Some proteins combine with other kinds of molecules such as carbohydrates, lipids, iron and other metals, or nucleic acids, to form glycoproteins, lipoproteins, hemoproteins, metalloproteins, and nucleoproteins respectively. The presence of these other biomolecules affects the protein properties. For example, a protein that is conjugated to carbohydrate, called a glycoprotein, would be more hydrophilic in character while a protein conjugated to a lipid would be more hydrophobic in character.
Protein Properties and Separation
Proteins are typically characterized by their size (molecular weight) and shape, amino acid composition and sequence, isolelectric point (pI), hydrophobicity, and biological affinity. Differences in these properties can be used as the basis for separation methods in a purification strategy (Chapter 4). The chemical composition of the unique R groups is responsible for the important characteristics of amino acids, chemical reactivity, ionic charge and relative hydrophobicity. Therefore protein properties relate back to number and type of amino acids that make up the protein.
Proteins are typically characterized by their size (molecular weight) and shape, amino acid composition and sequence, isolelectric point (pI), hydrophobicity, and biological affinity. Differences in these properties can be used as the basis for separation methods in a purification strategy (Chapter 4). The chemical composition of the unique R groups is responsible for the important characteristics of amino acids, chemical reactivity, ionic charge and relative hydrophobicity. Therefore protein properties relate back to number and type of amino acids that make up the protein.
Size:
Size of proteins is usually measured in molecular weight (mass) although occasionally the length or diameter of a protein is given in Angstroms. The molecular weight of a protein is the mass of one mole of protein, usually measured in units called daltons. One dalton is the atomic mass of one proton or neutron. The molecular weight can be estimated by a number of different methods including electrophoresis, gel filtration, and more recently by mass spectrometry. The molecular weight of proteins varies over a wide range. For example, insulin is 5,700 daltons while snail hemocyanin is 6,700,000 daltons. The average molecular weight of a protein is between 40,000 to 50,000 daltons. Molecular weights are commonly reported in kilodaltons or (kD), a unit of mass equal to 1000 daltons. Most proteins have a mass between 10 and 100 kD. A small protein consists of about 50 amino acids while larger proteins may contain 3,000 amino acids or more. One of the larger amino acid chains is myosin, found in muscles, which has 1,750 amino acids.
Separation methods that are based on size and shape include gel filtration chromatography (size exclusion chromatography) and polyacrylamide gel electrophoresis.
Amino Acid Composition and Sequence
The amino acid composition is the percentage of the constituent amino acids in a particular protein while the sequence is the order in which the amino acids are arranged.
Size of proteins is usually measured in molecular weight (mass) although occasionally the length or diameter of a protein is given in Angstroms. The molecular weight of a protein is the mass of one mole of protein, usually measured in units called daltons. One dalton is the atomic mass of one proton or neutron. The molecular weight can be estimated by a number of different methods including electrophoresis, gel filtration, and more recently by mass spectrometry. The molecular weight of proteins varies over a wide range. For example, insulin is 5,700 daltons while snail hemocyanin is 6,700,000 daltons. The average molecular weight of a protein is between 40,000 to 50,000 daltons. Molecular weights are commonly reported in kilodaltons or (kD), a unit of mass equal to 1000 daltons. Most proteins have a mass between 10 and 100 kD. A small protein consists of about 50 amino acids while larger proteins may contain 3,000 amino acids or more. One of the larger amino acid chains is myosin, found in muscles, which has 1,750 amino acids.
Separation methods that are based on size and shape include gel filtration chromatography (size exclusion chromatography) and polyacrylamide gel electrophoresis.
Amino Acid Composition and Sequence
The amino acid composition is the percentage of the constituent amino acids in a particular protein while the sequence is the order in which the amino acids are arranged.
Charge:
Each protein has an amino group at one end and a carboxyl group at the other end as well as numerous amino acid side chains, some of which are charged. Therefore each protein carries a net charge. The net protein charge is strongly influenced by the pH of the solution. To explain this phenomenon, consider the hypothetical protein in Figure 2.5. At pH 6.8, this protein has an equal number of positive and negative charges and so there is no net charge on the protein. As the pH drops, more H+ ions are available in the solution. These hydrogen ions bind to negative sites on the amino acids. Therefore, as the pH drops, the protein as a whole becomes positively charged. Conversely, at a basic pH, the protein becomes negatively charged. pH 6.8 is called the pI, or isoelectric point, for this protein; that is, the pH at which there are an equal number of positive and negative charges. Different proteins have different numbers of each of the amino acid side chains and therefore have different isoelectric points. So, in a buffer solution at a particular pH, some proteins will be positively charged, some proteins will be negatively charged and some will have no charge.
Each protein has an amino group at one end and a carboxyl group at the other end as well as numerous amino acid side chains, some of which are charged. Therefore each protein carries a net charge. The net protein charge is strongly influenced by the pH of the solution. To explain this phenomenon, consider the hypothetical protein in Figure 2.5. At pH 6.8, this protein has an equal number of positive and negative charges and so there is no net charge on the protein. As the pH drops, more H+ ions are available in the solution. These hydrogen ions bind to negative sites on the amino acids. Therefore, as the pH drops, the protein as a whole becomes positively charged. Conversely, at a basic pH, the protein becomes negatively charged. pH 6.8 is called the pI, or isoelectric point, for this protein; that is, the pH at which there are an equal number of positive and negative charges. Different proteins have different numbers of each of the amino acid side chains and therefore have different isoelectric points. So, in a buffer solution at a particular pH, some proteins will be positively charged, some proteins will be negatively charged and some will have no charge.
