Secretion of amino acids by bacteria: Physiology and mechanism

Abstract

Although representing a common property of microorganisms and being widely used for biotechnological purposes, solute secretion has been relatively poorly studied in terms of biochemistry. In this review, various examples of metabolite secretion processes by bacteria are discussed with the emphasis on the mechanisms of amino acid secretion by coryneform bacteria. Among the metabolic concepts which may be applied to explain the physiological meaning of metabolite secretion, mainly two concepts are dealt with, i.e. the so‐called 'overflow metabolism' on the one hand and the situation where non‐metabolizable intermediates are accumulated and finally secreted on the other. In the central part of this review, the different concepts are discussed which have been put forward to mechanistically explain amino acid secretion under particular metabolic conditions and in particular strains of bacteria, i.e. secretion mediated (i) by diffusion, (ii) by the participation of amino acid uptake systems, and (iii) by the use of specific secretion systems. These concepts are then applied to amino acid secretion in Corynebacterium glutamicum, and emplified by detailed studies on the mechanism and regulation of the secretion of lysine, isoleucine and glutamate by C. glutamicum.

Quantitative discrimination of carrier-mediated excretion of isoleucine from uptake and diffusion in Corynebacterium glutamicum

“…The isoleucine uptake system is expressed only above an internal isoleucine concentration of about 0.5 mM (1), presumably because it is advantageous for the cell to recover the small amounts of isoleucine which are constantly lost because of diffusion. This idea is known as the cyclic retention hypothesis (18,22). Under certain conditions, e.g., because of metabolic reasons (unlikely) or the availability of particular nutrients, such as growth on peptides (likely), the cytosolic isoleucine concentration exceeds a level of about 8 mM.…”

“…In contrast to uptake, amino acid excretion has not been studied extensively, although it is a common phenomenon for many, especially fermentative, bacteria (6,(17)(18)(19). Furthermore, it is of particular biotechnological importance, since the majority of amino acids are produced by microorganisms such as Escherichia coli, Serratia marcescens, and coryneform bacteria.…”

“…Furthermore, it is of particular biotechnological importance, since the majority of amino acids are produced by microorganisms such as Escherichia coli, Serratia marcescens, and coryneform bacteria. In the last few years, we have focused on the elucidation of mechanisms by which Corynebacterium glutamicum is able to excrete amino acids, e.g., lysine, isoleucine, or glutamate (18).…”

“…In general, three possible ways can be envisaged to be responsible for amino acid excretion (18). (i) Amino acids accumulated in the cytosol under particular metabolic conditions may pass through the plasma membrane by diffusion or by leaks through a physically altered membrane.…”

Functional analysis of sequences adjacent to dapE of Corynebacterium glutamicum reveals the presence of aroP, which encodes the aromatic amino acid transporter

“…As is known from the E. coli system, the C. glutamicum general aromatic amino acid uptake system functions according to a secondary mechanism. This was indicated by the fact that the addition of the uncoupler carbonyl cyanide m-chlorophenylhydrazone instantly stopped the L-tyrosine uptake under conditions under which the internal ATP level was confirmed not to be decreased significantly (8).…”