Amino Acids and Proteins

LESSON 4 Amino Acids and Proteins

Proteins are molecules of great size, complexity, and diversity. They are the source of dietary amino acids, both essential and nonessential, that are used for growth, maintenance, and the general well-being of man. These macromolecules, characterized by their nitrogen contents, are involved in many vital processes intricately associated with all living matter. In mammals and many internal organs are largely composed of proteins. Mineral matter of bone is held together by collagenous protein. Skin, the protective covering of the body, often accounts for about 10% of the total body protein.

Some protein function as biocatalysts (enzymes and hormones) to regulate chemical reactions within the body. Fundamental life process, such as growth, digestion and metabolism, excretion, conversion of chemical energy into mechanical work, etc, are controlled by enzymes and hormones. Blood plasma proteins and hemoglobin regulate the osmotic pressure and PH of certain body fluids. Proteins are necessary for immunology reactions. Antibodies, modified plasma globulin proteins, defend against the invasion of foreign substances of microorganisms that can cause various diseases, food allergies result when certain ingested proteins cause an apparent modification in the defense mechanism. This leads to a variety of painful, and occasionally drastic, conditions in certain individuals.

Food shortages exist in many areas of the world, and they are likely to

become more acute and widespread as the world’s population increases. providing

adequate supplies of protein poses a much greater problem than providing

adequate supplies of either carbohydrate or fat. Proteins not only are more

costly to produce than fats or carbohydrates but the daily protein requirement

per kilogram of bodyweight remains constant throughout adult life, whereas the

requirements for fats and carbohydrates generally decrease with age.

As briefly described above, proteins have diverse biological functions, structures, and properties. Many proteins are susceptible to alteration by a number of rather subtle changes in the immediate environment. Maximum knowledge of the composition, structure, and chemical properties of the raw materials, especially proteins, is required if contemporary and future processing of foods is to best meet the needs of mankind. A considerable amount of information is already available, although much of it has been collected by biochemists using a specific food component as a model system,

Amino Acids

Amino acids are the “building blocks” of proteins. Therefore, to understand the properties of proteins, a discussion of the structures and properties o f amino acids is required. Amino acids are chemical compounds, which contain both basic amino groups and acidic carboxyl groups. Amino acids found in proteins have both the amino and carboxyl groups on the a-carbon atom; a-amino acids have the following general structure:

		NH2
		│
R	─	C	─	COOH
		│
		H

At neutral pH values in aqueous solutions both the amino and the carboxyl groups are ionized. The carboxyl group loses a proton and obtains a negative charge, while the amino group gains a proton and hence acquires a positive charge. As a consequence, amino acids possess dipolar characteristics. The dipolar, or zwitterions, form of amino acids has the following general structure:

		NH3+⁺
		│
R	─	C	─	COO^-
		│
		H

Several properties of amino acids provide evidence for this structure: they are more soluble in water than in less polar solvents; when present in crystalline form they melt or decompose at relatively high temperatures (generally above 200): and they exhibit large dipole moments and large dielectric constants in neural aqueous solutions.

The R groups or side chains, of amino acids and proteins. these side chains may be classified in to four groups.

Amino acids with polar-uncharged (hydrophilic) r groups can hydrogenbond with water and are generally soluble in aqueous solutions. The hydroxyls of serine, heroine, and tyrosine; the sulfhydryl of thinly of cysteine, and the amides of asparagines and glutamine are the functional moieties present in r groups of the class of amino acids. Two of these, the toil of cysteine and the hydroxyl of tyrosine, are slightly ionized at PG 7 and can lose a proton much more readily than others in this class. The amides of asparagines and glutamine are readily hydrolyzed by acid or base to aspartic and glutamic acids, respectively.

Amino acids with nonpolar (hydrophobic) r groups are less soluble in aqueous solvents than amino acids with polar uncharged r groups. Five amino acids with hydrocarbon side chains decrease in polarity as the length of the side chain is increased. The unique structure of praline (and its hydoxylated derivative, hydroxyproline) causes this amino acid to play a unique role in protein structure.

The amino acids with positively charged (basic) r groups at ph 6-7 are lysine; argiine has a positively charged quanidino group. At ph 7.0 10% of the imidazole groups of histidine molecules are prorogated, but more than 50% carry positive at ph 6.0.

The dicarboxylic amino acids, asparic glutamic, possess net negative charges n the neutral ph range. An important artificial meal-flavoring food additive is the monosodium salt of glutamic acid.

Peptides

When the amino group of one amino acid reacts with the carboxyl group of another amino acid, a peptide bond is formed and a molecule of water is released. This can bond joins amino acids together to form proteins

╲

╱

NH3

COO

─C─

_ _-

COO

_{__-}

╱

╲

╱

╲

╱

—C—

COO

→

H3N

H2O

╱

╲

The peptide bond is slightly shorter than otter single c-n bonds. This indicates that the peptide bond has some characteristics of a double bond, because of resonance stabilization with the carbony1 oxygen. Thus group adjacent to the peptide bond cannot rotate freely, this rigidity of the peptide bond holds

│

C2	╲		N
	╲	╱	╲
		C	Cα
		║
		O

the six atoms in a single plane. the amino (_NH_) group does not ionize between ph o and 14 due to the double-bond properties of the peptide bond. In addition, r groups on amino acid residues, because of starch hindrance, force oxygen and hydrogen of the peptide bond to exist on a trans configuration. Therefore, the backbone of peptides and proteins has free rotation in two of the three bonds between amino acids.

