Amino acid permease activity

Regulation of amino acid permease activity as a function of nitrogen availability... 

The APFl gene product, a common factor of unknown function which increases the activity of amino acid permeases... 

A general amino acid permease (GAP) transports all of the amino acids. The ammonium ion inhibits and represses the GAP. The GAP therefore only appears to be active during the second half of fermentation, when the must no longer contains ammonium. It acts as a nitrogen scavenger towards amino acids (Cartwright et al, 1989). 

S. cerevisiae also has many specific amino acid permeases (at least 11). Each one ensures the transport of one or more amino acids. In Contrast to GAP, the ammonium ion does not limit their activity. From the beginning of the yeast log phase during the first stages of fermentation, these transporters ensure the rapid assimilation of must amino acids. 

A large number of amino acid transporters have been detected by isolating mutations which selectively inactivate one permease without altering enzyme activities involving the corresponding amino acid. Competitive inhibition, kinetics and regulatory behaviour have also been used as criteria to distinguish one transport system from another (see section 4.2). 

In contrast, less effort has apparently been directed to the transport of xenobiotics, and there is an intrinsic difficulty that in contrast to organisms that utilize carbohydrates or amino acids, suitable mutants defective in the metabolism of the substrate may not be available. This limitation makes it impossible to determine directly whether active transport is involved. Although the genes encoding permeases have been described quite frequently, details of their mechanisms have been less well documented ... 

Ovchinnikov 234 237) has shown that bovine rhodopsin, although quite different in amino acid sequence (348 residues), also forms seven transmembrane helices. This structural similarity between bacterial and mammalian light activated membrane proteins is remarkable. Since the two amino acid sequences have little in common it would appear that the necessary requirement is seven transmembrane helices to form a channel which is specific for proton migration. For example it has been suggested that a similar arrangement and function is performed by the lactose permease of E. coli237). 

During the process of nutrient assimilation, DIN is first actively transported across the cell membrane. This transport is mediated by species-specific enzymes called permeases that are present in the cell membrane. Once the inorganic nitrogen has crossed the cell membrane, it can participate in anabolic reactions. For example, ammonium helps build amino acids by first reacting with a-ketoglutaric acid to generate glutamic acid ... 

Synthesis of norepinephrine begins with the amino acid tyrosine, which enters the neuron by active transport, perhaps facilitated by a permease. In the neuronal cytosol, tyrosine is converted by the enzyme tyrosine hydroxylase to dihydroxyphenylalanine (dopa), which is converted to dopamine by the enzyme aromatic L-amino acid decarboxylase, sometimes termed dopa-decarboxylase. The dopamine is actively transported into storage vesicles, where it is converted to norepinephrine (the transmitter) by dopamine (3-hydroxylase, an enzyme within the storage vesicle. 

Whereas a major function of biological membranes is to maintain the status quo by preventing loss of vital materials and entry of harmful substances, membranes must also engage in selective transport processes. Living cells depend on an influx of phosphate and other ions, and of nutrients such as carbohydrates and amino acids. They extrude certain ions, such as Na+, and rid themselves of metabolic end products. How do these ionic or polar species traverse the phospholipid bilayer of the plasma membrane How do pyruvate, malate, the tricarboxylic acid citrate and even ATP move between the cytosol and the mitochondrial matrix (see figs. 13.15 and 14.1) The answer is that biological membranes contain proteins that act as specific transporters, or permeases. These proteins behave much like conventional enzymes They bind substrates and they release products. Their primary function, however, is not to catalyze chemical reactions but to move materials from one side of a membrane to the other. In this section we discuss the general features of membrane transport and examine the structures and activities of several transport proteins. 

The penetration of amino acids and sugars into the yeast activates membrane transport systems called permeases. The general amino acid... 

The penetration of ammonium and amino acids into the yeast cell activates numerous membrane proteinic transporters or permeases (Section 1.3.2). S. cerevisiae has at least two specific ammonium ion transporters (Dubois and Grenson, 1979). Then-activity is inhibited by several amino acids, in a non-competitive manner. 

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