Figure 1. Diversity of Escherichia coli respiratory chain enzymes. The respiratory chain comprises primary dehydrogenases (on the left), quinone species (middle) and terminal reductases (on the right). Expression of enzymes is regulated in response to the presence of reducing substrates and oxidants. Current topological models for the individual enzymes are represented with the cytoplasmic side of the membrane being ‘up’ in the figure. FAD, flavin adenine mononucleotide; FMN, flavin mononucleotide; b, haem b; o, haem o; NAD, nicotinamide adenine dinucleotide; DHAP, dihydroxyactone phosphate; TMAO, trimethylamine-N-oxide; DMSO, dimethylsulfide.

Figure 2. The electron-transfer chain comprising formate dehydrogenase (FdnGHI, left), a membrane-intrinsic quinone pool (centre) and nitrate reductase (NarGHI, right). This chain translocates protons by exploiting the scalar distribution of substrate-binding sites across the cytoplasmic membrane.

Figure 3. Membrane-bound transporter systems in the cytoplasmic membrane of bacteria. Metabolites are transported across the cytoplasmic membrane (grey box) by facilitated diffusion (example GlpF, the glycerol facilitator), proton-coupled permeases (example LacY, the lactose permease), binding protein-coupled ABC transporters (example HisJ QMP₂, the histidine permease) or by group translocation (example the phosphoenolpyruvate phosphotransferase system (PTS)). The F₀F₁ ATPase complex that generates ATP from the _H⁺ or consumes ATP to generate _H⁺ is also shown.