Solute Transport across Membranes:- Transporters Can Be Grouped into Superfamilies Based on Their Structures

We know from genomic studies that transporters constitute a significant fraction of all proteins encoded in the genomes of both simple and complex organisms. There are probably a thousand or more different trans porters in the human genome. A few hundred trans porters from various species have been studied with bio chemical, genetic, and electrophysiological tools, but investigators have determined the three-dimensional structures for only a handful of these. Examination of the many transporter genes reveals obvious sequence similarities among subsets of transporters. And as experience has shown, similar amino acid sequences in proteins generally reflect similar three-dimensional structures and, often, similar mechanisms of action. It is reasonable to hope that by determining the structure and mechanism of action of at least one member of each transporter family, we can learn much about the other members of the family—about their structures, substrate specificities, transport rates, and mechanisms of energy coupling. A phylogenetic tree in which proteins are grouped together based on sequence homologies has the potential to tell us much about the transport properties of individual proteins on that tree. When this phylogeny is combined with knowledge of structure, specificity, or mechanism, we have a very useful and relatively simple representation of the huge group of transporters (Table 11–3). Transporters can usefully be classified into super families, whose members have considerable similarity of sequence and might therefore be expected to share structural and functional properties. There are two very broad categories of transporters: carriers and channels (Fig. 11–29). Carriers bind their substrates with high stereospecificity, catalyze transport at rates well below the limits of free diffusion, and are saturable in the same sense as are enzymes: there is some substrate concentration above which further increases will not produce a greater rate of activity. Channels generally allow transmembrane movement at rates several orders of magnitude greater than those typical of carriers, rates approaching the limit of unhindered diffusion. Channels typically show less stereospecificity than carriers and are usually not saturable. Most channels are oligomeric complexes of several, often identical, subunits, whereas many carriers function as monomeric proteins. The classification as carrier or channel is the broadest distinction among transporters. Within each of these categories are superfamilies of various types, defined not only by their primary sequences but by their secondary structures. Some channels are constructed primarily of helical transmembrane segments, others have -barrel structures (Table 11–3). Among the carriers, some sim ply facilitate diffusion down a concentration gradient; they are the uniporter superfamily. Others (active trans porters) can drive substrates across the membrane against a concentration gradient, some using energy provided directly by a chemical reaction (primary active transporters) and some coupling uphill transport of one substrate with the downhill transport of another (secondary active transporters). We now consider some well-studied representatives of the main transporter superfamilies. You will encounter some of these trans porters again in later chapters in the context of the metabolic pathways in which they participate.

FIGURE 11–29 Classification of transporters. The numbers here correspond to the main subdivisions in Table 11–3.

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مواضيع ذات صلة

Solute Transport across Membranes:- Passive Transport Is Facilitated by Membrane Proteins

Membrane Dynamics:- Membrane Fusion Is Central to Many Biological Processes

The Composition and Architecture of Membranes:- Sphingolipids and Cholesterol Cluster Together in Membrane Rafts

The Composition and Architecture of Membranes:- Peripheral Membrane Proteins Are Easily Solubilized

The Composition and Architecture of Membranes:- A Lipid Bilayer Is the Basic Structural Element of Membranes

Reversible Binding of a Protein to a Ligand: Oxygen-Binding Proteins:- Cooperative Ligand Binding Can Be Described

Reversible Binding of a Protein to a Ligand: Oxygen-Binding Proteins:- Hemoglobin Undergoes a Structural Change on Binding Oxygen

Reversible Binding of a Protein to a Ligand: Oxygen-Binding Proteins:- Hemoglobin Subunits Are Structurally Similar to Myoglobin

Reversible Binding of a Protein to a Ligand: Oxygen-Binding Proteins: -Oxygen Is Transported in Blood by Hemoglobin

Reversible Binding of a Protein to a Ligand: Oxygen-Binding Proteins:- Protein Structure Affects How Ligands Bind

Membrane Dynamics:- Transbilayer Movement of Lipids Requires Catalysis

The Composition and Architecture of Membranes:- Lipids and Proteins Diffuse Laterally in the Bilayer