Separation techniques that are based on charge include ion exchange chromatography, isoelectric focusing and chromatofocusing.
Figure 2.5. The pI is the pH at which there is no net charge on the protein. At lower pH readings, there are more positive charges in the environment and therefore, the protein has an increased cationic character. The reverse is true at pH readings above the pI.
Hydrophobicity:
Literally, hydrophobic means fear of water. In aqueous solutions, proteins tend to fold so that areas of the protein with hydrophobic regions are located in internal surfaces next to each other and away from the polar water molecules of the solution. Polar groups on the amino acid are called hydrophilic (water loving) because they will form hydrogen bonds with water molecules. The number, type and distribution of nonpolar amino acid residues within the protein determines its hydrophobic character. (Chart of hydrophobicity or hydropathy)
A separation method that is based on the hydrophobic character of proteins is hydrophobic interaction chromatography.
Literally, hydrophobic means fear of water. In aqueous solutions, proteins tend to fold so that areas of the protein with hydrophobic regions are located in internal surfaces next to each other and away from the polar water molecules of the solution. Polar groups on the amino acid are called hydrophilic (water loving) because they will form hydrogen bonds with water molecules. The number, type and distribution of nonpolar amino acid residues within the protein determines its hydrophobic character. (Chart of hydrophobicity or hydropathy)
A separation method that is based on the hydrophobic character of proteins is hydrophobic interaction chromatography.
Solubility:
As the name implies, solubility is the amount of a solute that can be dissolved in a solvent. The 3-D structure of a protein affects its solubility properties. Cytoplasmic proteins have mostly hydrophilic (polar) amino acids on their surface and are therefore water soluble, with more hydrophobic groups located on the interior of the protein, sheltered from the aqueous environment. In contrast, proteins that reside in the lipid environment of the cell membrane have mostly hydrophobic amino acids (non polar) on their exterior surface and are not readily soluble in aqueous solutions.
Each protein has a distinct and characteristic solubility in a defined environment and any changes to those conditions (buffer or solvent type, pH, ionic strength, temperature, etc.) can cause proteins to lose the property of solubility and precipitate out of solution. The environment can be manipulated to bring about a separation of proteins- for example, the ionic strength of the solution can be increased or decreased, which will change the solubility of some proteins.
As the name implies, solubility is the amount of a solute that can be dissolved in a solvent. The 3-D structure of a protein affects its solubility properties. Cytoplasmic proteins have mostly hydrophilic (polar) amino acids on their surface and are therefore water soluble, with more hydrophobic groups located on the interior of the protein, sheltered from the aqueous environment. In contrast, proteins that reside in the lipid environment of the cell membrane have mostly hydrophobic amino acids (non polar) on their exterior surface and are not readily soluble in aqueous solutions.
Each protein has a distinct and characteristic solubility in a defined environment and any changes to those conditions (buffer or solvent type, pH, ionic strength, temperature, etc.) can cause proteins to lose the property of solubility and precipitate out of solution. The environment can be manipulated to bring about a separation of proteins- for example, the ionic strength of the solution can be increased or decreased, which will change the solubility of some proteins.
Figure 2.6: Ionic Strength and Protein Folding. This figure shows the effect of ion concentration on protein folding.
Biological Affinity (Function):
Proteins often interact with other molecules in vivo in a specific way- in other words, they have a biological affinity for that molecule. These molecular counterparts, termed ligands, can be used as “bait” to “fish” out the target protein that you want to purify. For example, one such molecular pair is insulin and the insulin receptor. If you want to purify (or catch) the insulin receptor, you could couple many insulin molecules to a solid support and then run an extract (containing the receptor) over that column. The receptor would be “caught” by the insulin bait. These specific interactions are often exploited in protein purification procedures. Affinity chromatography is a very common method for purifying recombinant proteins (proteins produced by genetic engineering). Several histidine residues can be engineered at the end of a polypeptide chain. Since repeated histidines have an affinity for metals, a column of the metal can be used as bait to “catch” the recombinant protein.
Proteins often interact with other molecules in vivo in a specific way- in other words, they have a biological affinity for that molecule. These molecular counterparts, termed ligands, can be used as “bait” to “fish” out the target protein that you want to purify. For example, one such molecular pair is insulin and the insulin receptor. If you want to purify (or catch) the insulin receptor, you could couple many insulin molecules to a solid support and then run an extract (containing the receptor) over that column. The receptor would be “caught” by the insulin bait. These specific interactions are often exploited in protein purification procedures. Affinity chromatography is a very common method for purifying recombinant proteins (proteins produced by genetic engineering). Several histidine residues can be engineered at the end of a polypeptide chain. Since repeated histidines have an affinity for metals, a column of the metal can be used as bait to “catch” the recombinant protein.
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