Proteins are found in all cells and carry out a variety of important cellular functions. Within any one cell there may be thousands of different proteins having a variety of sizes, structures, and functions. Proteins are also important structural components of the cell wall.
Proteins are the most complex and abundant of the macro molecules. Within cells, many proteins function as enzymes in the catalysis of metabolic reactions, while others serve as transport molecules, storage proteins, electron carriers, and structural components of the cell.
They are especially important in seeds, where they make up as much as 40 percent of the seed’s weight and serve to store amino acids for the developing embryo.
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Proteins are also important structural components of the cell wall. Because proteins and their building blocks, amino acids, form such a large component of plant life, plants serve as an important dietary source of the eight to ten essential amino acids for humans and other animals.
Amino Acids
Amino acids are the molecular building blocks of proteins. Amino acids all share a structure, with a central carbon atom, the alpha carbon, covalently bonded to a hydrogen atom, an amino group, a carboxylic acid group, and a group designated as an R group, which varies in structure from amino acid to amino acid.
It is the diverse nature of the R group that provides the protein with many of its structural and functional characteristics. Some R groups are either polar or electrically charged at physiological pH, making the R groups hydrophilic (water-loving). Other R groups are nonpolar and hydrophobic (water-avoiding).
The twenty standard amino acids the cell uses to synthesize its proteins are alanine, arginine, aspartate (aspartic acid), asparagine, cysteine, glutamate (glutamic acid), glutamine, glycine, histidine, isoleucine, leucine, lysine, methionine, phenylalanine, proline, serine, threonine, tryptophan, tyrosine, and valine.
Each of the twenty amino acids differs from the other nineteen in the structure of its R group. Once incorporated into a protein, a standard amino acid may undergo modification to create nonstandard amino acids and an even greater diversity of protein structures.
One of the more common nonstandard amino acids found in proteins is hydroxyproline, which is commonly found in plant cell-wall proteins. In addition to the twenty amino acids that build proteins, many non standard amino acids occur free in the cell and are not found in proteins. Canavanine, for example, occurs in the seeds of many legumes.
Based on the information in cellular deoxyribonucleic acid (DNA), the cell joins the twenty standard amino acids by peptide bonds in specific sequences, resulting in chains ranging from as few as two amino acids to many thousands. Shorter chains of amino acids are referred to as peptides or oligopeptides, while longer chains are referred to as polypeptides.
The term“protein” is usually reserved for those oligopeptides and polypeptides that have biological functions, because single polypeptides often do not have biological functions unless associated with other polypeptides.
Primary Structure
Proteins differ from one another in the sequences of their amino acids. The sequence of amino acids of a protein is called its primary structure. Mutations have been shown to result in the change of as few as one amino acid in a protein. Because DNA specifies a protein’s primary structure, protein sequence information is often used to study the evolutionary relationships among organisms.
Proteins are often complexed with other compounds in their biologically active state. These proteins are called conjugated proteins. Proteins complexed with metals, lipids, sugars, and riboflavin are called metalloproteins, lipoproteins, glycoproteins, and flavoproteins, respectively.
Glycoproteins (literally, “sugar proteins”) are important constituents of the plasma membrane. These sugar molecules can occur singly or in short, simple branched chains.
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| Amoni acids to proteins |
A protein chain may be folded into a variety of three-dimensional shapes. The three-dimensional shape a protein assumes is called its conformation and is determined by its amino acid sequence.
In order for a protein to be active, it must assume a certain conformation. Any alteration in its conformation may result in reduced activity. Denaturing agents alter the structure of a protein so that it loses its conformation, biological function, and activity.
Secondary Structure
The secondary structure refers to the local folding or conformation of the polypeptide chain over relatively short (fifty amino acids or so) stretches.
Two common secondary structures, the alpha helix and the beta sheet, occur regularly in proteins. On average, only about half of the polypeptide chain assumes the alpha or beta conformation,while the remainder exists in turns and random structures.
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Some proteins show only alpha structure, others only the beta structure, while still others show either a mixture of the two structures or neither secondary structure. Both the alpha and the beta structures increase the structural stability of the protein. The amino acid sequence determines whether a particular sequence of amino acids in a protein will assume the alpha or beta structure.
Tertiary Structure
The overall spatial orientation of the entire polypeptide chain in space is referred to as its tertiary structure. Generally, two tertiary structures are recognized. Fibrous or filamentous proteins are arranged as fibers or sheets, while globular proteins are arranged roughly as spherical or globular structures.
The amino acid sequence determines the overall folding of the protein tertiary structure. Fibrous proteins are primarily involved with structural functions, whereas globular proteins function as enzymes, transport molecules, electron carriers, and regulatory proteins.
Quaternary Structure
Some proteins are composed of more than one polypeptide chain. A protein composed of only one polypeptide is called a monomer,while proteins composed of two, three, four, and so on are referred to as dimers, trimers, and tetramers, and so on, respectively.
