Membrane Bounded Proteins and Functions

7/25/2019 Membrane Bounded Proteins and Functions

1/7

Membrane bounded proteins and Functions

Definition:

A membrane protein is a protein molecule that is attached to, or associated with the

membrane of a cell or an organelle. More than half of all proteins interact with

membranes.

Membrane proteins are proteins that interact with biological membranes. They are one of the

common types of protein along with soluble globular proteins, fibrous proteins, and disordered

proteins.They are targets of over 50% of all modern medicinal drugs. It is estimated that 20!0%

of all genes in most genomes encode membrane proteins. In addition to the lipid bilayer, the cell

membrane also contains a number of proteins.The lipid bilayer provides the structure for the cell

membrane, membrane proteins allow for many of the interactions that occur between cells.

Membrane proteins are easy to move with in the lipid bilayer as a result of its fluidity. "lthough

this is true for most proteins, they can also be confined to certain areas of the bilayer with

en#ymes. Membrane proteins perform various functions, and this diversity is reflected in the

significantly different types of proteins associated with the lipid bilayer.

Functions of Membrane bounded Proteins:

$ollowing are the functions of membrane proteins

&. 'tructural proteins are attached to microfilaments in the cytos(eleton which ensures

stability of the cell.2. )ell adhesion molecules allow cells to identify each other and interact. 'uch proteins are

involved in immune response.!. Membrane en#ymes produce a variety of substances essential for cell function.

*. Membrane receptor proteins serve as connection between the cell+s internal and eternal

environments.


2/7

5. Transport proteins play an important role in the maintenance of concentrations of ions.

These transport proteins come in two forms carrier proteins and channel proteins.

Types of membrane bounded proteins:

$ollowing are three main types of membrane bounded proteins.

&. Integral membrane proteins.

2. -eripheral membrane proteins

!. ipid/anchored proteins

1. ntegral membrane proteins: Integral membrane proteins are those which are permanently bound to the

lipid bilayer. These reuire a detergent 1such as '' or Triton 3/&004 or some other

apolar solvent to be displaced. "n integral membrane protein 1IM-4 is a type of

membrane protein that is permanently attached to the biological membrane. "ll

transmembrane proteins are IM-s, but not all IM-s are transmembrane proteins.IM-s

comprise a significant fraction of the proteins encoded in an organism+s genome.-roteins

that cross the membrane are surrounded by annular lipids 1see annular lipid shell4,

which are defined as lipids that are in direct contact with a membrane protein. 'uch

proteins can be separated from the biological membranes only using detergents, nonpolar

solvents, or sometimes denaturing agents.

There are three main categories of Integral membrane proteins.!" Transmembrane proteins:

Transmembrane proteins, are proteins that are permanently attached to the lipid membrane and

span across the membrane 1at least once4. The transmembrane regions of the proteins are either

beta/barrels or alpha/helical. The alpha/helical domains are present in all types of biological

membranes including outer membranes. The beta/barrels were found only in outer membranes of

6ram/negative bacteria, lipid/rich cell walls of a few 6ram/positive bacteria, and outer


3/7

membranes of mitochondria and chloroplasts. transmembrane proteins span from one side of a

membrane through to the other side of the membrane. $irm attachment of T- to biological

membrane is aided by a special class of membrane lipids, called annular lipid shell. Many T-s

function as gateways or loading doc(s to deny or permit the transport of specific substances

across the biological membrane, to get into the cell, or out of the cell as in the case of waste

byproducts. "s a response to the shape of certain molecules these freight handling T-s may

have special ways of folding up or bending that will move a substance through the biological

membrane.Transmembrane proteins are polytopic proteins that aggregate and precipitate in

water.

!A" #hodopsin:

7acterial rhodopsins are a family of bacterial opsins. They are retinal/binding proteins

that provide light/dependent ion transport and sensory functions to a family of halophilic

and other bacteria. They are integral membrane proteins with seven transmembrane

helices, the last of which contains the attachment point for retinal 1a conserved lysine4.

