[CELLMOL] Lecture 10-11

• Active transport is a process where solutes are moved against their concentration gradient, requiring an input of energy.
• Active transport is endergonic, characterized by a Gibs free energy of a positive value, meaning the process is non-spontaneous.
• Active transport establishes a non-equilibrium condition, maintaining a concentration gradient between the two sides of the membrane.
• For simple and facilitated diffusion, the end result is the establishment of equilibrium, when the concentration or number of solutes in one side of the membrane is more likely the same with the other side of the membrane.
• The protonation state determines whether the hydrogen will bind to it or if it will be deprotonated and exist in a negative state.
• Bacteriorhodopsin is analogous to the electron transport complex of the mitochondria and the photosystems of the chloroplast in that they are responsible for establishing a proton gradient which the Halobacterium will use for ATP synthase.
• pKa values are related to the pH of the molecule and determine the protonation state of the molecule.
• Active transport has intrinsic directionality, meaning there is direction in the transfer of solute, with the movement being against its concentration gradient.
• Direct active transport involves the coupling of ATP hydrolysis with moving solutes against their concentration gradient, using the energy provided by ATP hydrolysis to move the endergonic process of moving solutes against their concentration gradient forward.
• Active transport is an endergonic process, moving solutes against their concentration gradient is an endergonic process.
• Hydrolysis of ATP, the removal of a phosphate group, is a highly exergonic process, and when coupled with the process of moving solutes against their concentration gradient, the net delta g for both processes will be negative and hence the process is now thermodynamically favorable.
• Active transport performs three important cellular functions: uptake of essential nutrients, removal of wastes, and maintenance of nonequilibrium concentrations of certain ions.
• Active transport is directed, requiring input/spending energy to move solutes against their concentration gradient.
• The coupling of active transport to an energy source may be direct or indirect.
• Active transport mechanisms can be divided based on the sources of energy and whether or not two solutes are transported at the same time.
• Simple and facilitated diffusion have a direction, but solutes move without being directed as it is part of the process being spontaneous.
• The exergonic nature of ATP hydrolysis can be explained by considering several factors, including the fact that the products formed from ATP hydrolysis are more stable compared to the initial state, which is ATP.
• The vanadate ion, which shares a close structural similarity with the phosphate ion, can bind to the P-domain of P-type ATPases.
• Instead of inducing the necessary conformation changes that are associated with carrying out their function, the vanadate ion inhibits P-type ATPases and as a result, this protein is unable to carry out its function.
• Common structural motives for P-type ATPases include being made up of alpha helices and having several domains that are well-conserved: transmembrane domain, P-domain, N-domain, A-domain, and A-domain.
• The structure of P-type ATPases is highly conserved.
• Hydrolysis of ATP does not provide energy to drive the endergonic process into completion but it is also responsible for inducing specific conformation changes in the protein that will allow for the transport of solutes against their concentration gradient.
• P-type ATPases have subcategories: P1 ATPases transport heavy metal ions, P2 ATPases maintain gradients of ions, P3 ATPases acidify the medium, P4 ATPases act as flippases, and P5 ATPases are not well characterized but some are known to transport cations.
• The P2 ATPase — calcium pump, also known as the sarco-plasmic and endoplasmic reticulum calcium ATPase pump or SERCA pump, is a popular calcium pump.
• The transport mechanism for the SERCA pump is shown in figure 6.
• The change is enough for them to alter the pKa and hence alter their protonation state — and that is the reason why protons are translocated.
• In the transport mechanism involving Bacteriorhodopsin, what is crucial is the retinal because bacteriorhodopsin relies on light to move protons against their concentration gradient.
• Compounds with double bonds or triple bonds are efficient light absorbers because pi electrons that are found in the double or triple bonds are easy to excite.
• When the retinal molecules absorbs a photon, one of its double bonds isomerizes to a cis-form, activating the bacteriorhodopsin molecule.
• The retinal molecule is found between the a-helical structure of bacteriorhodopsin and it is covalently bonded to a specific lysine residue (Lys 216).
• The retinal chromophore is normally present in an “all-trans” conformation, covalently linked to a lysine side chain at position 216.
• The side chains attached at either ends of the double bonds are much closer to each other as opposed to the trans-configuration.
• The cis-configuration brings together the side chains attached at both ends of the double bond which increases the rotation energy barrier and once isomerization is completed, the pKa values of the adjacent Aspartate residues are altered, leading to the translocation of the bound proton to the Aspartate sidechain.
• F-type ATPases also use conserved Aspartate residues as the amino acids responsible for translocating protons.
• Very subtle changes in the microenvironment of the molecule lead to a drastic variation in the chemistry of the Aspartate residue.
• Bacteriorhodopsin is an integral membrane protein, meaning it is embedded in the membrane bilayer and it traverses the membrane bilayer.
• Bacteriorhodopsin has 7 a-helical membrane spanning segments forming a cylindrical shape.
• Bacteriorhodopsin uses energy from photons of light to drive active transport of protons out of the cell.
• The light-absorbing pigment retinal, related to vitamin A, is of paramount importance for the function of Bacteriorhodopsin.
• The isomerization into cis requires more energy because of steric hindrance.