Acidity in biology: Special reference to alkaliphilic microorganisms
Timo Korpela
https://urn.fi/URN:NBN:fi-fe2021042822424
Tiivistelmä
Water has simple
chemical formula, H2O, but it is far from simple substance. Water has physical and
chemical features which make life on the earth possible. Acidity, i.e. concentration of protons is an
important property typical to water solutions. Almost every chemical reaction
depends on protons, either directly or indirectly. The concept of acidity,
expressed by term pH, and some properties
of water will be briefly discussed.
pH in higher animals
and plants inside their cells is extremely strictly regulated. pH in human blood is 7.4.Deviation of over +/-
0.05 can be pathological. Human stomach can have pH 1-2 but after stomach pH it is increased in
small intestine to about 8. The purpose of these sharp changes is to kill microbes
in food. Most microbes die in pH 1-2 whereas those which do not die, probably
die at pH 8. This is important because enzymes produce rich nutrient medium into
the intestine and microbes could propagate uncontrollably.
The biggest
biodiversity centres around neutral pH indicating that it is favourable. When
pH of the surrounding deviates from neutral, the number of life forms decrease rather
sharply.
Our laboratory has
studied since 1980´s a soil bacterium Bacillus
alcalophilus. It grows optimally at pH around 10. A principal question is how such “alkaliphiles”
have “adapted” to high pH, i.e. to proton concentration about 1/1000 of that of neutral. Microbial adaptation could be separated two things:
microbes may secrete extracellular enzymes and other metabolites to their growth
medium which must be biologically active at pH 10 or more. Second, the microbes must absorb nutrients from surrounding, like
amino acids, sugars, vitamins etc. into cells.
Prof. Horikoshi´s
(Japan) research group has been pioneering the “alkaliphilic microbiology”
during >50 years. They and other groups have shown that pH inside alkaliphiles´
cells is lower than outside. Microbes have
proton pumps from out to in creating higher proton concentration inside the
cell. It follows that the cell wall must be specific to keep the extremely
volatile protons not leaking out of the cells. Proton pumps are certainly universal mechanism
to stay alive at high pH conditions in all organisms. Many evidences indicate
that pH inside alkaliphiles are not much over pH 9.5. But, this means proton concentration is still
several hundred times less than in neutral.
Second question is how
metabolism is working in low proton concentration. May be metabolism inside
cells is slower with the alkaliphiles? May be enzymes of alkaliphiles afford to
have lower specific activity because considerably less microbial species compete
for the same resources. Intracellular enzymes
are good objects for studying and comparing the metabolism. We purified to
homogeneity and characterized two pyridoxal-phosphate -dependent
aminotransferase enzymes from one
obligate and one facultative alkaliphiles and from Escherichia coli (neutralophile). Alkaliphilic enzymes had pH optima
of 9.5 (possibly reflecting cellular pH) but the optima were narrow compared to
E, coli. Surprisingly the specific
activities of all the three enzymes were practically the same when measured at their
pH optima. We consider that the substrate amino acid brings, on the average, one
proton to the active site with all of the three studied enzymes. In alkaline,
the active site promotes intake of protons but in neutral extra protons are
rejected.
Except in alkaline
soda lakes and related habitats, alkaliphiles are frequently found also in
neutral soils. It is considered that “microenvironment” in soil can create favourable
conditions to them. Our other studies indicate that alkaliphiles have special
role in nitrogen metabolism in soils. Alkaliphiles can rapidly detoxify, for
example, toxic nitrites. Alkaliphiles also produce various unique metabolites
into their surroundings and certainly have a role in soil ecology. Related discussion
could be made also for the acidophilic microbes.
Plants have optimal
growth at pH range of 4-8. The optimum pH depends on the plant species. pH
affects solubility of key nutrients and minerals in soil and pH is again rather
complex factor. Soil pH and mineral analyses belong also to the tools of the
modern sustainable agriculture.
Life of all higher
organisms and microbes are tightly interconnected with plants as well-known
with microbiome of humans and animals. Understanding these subtle relationships
with microbes involves the most modern science. Sustainable agriculture is therefore
knowledge-based responsible human behaviour. It has not been possible
previously because of lack of understanding but is currently emerging because
of breakthroughs in many branches of the sciences.
Kokoelmat
- Rinnakkaistallenteet [19207]