7acteriorhodopsin contains 8 transmembrane 9/ helices. The purple membrane of

:alobacterium halobium contains ordered sheets of bacteriorhodopsin, a protein of 2*;

amino acids residues which binds retnal, the same photosensitve pigment that is used


4/7

to capture light in our eyes. 7acteriorhodopsin uses the energy of light to pump protons

across the membrane.

8 transmembrane 9/ helices hoo(ed together by loops.

!$" Porins

-orins are beta barrel proteins that cross a cellular membrane and act as a pore through

which molecules can diffuse.


5/7

!" ntegral monotopic proteins:

Integral monotopic proteins are proteins that are permanently attached to the lipid

membrane from only one side and do not span across the membrane. Integral monotopicproteins, are permanently attached to the membrane from one side.

Three/dimensional structures of the following integral monotopic proteins have been determined

prostaglandin :2 syntheses & and 2 1cyclooygenases4

lanosterol synthaseand sualene/hopene cyclase

microsomalprostaglandin > synthase

carnitine ?/palmitoyltransferase2

There are also structures of integral monotopic domainsof transmembrane proteins

monoamine oidases" and 7

fatty acid amide hydrolase

mammalian cytochrome -*50 oidases

corticosteroid &&/beta/dehydrogenases
https://en.wikipedia.org/wiki/Cyclooxygenasehttps://en.wikipedia.org/wiki/Lanosterol_synthasehttps://en.wikipedia.org/wiki/Prostaglandin_E_synthasehttps://en.wikipedia.org/wiki/Carnitine_O-palmitoyltransferasehttps://en.wikipedia.org/wiki/Monoamine_oxidasehttps://en.wikipedia.org/wiki/Fatty_acid_amide_hydrolasehttps://en.wikipedia.org/wiki/Cytochrome_P450_oxidasehttps://en.wikipedia.org/wiki/11Beta_Hydroxysteroid_dehydrogenasehttps://en.wikipedia.org/wiki/Lanosterol_synthasehttps://en.wikipedia.org/wiki/Prostaglandin_E_synthasehttps://en.wikipedia.org/wiki/Carnitine_O-palmitoyltransferasehttps://en.wikipedia.org/wiki/Monoamine_oxidasehttps://en.wikipedia.org/wiki/Fatty_acid_amide_hydrolasehttps://en.wikipedia.org/wiki/Cytochrome_P450_oxidasehttps://en.wikipedia.org/wiki/11Beta_Hydroxysteroid_dehydrogenasehttps://en.wikipedia.org/wiki/Cyclooxygenase


6/7

!%".Peripheral membrane proteins:

-eripheral membrane proteins are proteins that adhere only temporarily to the biological

membrane with which they are associated. These molecules attach to integral membrane

proteins, or penetrate the peripheral regions of the lipid bilayer. The regulatory protein subunits

of manyion channelsand transmembrane receptors, for eample, may be defined as peripheral

membrane proteins. In contrast to integral membrane proteins, peripheral membrane proteins

tend to collect in the water/soluble component, or

fraction, of all the proteins etracted during a protein

purificationprocedure. -roteins with 6-I anchors are

an eception to this rule and can have purification

properties similar to those of integral membrane

proteins.The reversible attachment of proteins to

biological membranes has shown to regulate cell

signaling and many other important cellular events,

through a variety of mechanisms. $or eample, the close association between many en#ymesand

biological membranes may bring them into close proimity with their lipid substrate1s4.

Membrane binding may also promote rearrangement, dissociation, or conformational changes

within many protein structural domains, resulting in an activation of their biological activity.

"dditionally, the positioning of many proteins are locali#ed to either the inner or outer surfaces

or leaflets of their resident membrane. This facilitates the assembly of multi/protein complees

