Fundamentals of Biochemistry - A. A. Anisimov 1986

Carbohydrates
Carbohydrate formation in photosynthesis and chemosynthesis

6.5.1. Photosynthesis. Photosynthesis is a set of processes that capture solar energy in the form of chemical bonds within Organic compounds synthesized from inorganic substances.

¹ This section is presented in a highly abbreviated form because, in accordance with the curriculum of the Ministry of Higher and Secondary Specialized Education of the RSFSR, photosynthesis is to be covered in detail in lecture courses and textbooks on plant physiology.

Photosynthesis consists of two phases: the light phase (photophysical and photochemical stages) and the dark phase. During the light phase, solar energy is absorbed by chlorophyll and transferred to a "reaction center," where Chemical Reactions Involving electron transport between various carriers and coupled phosphorylation generate reducing and energy equivalents (NADPH and ATP). In addition to light, the normal functioning of light-phase reactions requires chlorophyll and Water (or another hydrogen donor). Various chlorophylls are capable of absorbing quanta of light of a specific wavelength and transitioning into an excited state. Returning to the ground state results in the release of energy, which, through a series of intermediate steps, is stored in the form of ATP and NADPH.

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Fig. 6.6. Structure of chlorophyll a:

S—IV — pyrrole rings; dashed lines indicate coordination bonds

All currently known photosynthetic organisms contain chlorophylls—green magnesium-porphyrin pigments. Over ten types of chlorophylls are known, differing in The Nature of the chemical groups attached to the pyrrole structures of the porphyrin ring, their coloration, and their distribution among living organisms. For example, all green plants contain chlorophylls a and b (Fig. 6.6), diatoms contain chlorophyll c, and red Algae contain chlorophyll d. The Cells of purple Bacteria contain bacteriochlorophylls a and b, whereas green bacteria contain bacteriochlorophylls c and d. An important property of chlorophyll molecules is their ability to interact with Proteins and with one another, forming aggregated forms with distinct absorption spectra.

Along with green pigments, METABOLISM/14.html">Chloroplasts and chromatophores contain carotenoids—yellow and orange pigments of polyisoprenoid nature. Carotenoids can be classified into several groups based on their structure: true carotenoids, hydroxyl-containing carotenoids, and carotenoids containing carbonyl groups. The main representatives in higher plants are ß-carotene and xanthophyll (Fig. 6.7). It is hypothesized that carotenoids absorb light rays not captured by chlorophyll and transfer their energy to chlorophyll molecules. Evidence suggests that carotenoids protect chlorophyll molecules from degradation during photooxidation, play a specific role in sexual reproduction during pollen germination and pollen tube growth in higher plants, and are involved in gamete maturation in algae and Fungi.

Fig. 6.7. Structure of ß-carotene

The third group of pigments is phycobilins. These are red and blue pigments (phycoerythrins, phycocyanins) found in the chromatophores of certain algae. The Chemical Structure of phycobilins is based on the same tetrapyrrole framework, but the pyrrole groups are arranged linearly. Phycobilins absorb light energy in the green and yellow Regions of the spectrum and transfer it to the chlorophyll molecule, where it is utilized in The process of photosynthesis. The presence of phycobilins in algae is an evolutionary adaptation for absorbing the sunlight wavelengths that penetrate deep into sea water. The Optical Properties of chlorophyll and its role as a photosensitizer¹ were first investigated by the great Russian physiologist K. A. Timiryazev. Later, major contributions to this problem were made by A. N. Terenin and A. A. Krasnovsky.

¹ Photosensitizers are substances that absorb light energy and transfer it to a particular colorless molecule.

The Dark Phase of photosynthesis involves the fixation and reduction of CO₂ to form CARBOHYDRATES and other End products of photosynthesis. Light is not required at this stage; instead, the reducing and energy equivalents generated during the light phase are utilized. During the dark phase, the hydrogen atoms supplied by the light reactions are used to reduce CO₂ to carbohydrates According to the overall equation of photosynthesis:

Approximately 160 kJ of Energy is stored per mole of synthesized carbohydrate.

The reduction of CO₂ begins with its attachment to the five-carbon acceptor ribulose-1,5-bisphosphate (RuBP). The resulting six-carbon compound is highly unstable. Experiments by M. Calvin using radioactive carbon dioxide (¹⁴CO₂) established that within 2 seconds of photosynthesis, the first fixed labeled compound is not a C6-compound, but 3-phosphoglyceric acid (PGA), with the label detected exclusively in the carboxyl group. The carboxylation reaction is catalyzed by ribulose bisphosphate carboxylase (RuBisCO). Next, the resulting PGA is phosphorylated by the enzyme phosphoglycerate kinase using ATP and converted into 1,3-bisphospho-D-glyceric acid, which is more reactive and more easily reduced to glyceraldehyde-3-phosphate (G3P).