If a few amino acids are joined together by peptide bonds the compound is called a” most natural peptides are formed by the partial hydrolytic of proteins; however, a few peptides are important metabolites. Ansetime and carnosine are two derivatives of histamine that are found in muscles pf animals. The biochemical function of these peptides is not understood.

Glutathione occurs in mammalian blood, yeast, and especially in tissues of rapidly dividing cells. It is thought to function in oxidative metabolism and detoxification.

Duirng oxidation, two moletcules of glutathiune join vin a disulfide bridge (-S-S) between two cysteine is not found in proteins.

Other peptides functino as antibodies and hormones. Oxytocin and hormones. Oxytocin and vasopressin are examples of peptide hormones.

Protein structure

Proteins perform a wide variety of biological functions and since they are composed of hundreds of amino acids, their structures are much mere complex than those of peptides.

Enzymes are globular proteins produced in living matter for the special purpose of catalyzing vital chemical reactions that otherwise do not occur under physiological conditions. Hemoglobin and myoglobin are hemo-containing proteins that transport oxygen and carbon dioxide in the blood and muscles. The major muscle proteins, actin and myosin, convert chemical energy to mechanical work, while proteins in tendons (collagen and elastim) bind muscles to bones, skin, hairy fingernails, and toenails are pertinacious protective substance. The food scientist is concerned about proteins in foods since knowledge of protein structure and behavior allows him to more ably manipulate foods for the benefit mankind.

Nearly an infinite number of proteins could be synthesized from the 21natural occurring amino acids. However, it has been estimated that only about 2000 different proteins exist in nature. The number is greater than this if one considers the slight variations found in proteins from different species.

The linear sequence of amino acids in protein is referred toast “primary structure “. In a few proteins the primary structure has been determined and one protein (ribonuclease) has been synthesized in the laboratory. It is the unique sequence of amino acids that imparts many of the fundamental properties to different protein and tertiary structures. If the protein contains a considerable number of amino acids with hydrophobic groups, its solubility in aqueous solvents is probable less than that of proteins containing amino acids with many hydrophilic groups.

If the primary structure of the protein were not folded, protein molecules would be excessively long and thin. A protein having a molecular weight of 13,000 would be 448 a thick. This structure allows excessive interaction with other substances, and it is not found in nature The three-dimensional manner in which relatively close members of the protein chain are arranged is referred to as” secondary structure.”

examples or secondary structure are the a-helix of wool, the pleated-sheet configuration of silk, and the collagen helix.

The native structure of a protein is that structure which possesses the lowest feasible free energy. Therefore, the structure of a protein is not random but somewhat ordered. when the restrictions of the peptide bond are superimposed on a polyamino acid chain of a globular protein, a right handed coil, the ∝-helix, appears to be one of the most ordered and stable structures feasible.

the ∝-helix contains 3.6 amino acid residues per turn lof the protein backbone, with the r groups of the amino acids extending outward from the axis of the helical structure, hydrogen bonding can occur between the nitrogen of one peptide bond and the oxygen of another peptide bond four residues along the protein chain, the hydrogen bonds are nearly parallel to the axis of the helix, lending strength to the helical structure, since this arrangement allows each peptide bond to form a hydrogen bond, the stability of the structure greatly enhanced. The coil of the helix is sufficiently compact and stables that even substances with strong tendencies to participate in hydrogen bonding, such as water, cannot enter the core.

A secondary saturation found in many fibrous proteins is the β-pleated sheet configuration. In this configuration the peptide backbone forms a zigzag pattern, with the r groups of the amino acids extending alive and below the peptide chain. Since all peptide bonds are available for hydrogen bonding, this configuration allows maximum cross-linking between adjacent polypeptide chains and thus good stability. Both parallel-pleated sheet, where the polypeptide chains run in opposite directions, are possible. Where groups are bulky or have little charges, the interactions of the r groups do not allow the pleated-sheet configuration to exist. silk and insect fibers are the best examples of theβ-sheet, although feathers of birds contain a complicated form of these configuration.

Another type of secondary structure of fibrous proteins is the collagen helix. collagen is the most abundant protein in higher vertebrates, accounting for one-third of the total body protein, collagen resists stretching, is the major component of tendons, and contains one-third glycine and one-fourth proline or hydroxyprolinethe rigid r groups, and the lack of hydrogen bonding by peptide linkages involving proline and hydroxyproline, prevents formation of an ∝-helical structure and forces the collagen polypeptide chain into an odd kinked-type helix. Peptide bonds composed of glycine form interchain hydrogen bonds with two other collagen polypeptide chains, and this results in a stable triple helix. This triple-helical structure is called “tropocollagen” and it has a molecular weight of 3000,000 Daltons.

The manner in, which large portions of it protein chain are arranged is referred to as tertiary structure. This involves folding of regular unts of the secondary structure as well as the structuring of areas of the peptide chain that are devoid of secondary structure. for example, some proteins contain areas where ∝-helical structure exists and other areas where this structure cannot form. depending on the amino acid sequence, the length of the ∝-helical portions are held together by hydrogen bonds formed between r groups, by salt linkages, by hydrophobic interactions, and by covalent disulfide(-s-s-0 linkages.

The structures discussed so far have involved only a single peptide chain. The structure formed when individual (subunit) polypeptide chains interact to form a native protein molecule is referred to as “quaternary structure”. The bonding mechanisms that hold protein chains together are generally the same as those involved in tertiary structure, with the possible exception that disulfide bonds do not assist in maitaining the quaternary structures of proteins