by increasing the probability of any appropriate proteinprotein interactions.
https://en.wikipedia.org/wiki/Proteinhttps://en.wikipedia.org/wiki/Biological_membranehttps://en.wikipedia.org/wiki/Biological_membranehttps://en.wikipedia.org/wiki/Integral_membrane_proteinhttps://en.wikipedia.org/wiki/Integral_membrane_proteinhttps://en.wikipedia.org/wiki/Lipid_bilayerhttps://en.wikipedia.org/wiki/Ion_channelhttps://en.wikipedia.org/wiki/Ion_channelhttps://en.wikipedia.org/wiki/Transmembrane_receptorhttps://en.wikipedia.org/wiki/Protein_purificationhttps://en.wikipedia.org/wiki/Protein_purificationhttps://en.wikipedia.org/wiki/GPI_anchorhttps://en.wikipedia.org/wiki/Integral_membrane_proteinhttps://en.wikipedia.org/wiki/Integral_membrane_proteinhttps://en.wikipedia.org/wiki/Cell_signalinghttps://en.wikipedia.org/wiki/Cell_signalinghttps://en.wikipedia.org/wiki/Enzymehttps://en.wikipedia.org/wiki/Substrate_(biochemistry)https://en.wikipedia.org/wiki/Conformational_changehttps://en.wikipedia.org/wiki/Proteinhttps://en.wikipedia.org/wiki/Biological_membranehttps://en.wikipedia.org/wiki/Biological_membranehttps://en.wikipedia.org/wiki/Integral_membrane_proteinhttps://en.wikipedia.org/wiki/Integral_membrane_proteinhttps://en.wikipedia.org/wiki/Lipid_bilayerhttps://en.wikipedia.org/wiki/Ion_channelhttps://en.wikipedia.org/wiki/Transmembrane_receptorhttps://en.wikipedia.org/wiki/Protein_purificationhttps://en.wikipedia.org/wiki/Protein_purificationhttps://en.wikipedia.org/wiki/GPI_anchorhttps://en.wikipedia.org/wiki/Integral_membrane_proteinhttps://en.wikipedia.org/wiki/Integral_membrane_proteinhttps://en.wikipedia.org/wiki/Cell_signalinghttps://en.wikipedia.org/wiki/Cell_signalinghttps://en.wikipedia.org/wiki/Enzymehttps://en.wikipedia.org/wiki/Substrate_(biochemistry)https://en.wikipedia.org/wiki/Conformational_change


7/7

& 'ipid Anchored proteins:

ipid/anchored proteins 1also (nown as lipid/lin(ed proteins4 areproteinslocated on the surface

of the cell membranethat are covalentlyattached to lipidsembedded within the cell membrane.

These lipids insert and assume a place in the bilayer structure of the membrane alongside the

similar fatty acid tails. The lipid/anchored protein can be located on either side of the cell

membrane.Thus, the lipid serves to anchor the protein to the cell membrane.The lipid groups

plays a role in protein interaction and can contribute to the function of the protein to which it is

attached. $urthermore, the lipid serves as a mediator of membrane associations or as a

determinant for specific protein/protein interactions. $or eample, lipid groups can play an

important role in increasing molecular hydrophobicity. This allows for the interaction of proteins

with cellular membranes and protein domains.?verall, there are three main types of lipid/

anchored proteins which include prenylated proteins, fatty acylated proteins and

glycosylphosphatidylinositol/lin(ed proteins 16-I4. " protein can have multiple lipid groups

covalently attached to it, but the site where the lipid binds to the protein depends both on thelipid group and protein.
https://en.wikipedia.org/wiki/Proteinhttps://en.wikipedia.org/wiki/Proteinhttps://en.wikipedia.org/wiki/Proteinhttps://en.wikipedia.org/wiki/Cell_membranehttps://en.wikipedia.org/wiki/Covalent_bondhttps://en.wikipedia.org/wiki/Covalent_bondhttps://en.wikipedia.org/wiki/Lipidhttps://en.wikipedia.org/wiki/Fatty_acidhttps://en.wikipedia.org/wiki/Hydrophobehttps://en.wikipedia.org/wiki/Proteinhttps://en.wikipedia.org/wiki/Cell_membranehttps://en.wikipedia.org/wiki/Covalent_bondhttps://en.wikipedia.org/wiki/Lipidhttps://en.wikipedia.org/wiki/Fatty_acidhttps://en.wikipedia.org/wiki/Hydrophobe

Membrane Bounded Proteins and Functions

Documents

Transcript of Membrane Bounded Proteins and Functions