Proteins odors

Keywords Odorant-binding proteins Odorant-degrading enzymes ... 

The biochemistry of odor detection involves at least three types of protein odor receptors (ORs) odorant binding proteins (OBPs) and odor degrading enzymes (ODEs). ORs are expressed by olfactory receptor neurons (ORNs) and localized in the membranes of the ciliated dendrites (Figure 14.1). The result of detection is translated into neuronal electrical activity by transductory proteins. But while transductory proteins are more or less common for all olfactory neurons, differential expression of ORs, OBPs and ODEs allows the neurons to detect specific odor molecules. 

The olfactory receptors are mned broadly because each receptor type responds to a range of odorants, and each odorant fires a range of receptor types (16). For some receptor protein/odorant combinations, the binding affinity is concentration dependent (40, 41), and this can correlate with observed changes in character as concentration varies (2, pp. 71-74). 

Properties YIsh. to amber cl. or si. hazy liq. faint, char, protein odor sol. clear pH 5.5-6.5 (10% aq.) 25% min. dry matter Use Level 14% (skin and hair cleansing) 24% (skin and hair care)... 

Pish protein concentrate and soy protein concentrate have been used to prepare a low phenylalanine, high tyrosine peptide for use with phenylketonuria patients (150). The process includes pepsin hydrolysis at pH 1.5 ptonase hydrolysis at pH 6.5 to Hberate aromatic amino acids gel filtration on Sephadex G-15 to remove aromatic amino acids incubation with papain and ethyl esters of L-tyrosine and L-tryptophan, ie, plastein synthesis and ultrafiltration (qv). The plastein has a bland taste and odor and does not contain free amino acids. Yields of 69.3 and 60.9% from PPG and soy protein concentrate, respectively, have been attained. 

The future in research will certainly lead to a better understanding of how odors are recognized, sorted, and classified. Studies promise, among other things, to determine whether perceptually similar, but stmcturaHy different, odors share the same class of receptor proteins, whether responses to odors can be modified, and possibly why olfactory neurons regenerate but other neurons do not. 

The G-proteins are heterotrimers made of three families of subunits, a, P, and y, which can interact specifically with discrete regions on G-protein-coupled receptors. This includes most receptors for neurotransmitters and polypeptide hormones (see Neuroregulators). G-protein-coupled receptors also embrace the odorant receptor family and the rhodopsin-linked visual cascade. 

Amines occur widely in all living organisms. Trimethylamine, for instance, occurs in animal tissues and is partially responsible for the distinctive odor of fish, nicotine is found in tobacco, and cocaine is a stimulant found in the South American coca bush. In addition, amino acids are the buildingblocks from which all proteins are made, and cyclic amine bases are constituents of nucleic acids. 

Cormier and Dure (1963) found another type of luciferin and called it protein-free luciferin. Protein-free luciferin was found in the vapor condensate of freeze-drying whole animals, and also in the 3 5-56 % ammonium sulfate fraction of the crude extract noted above. The protein-free luciferin behaved like an aromatic or heterocyclic compound and it was strongly adsorbed onto Sephadex and other chromatography media, requiring a considerable amount of solvent to elute it. The luminescence reaction of protein-free luciferin in the presence of luciferase required a 500-times higher concentration of H2O2 compared with the standard luciferin preparation. Both types of the luciferin preparation had a strong odor of iodoform. 

As the name implies, the odor of urine in maple syrup urine disease (brancbed-chain ketonuria) suggests maple symp or burnt sugar. The biochemical defect involves the a-keto acid decarboxylase complex (reaction 2, Figure 30-19). Plasma and urinary levels of leucine, isoleucine, valine, a-keto acids, and a-hydroxy acids (reduced a-keto acids) are elevated. The mechanism of toxicity is unknown. Early diagnosis, especially prior to 1 week of age, employs enzymatic analysis. Prompt replacement of dietary protein by an amino acid mixture that lacks leucine, isoleucine, and valine averts brain damage and early mortality. 

A thiol contains an —SH group covalently bonded to carbon. Sulfur is just below oxygen in the periodic table, so a thiol is somewhat similar to an alcohol. Still, the chemical and physical properties of thiols differ significantly from those of alcohols. For example, whereas alcohols have inoffensive odors, thiols smell bad. The stench of skunk scent is due to thiols, including 3-methylbutanethiol. Thiols are important in proteins because of their abilities to form S—S linkages, which we describe in Section 13-1. 

By far the most studied family of the G-protein-coupled receptors are the rhodopsin-like receptors. These are also the largest group of receptors in number as they include receptors not only for the monoamines, nucleotides, neuropeptides and peptide hormones, but they also include the odorant receptors which number several hundreds of related receptors. These receptors have short N-termini, a conserved disulphide bridge between the TM2-TM3 and TM4—TM5 extracellular domains, and variable-length C-termini. In some cases the C-terminus is myristolyated which by tying the C-terminus to the cell membrane generates a fourth intracellular loop. 

In the posterior glands of the nasal septum and in the vomeronasal glands an odorant-binding protein (OBP-II), as expected, increases... 

Bianchet M., Bains G., Pelosi P., Pevsner J., et al. (1996). The three-dimensional structure of bovine odorant binding protein and its mechanism of odour recognition. Nature Struct Biol 3, 934-939. 

Briand L., Huet J., Perez V., Lenoir G., et al. (2000). Odorant and pheromone binding by aphrodisin, a hamster aphrodisiac protein. FEBS Letts 476, 179-185. 

Krishna N., Getchell M., Margolis F. and Getchell T. (1995). Differential expression of vomeromodulin and odorant-binding protein, putative pheromone and odorant transporters, in the developing rat nasal chemosensory mucosae. J Neurosci Res 40, 54-71. 

Lobel D., Marchese S., Krieger J., Pelosi P., et al. (1998). Subtypes of odorant-binding proteins heterologous expression and ligand binding. Europ J Biochem 254, 318-324. 

Preti G., Spielman A., Zeng X.-N. and Leyden J.J. (1995). The characteristic female axillary odors and their precursor proteins qualitative compraison to males. Chem... 

Rama Krishna N.S., Getchell M.L. and Getchell T.V. (1994). Expression of the putative pheromone and odorant transporter vomeromodulin mRNA arid protein in nasal chemosensory mucosae. J Neurosci Res 39, 243-259. 

Spielman A., Sunavala G., Harmony J., Stuart W., et al. (1998). Identification and immunohistochemjcal localization of protein precursors to human axillary odors in apocrine glands and secretions. Arch Dermatol 134, 813-818. 

---