Neilands Microbial Iron Metabolism

A Comprehensive Treatise
1. Auflage 2014
ISBN: 978-1-4832-7481-2
Verlag: Elsevier Science & Techn.
Format: EPUB
Kopierschutz: 6 - ePub Watermark

E-Book, Englisch, 616 Seiten

Microbial Iron Metabolism: A Comprehensive Treatise provides a comprehensive treatment of microbial iron metabolism. It aims to contribute to an increased understanding of the path of iron in microbial species and, eventually, in the plant and animal. The book is organized into five parts. Part I describes some features of iron and its function in the microbial world. These include a historical sketch of the recognition of the importance of iron in cellular physiology; a description of certain physical properties of ferrous and ferric ions; and a list of various known biocoordination derivatives grouped by ligand atom. Metabolism under iron-limited conditions is also examined. Part II presents studies on iron transport, biosynthesis, and storage in microorganisms. Part III examines iron enzymes and proteins, including ferredoxin, rubredoxin, nitrogenase, and hydrogenase. Part IV deals with reactions of inorganic substrates. Part V presents a study on the role of bacterial iron metabolism in infection and immunity.

Autoren/Hrsg.

Neilands, J. B.

Leseproben

Chapter 2

METABOLISM IN IRON-LIMITED GROWTH

P. ANN LIGHT and ROGER A. CLEGG

Publisher Summary

This chapter provides an overview of metabolism in iron-limited growth. The primary aim of many experimental investigations into the metabolism of microorganisms, whose growth is limited by iron availability, is to learn something of the role played by this trace metal in the cell under more normal conditions. All possible effects of iron deficiency have been described in various organisms on those enzymes containing either heme or nonheme iron prosthetic groups. Thus, it may result in a diminished or lack of synthesis of an enzyme, accompanied, in some instances, by the synthesis of a new enzyme to perform the function of the lost iron protein. Alternatively, only the production of the iron prosthetic group may be inhibited by iron deficiency, with the synthesis of the apoprotein continuing unchecked. The mechanisms of the effects of iron deficiency on respiratory iron proteins have been partially elucidated in the case of the heme proteins but remain entirely obscure for the nonheme iron proteins. However, the enzymes responsible for the assembly of the iron–sulfur prosthetic group and its incorporation into the apoprotein have not been identified. It is by no means clear that these processes, , are enzyme-mediated, as they can occur spontaneously .

I Introduction

II Iron-Limited Growth

A Growth of Cells

B The Batch Culture

C Continuous Culture

III Continuous Culture

IV Effects of Iron Deficiency and Iron-Limitation

A Effects on Enzyme Activities

B Effects on Morphology

C Effects on Growth Rate

D Effects on Growth Efficiency

V Summary

References

I INTRODUCTION

Iron has long been recognized as an essential constituent of defined media for the growth of fungi (Bortels, 1927; Elvehjem, 1931) and bacteria (Waring and Werkman, 1943). Without exception, iron-deficient batch cultures yield a decreased quantity of cellular material (Joslyn, 1941; Waring and Werkman, 1943; Olson and Johnson, 1949) and a decreased concentration within the iron-deficient cells of enzymes having iron porphyrin prosthetic groups (Elvehjem, 1931; Yoshikawa, 1937; Waring and Werkman, 1943; Healy 1955; Padmanaban and Sarma, 1965). The concentration of iron-binding pigments (Neilands, 1966), if secreted by the organism, may increase under iron-deficient culture conditions and porphyrin metabolism may become abnormal (Lascelles, 1962). In addition to these effects, which bear an obvious relationship to iron uptake and utilization, iron deficiency has been observed to induce changes in the activities of certain enzymes whose regulation by iron serves a function which is not readily obvious to the investigator. Two examples of such regulation are the 24-fold stimulation of NADase activity in caused by iron deficiency (Healy 1955) and the influence of the concentration of iron in the growth medium on the excretion of riboflavin by some yeasts of the genus (Tanner 1945).

