IX. Proteins

Proteins

What Are Proteins?

The word proteins was coined by a Dutch chemist G. J. Mulder and originated from the Greek word “proteios”, meaning first or most important. Proteins are organic compounds made up of different building blocks (basic units) called amino acids joined together by peptide bonds (Figure 9.1). A dipeptide contains one peptide bond and two amino acids, whereas a tripeptide contains three amino acids and two peptide bonds. A peptide with more than ten amino acids is called a polypeptide. Proteins are essentially large polypeptides. The structure of a protein is determined first by the sequence of individual amino acids it has in the polypeptide chain. This is also called the primary structure of the protein.

Protein Functions: Proteins are vital for life and are the major structural components of animal tissues (e.g., skin, muscles, wool, feather, tendons, eggs). In addition, proteins are also involved in biochemical (e.g., enzymes), immunological (e.g., immunoglobulins), transportational (e.g., lipoproteins), and other regulatory (e.g., hormones) activities. Proteins can also provide energy when needed.
Many of the structures in animal tissue (e.g., muscle) and metabolic reactions (e.g., enzymes, hormones) are catalyzed by proteins. Therefore, protein synthesis is essential for maintaining life process. Provision of adequate dietary protein and amino acids are essential for maintaining growth, health, and productivity in food-producing animals. Intestinal microflora can synthesize proteins from nonprotein sources in ruminant animals.

Protein requirements vary with life stages and are high during phases of fast growth in young animals and during pregnancy and lactation. Like other macronutrients, proteins contain carbon, oxygen, and hydrogen. In addition, proteins also contain nitrogen and sulfur (in some amino acids). It is the nitrogen that makes proteins very unique in animal nutrition with respect to its digestibility, metabolism, and disposal within the animal body.

Classification of Proteins

Proteins can be classified based on their shape; solubility in water, salt, acid, base, or alcohol; or according to the nature of the prosthetic group.

Classification Based on Solubility and Prosthetic Group

1. 1. Globular proteins: soluble in water or dilute acids, bases, or alcohol
      1. Albumin (water soluble; present as albumen in egg white; in blood circulation, it performs various functions [e.g., as a carrier of lipids])
      2. Globulin (soluble in dilute neutral solutions; functions as part of the immune system in body defense [e.g., immunoglobulins]
   2. Fibrous proteins: insoluble in water and are resistant to digestive enzymes
      1. Keratins (e.g. wool, hair, feather, hooves, horn)
      2. Collagen (can be converted to gelatin when heated; present in bone, teeth, tendons, and soft connective tissue)
   3. Conjugated proteins: contain other nonprotein compounds in structure. Some examples follow:
      1. Lipoproteins (lipid-carrying protein)
      2. Hemoprotein (proteins with heme units)
      3. Glycoproteins (proteins with sugar)
      4. Nucleoprotein (proteins bound to nucleic acid

These proteins have limited nutritional value but are important in biochemical, structural, and other metabolic functions. For example, feather meal is high in protein (keratin) but very low in digestibility and is of limited use in animal nutrition as a feed ingredient. Amino acids in the polypeptide chain in feather meal form disulfide bonds (-S-S-), which twist the polypeptide chain into a specific coiled structure such as helix or sheet. This is called a secondary structure. These bonds account for the tough physical properties of hooves and horns and their low digestibility.

The disruption of secondary structure by heat treatment causes denaturation of the proteins (e.g., egg white coagulation during cooking). Certain antinutritional factors in feed (e.g., trypsin inhibitor in soybean meal) are proteins. Heat processing denatures trypsin inhibitor in soybean meal and can enhance digestibility.

Amino Acids: Amino acids are the building blocks of proteins. There are more than 300 different amino acids known to exist in nature. Out of these, about 20 amino acids are important constituents of animal proteins and are associated with muscles, connective tissues, skin, feathers, horns, blood, enzymes, and hormones. There about 10 amino acids that should be present in the diet of animals because animal tissues cannot synthesize them or cannot make the adequate amount needed for metabolic functions; these are called essential amino acids. A few other amino acids such as citrulline and ornithine do not occur in animal tissues but are involved in cellular metabolic functions.

All amino acids by definition contain at least one amino group (-NH2) and one carboxyl group (–COOH) on the C atom adjacent to the carboxyl group (Figure 9.2). An exception to this is proline (imino acid), which is lacking a free amino group. The general structure of an amino acid is shown below by the amino acid glycine, the simplest of the amino acids (Figure 9.2). The R group (shown in the red circle) in amino acids varies for different amino acids. The R group is the remainder of the molecule or any other group attached to the C atom. In the case of glycine, it is an H group. The amino group (NH2) provides basic properties to the amino acid, and the carboxyl (COOH) group provides acidic properties. Amino acids important in animal nutrition are alpha (α) amino acids, which are carboxylic acids with an amino group on the α-carbon (or the first carbon attached to a functional group). A list of amino acids important in animal nutrition, their essentiality and classification are shown in Table 9.1.

Amino acids can exist in two isomeric forms, the D- and L-isomers. The D- and L-amino acids differ in their configuration of groups around the asymmetric α-carbon. Only L-amino acids are used in protein synthesis, except methionine, where both D- and  L-amino acids can be used by the animal. DL methionine is commonly used as an amino acid supplement in animal feeds.

 All amino acids except glycine contain an asymmetric α-carbon (with four different chemical groups attached to it). Compounds with asymmetric carbons can exist as isomers.

Essential Amino Acids: Animal body can synthesize some amino acids in sufficient amounts. However, animals cannot synthesize some amino acids, or not in the amount that is needed for body requirements. Such amino acids need to be provided through diet in monogastric animals; these are called essential (indispensable) amino acids. Pigs, dogs, and humans need a total of 10, and chickens and cats need a total of 11 essential amino acids. A list of essential amino acids needed by monogastric animals is shown below.
It should be borne in mind that other nonessential amino acids are also physiologically important for metabolic functions in the body and are made from other precursors available through diet (e.g., carbohydrates, nonprotein nitrogenous substances).
The need for essential amino acids varies in animals. For example, horses need essential amino acids, whereas ruminant animals (e.g., cattle, sheep, goats) generally do not have a requirement of essential amino acids as they are synthesized by rumen microbes.