The nucleus is surrounded by the nuclear membrane.
ribosome - small organelles composed of RNA-rich cytoplasmic granules that are sites of protein synthesis.
rough endoplasmic reticulum - (rough ER) a vast system of interconnected, membranous, infolded and convoluted sacks that are located in the cell's cytoplasm (the ER is continuous with the outer nuclear membrane).
In the body, the iron in the heme is coordinated to the fournitrogen atoms of the porphyrin and also to a nitrogen atom froma histidine residue (one of the amino-acid residues inhemoglobin) of the hemoglobin protein (see Figure 4). The sixthposition (coordination site) around the iron of the heme isoccupied by O2 when the hemoglobin protein isoxygenated.
The Golgi body packages proteins and carbohydrates into membrane-bound vesicles for "export" from the cell.
lysosome - (also called cell vesicles) round organelles surrounded by a membrane and containing digestive enzymes.
Rough ER transports materials through the cell and produces proteins in sacks called cisternae (which are sent to the Golgi body, or inserted into the cell membrane).
smooth endoplasmic reticulum - (smooth ER) a vast system of interconnected, membranous, infolded and convoluted tubes that are located in the cell's cytoplasm (the ER is continuous with the outer nuclear membrane).
To understand the oxygen-binding properties of hemoglobin, wewill focus briefly on the structure of the protein and the metalcomplexes embedded in it.
It contains enzymes and produces and digests lipids (fats) and membrane proteins; smooth ER buds off from rough ER, moving the newly-made proteins and lipids to the Golgi body, lysosomes, and membranes.
vacuole - fluid-filled, membrane-surrounded cavities inside a cell.
The shape change in the heme group has important implicationsfor the rest of the hemoglobin protein, as well. When the ironatom moves into the porphyrin plane upon oxygenation, thehistidine residue to which the iron atom is attached is drawncloser to the heme group. This movement of the histidine residuethen shifts the position of other amino acids that are near thehistidine (Figure 6). When the amino acids in a protein areshifted in this manner (by the oxygenation of one of the hemegroups in the protein), the structure of the interfaces betweenthe four subunits is altered. Hence, when a single heme group inthe hemoglobin protein becomes oxygenated, the whole proteinchanges its shape. In the new shape, it is easier for the otherthree heme groups to become oxygenated. Thus, the binding of onemolecule of O2 to hemoglobin enhances the ability ofhemoglobin to bind more O2 molecules. This property ofhemoglobin is known as "cooperative binding."
Note: The coordinates for the hemoglobin protein (in this and subsequent molecular representations of all or part of the protein) were determined using x-ray crystallography, and the image was rendered using SwissPDB Viewer and POV-Ray (see ).
This figure shows the heme group and a portion of the hemoglobin protein that is directly attached to the heme. When hemoglobin is deoxygenated (left), the heme group adopts a domed configuration. When hemoglobin is oxygenated (right), the heme group adopts a planar configuration. As shown in the figure, the conformational change in the heme group causes the protein to change its conformation, as well.
How do CO2 and H+ promote the release ofO2 from hemoglobin? These species help forminteractions between amino-acid residues at the interfaces of thefour subunits in hemoglobin. These interactions are called "salt bridges,"because they are between positively-charged and negatively-charged amino-acidresidues on different subunits of the same protein (Figure 8). When"salt bridges" form, the subunits are held in a position that"tugs on" the histidine that is attached to the heme iron. (See Figure5.) This favors the domed configuration, which is the deoxygenated form ofhemoglobin.
This is a molecular model of hemoglobin with the subunits displayed in the ribbon representation. A ribbon representation traces the backbone atoms of a protein and is often used to represent its three-dimensional structure. The four heme groups are displayed in the ball-and-stick representation.
On the left is a schematic diagram of the interface of two subunits of the deoxygenated hemoglobin protein. In the presence of CO2 and H+ (e.g., in the muscles), charged groups are formed on the amino acid residues lining the subunit interface. These charged groups are held together by ionic interactions, forming "salt bridges" between the two subunits, and stabilizing the deoxygenated form of hemoglobin.