The purpose of this review is to compare the structural and functional properties of a group of proteins that share the feature of being able to respond to light energy by way of a bound retinoid chromophore. It may be useful, by way of introduction, to place these systems into a wider biological context. The carotenoids and their bisected relatives, the retinoids, comprise one of several important groups of substances which share an important metabolic ancestor, isopentenyl pyrophosphate. The prenyl moiety of this compound also finds its way into an incredible variety of natural substances ranging from many of the plant-based oils, spices, toxins, rubber and hormones to animal products such as pheromones and steroid hormones. They include the polyprenyl alcohols so important to all living organisms as carriers in the process of glycosylation. The carotenoid/retinoid group is usually identified with the utilization of light in some way, principally because the former are often responsible for the bright colouration exhibited by plants and animals while the latter play an important role in vision, and both are involved in capture of light energy. In most cases, the biological roles of the carotenoids and retinoids are mediated through proteins to which they bind with varying affinities. Their resonating conjugated double bond structure readily allows the absorption of energy in the 300-400 nm region and part of their biological usefulness lies in the ease with which absorbance maxima can be shifted by association with protein. Two instructive examples of this can be seen in a-crustacyanin

/pdf/the-opsin-family-of-proteins-4idgd02ray.pdf

The opsin family of proteins.

Linking regions between helices in bacteriorhodopsin revealed.

Complex compounds of platinum (II) and (IV) with amino acids, peptides and their derivatives

The biotin-binding site on avidin has been labeled with biotin conjugated to undecagold, an organometallic cluster compound containing 11 gold atoms in a core angestroms in diameter. Examination of unstained specimens by scanning transmission electron microscopy reveals the labeled sites directly, without computational averaging or filtering of the images. This approach should be widely applicable for determining the locations of subunits and functional site in biological macromolecules at a resolution at a resolution in range of 15 angstroms.

https://science.sciencemag.org/content/sci/218/4569/290.full.pdf

Biospecific labeling with undecagold: visualization of the biotin-binding site on avidin.

Undecagold clusters for site-specific labeling of biological macromolecules: simplified preparation and model applications.

A platinum-containing derivative of bacteriorhodopsin has been prepared by treating purple membranes with glycyl-L-methionatoplatinum. Low-dose electron diffraction was used to identify Pt binding sites in the 5.6 A resolution reconstruction of the bacteriorhodopsin unit cell in projection. This is a necessary first step in the use of the Pt derivative for identifying the parts of the amino acid sequence corresponding to the α helices in the bacteriorhodopsin structure and for obtaining phases for reflections out to 3.5 A resolution by the method of heavy-atom isomorphous replacement. The largest peak in a Fourier difference map between platinum-labeled and native purple membrane is larger than the spurious features expected to arise from errors in measurements of diffraction intensities.

https://www.pnas.org/content/pnas/78/5/2947.full.pdf

J. W. Wiggins

Papers

Location of platinum binding sites on bacteriorhodopsin by electron diffraction.