The chapter "Microbial and Animal Rhodopsins: Structures, Functions and Molecular Mechanisms" by Oliver P. Ernst, David T. Lodowski, Marcus Elstner, Peter Hegemann, Leonid S. Brown, and Hideki Kandori provides an in-depth analysis of the structures, functions, and molecular mechanisms of microbial and animal rhodopsins. The authors highlight the conservation of key amino acids in the transmembrane (TM) helices of microbial rhodopsins, which are crucial for their activity and selectivity. Despite varying global sequence identities, a subset of residues remains conserved, particularly in the retinal binding pocket. The chapter also discusses the functional conversion between light-driven proton and chloride pumps, noting that a single amino acid replacement can transform a proton pump into a chloride pump, while the reverse conversion is not possible due to the lack of strongly hydrogen-bonded water molecules. Additionally, the chapter explores the sequence and motif conservation in G protein-coupled receptors (GPCRs) and the role of ordered water molecules in receptor function, using the bovine Rho structure as a reference. The text emphasizes the importance of specific residues and motifs in maintaining the active state of the receptor.The chapter "Microbial and Animal Rhodopsins: Structures, Functions and Molecular Mechanisms" by Oliver P. Ernst, David T. Lodowski, Marcus Elstner, Peter Hegemann, Leonid S. Brown, and Hideki Kandori provides an in-depth analysis of the structures, functions, and molecular mechanisms of microbial and animal rhodopsins. The authors highlight the conservation of key amino acids in the transmembrane (TM) helices of microbial rhodopsins, which are crucial for their activity and selectivity. Despite varying global sequence identities, a subset of residues remains conserved, particularly in the retinal binding pocket. The chapter also discusses the functional conversion between light-driven proton and chloride pumps, noting that a single amino acid replacement can transform a proton pump into a chloride pump, while the reverse conversion is not possible due to the lack of strongly hydrogen-bonded water molecules. Additionally, the chapter explores the sequence and motif conservation in G protein-coupled receptors (GPCRs) and the role of ordered water molecules in receptor function, using the bovine Rho structure as a reference. The text emphasizes the importance of specific residues and motifs in maintaining the active state of the receptor.