16.4: A Diversity of Membrane Proteins Functions

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Membrane protein functions include doing the following:

serving as receptors for hormones or neurotransmitters.
working as immune-system antibodies (immunoglobulins) to recognize foreign substances (antigens).
acting as cell-recognition molecules that bind cells together.
forming cell membrane structures that directly pass chemical information between cells.
anchoring cells to extracellular surfaces like connective tissue.
anchoring the plasma membrane to proteins of the cytoskeleton (e.g., actin).
facilitating molecular transport (entry or exit of substances into or out of cells).
working as enzymes to catalyze crucial reactions in cells. • Electron transport (in mitochondria and chloroplpasts).

Figure 16.17 summarizes some membrane protein functions.

Screen Shot 2022-05-24 at 4.53.30 PM.png — Figure 16.17: Examples of the many functions of membrane proteins.

Transmembrane proteins perform most of the functions illustrated here. However, peripheral membrane proteins also play vital roles in membrane function. Cytochrome c is an example. It is a redox component loosely bound to the rest of the electron transport system in the mitochondrial cristal membrane. Other peripheral membrane proteins may serve to regulate transport or signaling activities of transmembrane-protein complexes or may mediate connections between the membrane and cytoskeletal elements. As shown here, peripheral proteins do not penetrate membranes. They bind reversibly to the internal or external surfaces of the biological membrane with which they are associated. Shortly, we’ll take a closer look at what holds membrane proteins in place and how they perform their unique functions. Check out major membrane-protein functions, actions, and cellular locations in Table 16.1, below.

Table 16.1

Some Functions of Membrane Proteins

Basic Function	Specific Actions	Examples
Electron transport	Oxidizes reduced energy substrates (e.g., NADH, \(\rm FADH_2\), NADPH)	Mitochondrial respiration, photosynthesis
Facilitated transport, facilitated diffusion	Regulate diffusion of substances across membranes along a concentration gradient	\(\rm Ca^{++}\) and other ion channels, glucose transporters
Active transport	Use energy to move ions from low to high concentration across membranes	Mitochondrial protein pumps, the Na+/K+ ion pump in neurons
Signal transduction	Conveys information from molecular signals to cytoplasm (e.g., for hormones that can’t enter cells), leading to a cellular response	Protein hormone and growth factor signaling, antibody/antigen interactions, cytokine mediation of inflammatory responses.
Cell-cell interactions	Cell-cell recognition and binding to form tissues	Formation of desmosomes, gap junctions, and tight junctions
Anchors to cytoskeleton	Link membrane proteins to cytoskeleton	Formation of desmosomes, gap junctions, and tight junctions
Enzymatic	Usually multifunctional proteins with enzymatic activities	F1 ATP synthase, which uses proton gradient to make ATP; adenylyl cyclase, which makes cAMP during signal transduction; note that some receptor proteins are linked to enzymatic domains in the cytoplasm.

288-2 Diversity of Membrane Protein Structure & Function

289-2 Pore Proteins May Cross the Membrane Many Times

290-2 Red Blood Cell (Erythrocyte) Membrane Protein Functions