrane protein.

 

I understand the information here may appear bland. This is because it has been generalized for most process within the cell and cell biologists study the various pathways that exist which can be very overwhelming. Here a brief overview is being discussed such that when you choose to do more detailed study, this generic pattern would have provided you with a basic background.

When protein sorting occurs, they are imported into different places via one of the following three ways:

1-    Transport through nuclear pores: Nuclear pores are large complex structures in the nuclear envelope (double-membrane) that regulate movement of macromolecules (proteins and nucleic acids) in and out of the nucleus.

The nuclear envelope has many nuclear pores termed nuclear pore complex (NPC).

Nuclear Pore Complex

In the nucleoplasmic face of the NPC, a nuclear ring supports eight basket filaments joined by the terminal ring forming a structure called the nuclear basket. In the cytoplasmic face, the NPC’s cytoplasmic ring supports eight cytoplasmic filaments. In the center of the NPC, there are the central plug or transporter (protein) and spoke which connect the two ring subunits together.

Nuclear Pore Complex2. Click on image for credit.

This complex provides the nucleus with the ability to selectively allow entry and exit of molecules. Ions, metabolites, and small proteins can pass freely and nonselectively between the nucleus and the cytosol.

Large molecules, however, and macromolecular complexes carry sorting signals called nuclear localization signals (NLS) to which a nuclear import receptor protein (NIRP) will bind and direct through the pore. The NIRP are also called nuclear transport receptors (NTR) and leads the proteins (synthesized in the cytoplasm but required for the nucleus) into the NPC,

The transport process via NPCs.

The NIR attaches to the NLS of the protein that wants to enter the nucleus, forming a NIR-protein complex, and delivers it in through the NPC.

Once inside, one of the proteins of the NPC is an enzyme named Ran-GTPase which functions to add the protein complex Ran-GTP to the NIR-protein complex and causes conformational changes in the latter’s polypeptide chain leading to the dissociation of the NIR from the protein intended for the nucleus. Ran-GTP is a complex of Ran protein attached to guanosine triphosphate (a nucleoside).

The NIR-Ran-GTP complex formed, then, is powered by GTP hydrolysis to GDP (guanosine diphosphate– di meaning two, as in only two phosphate groups remain attached to the guanosine nucleoside molecule) and exit the nucleus through the NPC. The Ran-GTP is thus converted to Ran-GDP which, once the NIR-Ran-GDP complex exits the nucleus, will dissociate from the NIR.

Nuclear export works similar to nuclear import, but in reverse and proteins contain a nuclear-export signal (NES).

In terms of permeability, water, dissolved gases such as carbon dioxide and oxygen and lipid solid molecules simply diffuse across the phospholipid bilayer because the fluidity permits them.

Water soluble anions (negative ions) generally pass through small horns less than .8 nm in diameter (.0000000008m; 1m=1000mm, 1mm=1000µ-micrometer, 1µm= 1000nm). However, all other larger molecules require carrier molecules or proteins to transport them through the membrane.

 

The following are an overview of the four different ways in which atoms and molecules cross the biomembrane, which is also called transmembrane transport:

 

  • Simple diffusion; is the process by which small molecules and ions simply, because of their tendency to spontaneously move around, especially from a region where they are highly concentrated towards a region where they are less, diffuse through the phospholipid bilayer of the biomembrane.

 

  • Facilitated diffusion; is the process by which, apart from water and ions, specific molecules such as monosaccharides and amino acids, diffuse through channel proteins down their concentration gradient. This assisted diffusion provides them with a special hydrophilic pathway since some molecules might resist the hydrophobic core of the phospholipid bilayer. Some specific ion channels remain open much of the time and are called nongated channels and others only open in response to specific chemical or electrical signals and referred to as gated channels, including the voltage-gated channels of the nervous system which transmit action potentials (nerve impulses) to the brain.

 

  • Active Transport; is the transmembrane transport that occurs when channel proteins, such as the enzyme ATPases (enzymes usually have the suffix –ase) use the energy of ATP hydrolysis (breaking one phosphate group from the nucleotide Adenosine triphosphate) to move ions or small molecules across a membrane against their chemical concentration gradient or electric potential gradient or both.

 

General Principle of ATP hydrolysis. Breaking bonds usually give off energy.

 

ATP-powered pumps which requires energy, is coupled to the hydrolysis of ATP and the overall reaction– ATP hydrolysis and the “uphill” movement of ions or small molecules- is energetically favorable. All ATP-powered pumps are transmembrane proteins with one or more binding sites for ATP located on the cytosolic face of the membrane.

Sodium/Potassium ATP Powered Pump.

Although these proteins commonly are called ATPases, they normally do not hydrolyze ATP into ADP and Pi unless ions or other molecules are simultaneously transported. Because of this tight coupling between ATP hydrolysis and transport, the energy stored in the phosphahydride bond (bond between two phosphate groups) is not dissipated but rather used to move ions or other molecules uphill against an electrochemical gradient.

 

  • Cotransporter; are also enzymes. They mediate coupled reactions in which an energetically unfavorable reaction, such as the movement of molecules or ions against their concentration gradient, is coupled to an energetically favorable reaction such as the passive transport of molecules down their concentration gradient. The co-transporter will use the energy of the passive movement to actively transport other molecules up their concentration gradient. Therefore, unlike ATP, two molecules have to always move through the co-transporter at one time.

 

This Cotransporter is a Symporter. More below. Click on image for credit.

 The co-transporter is sometimes referred to as secondary-active transporter because it uses the energy stored into an electrochemical gradient.

Cotransporters fall into two types:

1-            Antiporter: in which the movement of both molecules (energy favorable and unfavorable) are in opposite directions.

2-            Symporter: in which the movement of both molecules are in the same direction (see above image). This should not confuse you because the same principle applies- one molecule will be moving down its concentration gradient, and the energy in this will be used to actively move another molecule up its concentration gradient but in the same direction, similar to the Sodium/Potassium pump but without the need for ATP hydrolysis because the Cotransporters are smarter and more efficient than Active transporters or ATPases.