The reduction reaction involves NADPH produced in the light stage and the enzyme glyceraldehyde-3-phosphate dehydrogenase. Part of the formed G3P molecules is converted into dihydroxyacetone phosphate (DHAP) through the action of the enzyme Triosephosphate isomerase. These two triose phosphates condense under The Influence of aldolase. Fructose-1,6-bisphosphate (FBP) is formed first, followed by fructose monophosphates (F6P) and glucose monophosphates (G6P), and finally sucrose and starch. For the photosynthetic process to continue, continuous regeneration of RuBP is required; therefore, 6 molecules of RuBP participate in one turn of the cycle, fixing 6 molecules of CO₂. The resulting 12 molecules of PGA, and subsequently 12 molecules of G3P following its reduction, are utilized as follows:

5G3P → 5DHAP

3G3P + 3DHAP → 3FBP → 3F6P

2G3P + 2F6P → 2 Xylulose-5P + 2 Erythrose-4P

2G3P + 2 Sedoheptulose-7P → 2 Xylulose-5P + 2 Ribose-5P

Thus, two molecules of glyceraldehyde-3-phosphate form one hexose molecule, which exits the cycle, while 10 molecules are used to regenerate six molecules of ribulose-1,5-bisphosphate, completing the cycle.

Fig. 6.8 illustrates all the Reactions of the dark phase of photosynthesis. Much of the credit for developing this pathway belongs to the American biochemist M. Calvin and his coworkers (1957), which is why it is known as The Calvin Cycle.

Fig. 6.8. Biochemical transformations of carbon in photosynthesis (the Calvin cycle):

numbers indicate the Enzymes catalyzing these conversions; 1 — ribulose-1,5-bisphosphate carboxylase, 2 — phosphoglycerate kinase, 3 — glyceraldehyde-3-phosphate dehydrogenase, 4 — triosephosphate isomerase, 5 — aldolase, 6 — phosphatase, 7 — transketolase, 3 — aldolase, 9 — phosphatase, 10 — transketolase, 11 — phosphoketopentose epimerase, 12 — ribosephosphate isomerase, 13 — phosphoribulokinase; see text for other designations.

The diagram shows three pathways leading to RuBP regeneration, ensuring a continuous supply of this compound and preventing any limitations in CO₂ fixation. The overall equation of the Calvin cycle is as follows:

6RuBP + 6СО₂ + 12NADPH + 18ATP + 12H+ → 6RuBP +

+ 12NADP⁺ + 18ADP + 18H₃РО₄+ Hexose

For a long time, it was believed that carbohydrates were the sole organic products of photosynthesis, and that all other plant metabolites were formed from carbohydrates via non-photosynthetic reactions. However, as early as the beginning of the 20th century, prominent Russian plant physiologists V. V. Sapozhnikov and F. N. Krasheninnikov presented compelling arguments for the synthesis of Amino Acids and Proteins during photosynthesis.

Experimental proof of amino acid formation during photosynthesis (using radiotracers) was obtained in the USSR by A. A. Nichiporovich and his coworkers. Today, it is well established that chloroplasts synthesize a wide range of compounds alongside carbohydrates as direct products of photosynthesis. PGA, the very first product of photosynthesis, can be converted into Serine via amination. PGA is readily transformed into phosphoenolpyruvate (PEP) and then into Pyruvate, which serves as a precursor for various organic acids; its amination yields Alanine.

Carboxylation of PEP leads to the synthesis of oxaloacetate, which can be further reduced to malate or aminated to aspartate. The interaction of PEP with erythrose-4-phosphate, another intermediate of the Calvin cycle, yields shikimic acid—a precursor for The Biosynthesis of aromatic amino acids, as well as various phenols, Tannins, Glycosides, etc.

In some plants, oxaloacetate and malate are initially formed as primary photosynthetic products via PEP carboxylation mediated by PEP carboxylase. Since these primary products contain four carbon atoms, this pathway is referred to as The C4 pathway of photosynthesis, in contrast to the Calvin cycle where the resulting PGA contains three carbon atoms (the C3 pathway). The C4 pathway, which involves two distinct Cell types and Two Types of chloroplasts, is known as the cooperative pathway. It was discovered and investigated in the 1960s by Yu. S. Karpilov (USSR), M. Hatch, and C. Slack (Australia).

6.5.2. Chemosynthesis. Heterotrophic CO₂ fixation. The renowned Russian microbiologist S. N. Winogradsky demonstrated (1885–1895) that organic matter in nature is synthesized not only via photosynthesis in green plants, but also by chlorophyll-free bacteria. These bacteria derive the energy required for organic synthesis from The oxidation of various Inorganic Compounds, such as Fe, N, S, H, Sb, and Mn. This process is termed chemosynthesis. Some chemosynthetic bacteria utilize simple organic substances—such as methane, methanol, etc.—as hydrogen Donors.

Studies on the chemistry of labeled carbon dioxide assimilation (¹⁴СО₂) by various chemosynthesizing bacteria have shown that phosphoglyceric acid is the first stable product of chemosynthesis, while The addition of CO₂ to ribulose bisphosphate—i.e., the Calvin cycle—serves as the primary mechanism of CO₂ assimilation. In many chemosynthetic bacteria, the Calvin cycle is the main, though not exclusive, pathway for organic matter production.

Thus, both photosynthesis and chemosynthesis serve as the primary sources of organic matter on Earth.

As early as 1914, A. F. Lebedev suggested that heterotrophic organisms could partially assimilate carbon not only from preformed organic compounds but also from CO₂ by fixing the latter onto certain keto acids. In recent years, this hypothesis has received experimental confirmation through the studies of A. L. Kursanov, H. Wood, S. Ochoa, and others.

However, as noted previously, a fundamental difference exists between autotrophs and heterotrophs: the former are capable of synthesizing organic matter entirely from inorganic precursors (CO2 and H₂O), whereas the latter assimilate CO₂ while utilizing preformed organic compounds, such as pyruvate.