In some instances, cultivation of a microorganism under conditions in which iron can be unequivocally defined to be the growth-limiting nutrient (see this chapter, Section II) produces effects similar to those that result from its batchwise cultivation in iron-deficient media. An example of this is afforded by similar effects on mitochondrial function are induced during its iron-limited continuous culture (Light and Garland, 1971) and by batch culture in iron-deficient media (Ohnishi 1969; Ohnishi and Schleyer, 1969). More commonly however, only data relating to the effects of iron deficiency in batch cultures are available, consequently, many of the investigations to which reference is made in this chapter concerned with metabolism in iron-limited growth, were performed using iron-deficient batch cultures in which iron was not demonstrated to have been, and may indeed not have been, growth limiting.

The chemistry of iron and its complexes (see Chapter 1) ideally suits it to performing a catalytic function in electron transfer reactions (Williams, 1968). Our interest in metabolism under iron-limited conditions arises from our use of these conditions in an attempt to clarify the role of nonheme iron proteins in respiratory chain oxidation and energy conservation in The results presented and cited in this chapter are, therefore, not comprehensive in the field of metabolism but are a selection reflecting the bias of the authors.

II IRON-LIMITED GROWTH

A Growth of Cells

Most unicellular organisms which are amenable to life in the laboratory are able to grow in submerged liquid culture and increase in number either by binary fission, as with the Eubacteriales, or by a modification of this process, such as budding or the formation of simple unicellular branches. The yeasts, which are the chief subject of interest in this chapter, reproduce themselves by budding.

All microorganisms growing in submerged culture absorb their nutrient requirements from the surrounding medium. Thus, for growth to occur satisfactorily, the medium must contain all necessary chemical elements in a form suitable for utilization by the organism; the hydrogen ion concentration, temperature, oxygen tension, and all other environmental factors must, of course, also be favorable for growth of the organism.

The rate at which growth actually occurs may be defined in a number of ways, the most common of which is called the specific growth rate. This is defined as the rate at which unit mass of cell increases in unit time:

=(1/x)(dx/dt)=loge2/doublingtime 1

1

where is the mass of cell material and is time. Doubling time is defined as the length of time taken for the cell mass of the culture to double.

Growth rate is difficult to determine by simple observation of individual growing cells. Very easy to assess, however, is the time taken for consecutive divisions, and, therefore, rates of increase in size of culture are usually expressed in terms of cell division times, generation times, and doubling times, which although not necessarily identical in the strictest sense, are dependent upon, represent, and can be determined by observation of the time required for one cell to become two.

Monod (1942) showed that the Michaelis-Menten equation for enzyme-catalyzed reactions applied well to microbial growth rate studies, and his modified equation

=µmax[s/(Ks+s)] 2

2

is now commonly assumed as the starting point of discussions on microbial growth rate (Fig. 1), where µ is the exponential growth rate, µmax is the maximum possible growth rate for that organism under those conditions, is the concentration of growth-limiting substrate in the culture, and is a saturation constant numerically equal to the concentration of substrate which must be present in the culture to allow growth to proceed at one-half the maximum rate possible (i.e., 1/2 µmax).

Fig. 1 Relationship between exponential growth rate (µ) and concentration of growth-limiting substrate (). , saturation constant; µmax, maximum growth rate.

Monod (1942) also showed that the conversion efficiency of substrate to cell material in any given system was constant, or approximately so:

3

3

Substrate utilized × Y = cell material produced (3) where Y is the yield constant. From Eqs. (2) and (3) it was thus possible to predict the course of growth and product weight of a culture of known composition.

B The batch culture

In the laboratory the most usual way to grow microorganisms is by the batch culture method. A single organism (or, more commonly, a small number of organisms) is introduced into a volume of sterile medium. After an initial lag phase, during which the cells are adjusting to their new environment and cell division proceeds at a rate less than maximal, growth rate of individual cells increases and at the same time becomes more uniform as between individual cells in the culture, until the whole population is dividing with remarkable regularity. Cell numbers and culture weight increase exponentially at this time, and hence this state of the culture is known as the exponential or logarithmic phase. However, it can be seen from Eq. (2) that, since nutrient is being consumed, the growth...