Info

Secondary active transporters

Uniporters

L3J Primary active transporters

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

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 simply facilitate diffusion down a concentration gradient; they are the uniporter superfamily. Others (active transporters) 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 su-

perfamilies. You will encounter some of these transporters again in later chapters in the context of the metabolic pathways in which they participate.

The Glucose Transporter of Erythrocytes Mediates Passive Transport

Energy-yielding metabolism in erythrocytes depends on a constant supply of glucose from the blood plasma, where the glucose concentration is maintained at about 5 mM. Glucose enters the erythrocyte by facilitated diffusion via a specific glucose transporter, at a rate about 50,000 times greater than the uncatalyzed diffusion rate. The glucose transporter of erythrocytes (called GLUT1 to distinguish it from related glucose transporters in other tissues) is a type III integral protein (Mr ~45,000) with 12 hydrophobic segments, each of which is believed to form a membrane-spanning helix. The detailed structure of GLUT1 is not yet known, but one plausible model suggests that the side-by-side assembly of several helices produces a transmembrane channel lined with hy-drophilic residues that can hydrogen-bond with glucose as it moves through the channel (Fig. 11-30).

The process of glucose transport can be described by analogy with an enzymatic reaction in which the "substrate" is glucose outside the cell (Sout), the "product" is glucose inside (Sin), and the "enzyme" is the transporter, T. When the rate of glucose uptake is measured

Outside

Outside

Inside

COO-

FIGURE 11-30 Proposed structure of GLUT1. (a) Transmembrane helices are represented as oblique (angled) rows of three or four amino acid residues, each row depicting one turn of the a helix. Nine of the 12 helices contain three or more polar or charged amino acid residues, often separated by several hydrophobic residues. (b) A helical wheel diagram shows the distribution of polar and nonpolar residues on the surface of a helical segment. The helix is diagrammed as though observed along its axis from the amino terminus. Adjacent residues in the linear sequence are connected with arrows, and each residue is placed around the wheel in the position it occupies in the helix; recall that 3.6 residues are required to make one complete turn of the

Inside

COO-

FIGURE 11-30 Proposed structure of GLUT1. (a) Transmembrane helices are represented as oblique (angled) rows of three or four amino acid residues, each row depicting one turn of the a helix. Nine of the 12 helices contain three or more polar or charged amino acid residues, often separated by several hydrophobic residues. (b) A helical wheel diagram shows the distribution of polar and nonpolar residues on the surface of a helical segment. The helix is diagrammed as though observed along its axis from the amino terminus. Adjacent residues in the linear sequence are connected with arrows, and each residue is placed around the wheel in the position it occupies in the helix; recall that 3.6 residues are required to make one complete turn of the

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1 Ser

Essentials of Human Physiology

Essentials of Human Physiology

This ebook provides an introductory explanation of the workings of the human body, with an effort to draw connections between the body systems and explain their interdependencies. A framework for the book is homeostasis and how the body maintains balance within each system. This is intended as a first introduction to physiology for a college-level course.

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