Protein Functions Plants

Proteins also comprise a significant percentage of the plant cell by weight; it has been estimated that a typical plant cell contains 5000 to 10,000 different polypeptides and millions of individual protein molecules.7 Some proteins are structurally and/or functionally related to mammalian proteins as they fulfill similar biochemical roles in the plant cell. Examples of biological functions fulfilled by proteins within plant cells are as follows:

• Structural: structural proteins maintain the integrity of plant cell walls, cytoskeleton, etc. Examples are actin microfilaments and microtubules of tubulin that form the cytoskeleton, glycoproteins in the cell wall, etc.

• Defense: plants have developed a sophisticated array of pathogenesis proteins that defend the plant against bacterial, fungal, or viral infection. Some of these proteins also are effective in protecting plants against insect feeding or infection by plant pathogens. Examples of pathogenesis-proteins include protease inhibitors, defensins, thionins, chitinases, lectins, ribosomal inactivating proteins, etc.8 A few members of these pathogenesis-proteins have the distinction of being toxic to mammals and will be discussed in later chapters.

• Motor function: although plants do not contain skeletal muscle composed of complexes of actin/myosin, they do contain myosin, kinesin, and dynein proteins that facilitate movement of chromosomes during cell division and transport of molecules through the cytoplasm and the movement of vesicles along microtubules.9

• Metabolism: as in mammals, protein enzymes catalyze a myriad of biochemical reactions in plant cells. Some of these reactions are similar to those that occur in mammalian cells, whereas others are different, such as enzymes like sucrase, desaturases, nitrogenase, cellulose synthase, etc.

Certain biochemical functions are unique to plant cells and have no correlates in mammals; these include:

• Photosynthesis: plant proteins that facilitate transfer of energy from light into plant cell metabolism.10,11 The enzyme called rubisco (ribulose 1,5-biphosphate carboxylase/oxygenase) is one of the most abundant proteins in the world, as it is present in nearly all plant cells.12 It is enzyme-involved in photosynthesis by helping to convert CO2 to sugars that are essential to plant survival. Some have considered this enzyme the most important of all enzymes since it is involved in the first step in photosynthesis, which sustains the plant life other organisms depend upon for food.9

TABLE 1.1

Molecular Weights of Various Mammalian and Plant Proteins

TABLE 1.1

Molecular Weights of Various Mammalian and Plant Proteins

Molecular Weight

Protein

source

(Daltons)16

Insulin

Mammal

6000

Lysozyme

Mammal

15,000

Albumin

Mammal

69,000

IgG immunoglobulin

Mammal

150,000

Factor VIII (coagulation)

Mammal

285,000

IgM immunoglobulin

Mammal

950,000

Plant

Zeins

Plant (maize)

10,000-58,000

Vicilin

Plant (garden pea)

186,000

Glycinin

Plant (soybean)

330,000

Rubisco

Plant

560,000

Pyruvate dehyrdrogenase

Plant

5,086,000

protein complex

• Storage proteins: provide a reserve of food (proteins) to support the germination of the seed and growth of the plant during early growth. Seed proteins have been divided into four classes based on their water solubility: albumins (barley, oats, wheat, etc.), globulins (wheat, maize, etc.), glu-telins (wheat), and prolamins (barley, wheat, maize, etc.). Storage proteins also provide essential food for humans and farm animals.11

Proteins are considered to be macromolecules since their size and molecular weight are quite large compared to other small molecules such as glucose and individual amino acids, whose molecular weight ranges from 75 to 300 Daltons. Most proteins consist of 50 to 2000 amino acids.1 The molecular weight of mammalian and plant cell proteins ranges considerably, as shown in Table 1.1.

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