Written for a broad* cross-disciplinary audience* the Encyclopedia of Biological Chemistry addresses the fundamental discipline of biological chemistry underlying virtually all of the life sciences. This compilation of more than 500 different entries encompasses all aspects of biochemistry* as well as the extensions of this subject into the related fields of molecular biology* cell biology* genetics and biophysics. This comprehensive encyclopedia covers all areas of biological chemistry written by more than 500 selected international experts. Articles are generously illustrated including more than 800 images in four-color. Each entry contains a clear* concise review of the topic along with illustrations* a glossary of technical terms and a section for additional reading. Each entry further contains general background and term definitions as well as a comprehensive review of the current research in the field. Students* science journalists and scientists seeking a concise introduction to specific topics will appreciate the clear* tabular format of each entry.







Topology

Most, but not all, ABC proteins are ABC transporters.

Each of those molecules contains, or is associated to, one

or two cytoplasmic ATP-binding domains named nucleotide

binding domains (NBDs) (Figure 1) and one or two

transmembrane domains (TMDs) (Figure 2). Each TMD

comprises usually six

a-helix spans. Association of one

TMD to one NBD results in a half-size ABC transporter;

however, they are believed to function as homo- or

heterodimers so that the minimal functional organization

of an ABC transporter is considered to be TMD–NBD–

TMD–NBD or NBD–TMD–NBD–TMD. In eukaryotes,

two TMDs and two NBDs are often associated in

one single molecule called full-sized ABC transporter.

The topological relation between NBD(s) and TMDs is

variable (Figure 2). In bacteria two NBDs often associate

with two TMDs either as four single subunits encoded by

the same operon or in various combinations of fused

subunits. Association of other proteins may occur. The

most prominent associated bacterial protein is the

periplasmic solute-binding receptor, which in gramnegative

bacteria is found in the periplasm, and in

gram-positive bacteria is present often as a lipoprotein,

bound to the external membrane surface via electrostatic

interactions (Figure 3). The three domains of the bacterial

ABC uptake transporters: namely the periplasmic



binding receptor, the cytoplasmic NDB, and the membrane

TMD are believed to have arisen from a common

ancestral ABC transporter in which these three proteins

were already present. However, during evolution, the

sequence of the periplasmic solute-binding receptors

diverges more rapidly than that of the TMDs, while that

of NBDs is the least divergent. Thus, all NBDs

are homologous, but this is not true for the TMDs or

the receptors. Nevertheless, the phylogenetic clustering

patterns in bacterial ABC from different species are

generally the same for all three types of proteins, despite

their variable rate of evolution.

The topology of some eukaryotic ABC effluxers can

be complex as additional TM spans occur in some

systems (Figure 3) as well as extra cytoplasmic domains

of presumed regulatory function.







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