Proteins

explain the structure and classification of proteins and amino acids

A protein is an organic compound containing C, O, N, H, and sometimes S and P. The primary structure of a protein is its sequence of amino acids which are linked together by peptide bonds. There are 20 amino acids making proteins that are naturally occurring. An amino acid has a carbon center (which is also called α-carbon) with a hydrogen (H), an amino group (-NH2), a carboxyl group (-COOH), and a side chain (which is also called a R-group). A side chain is unique for each amino acid. Amino acids can form a peptide when they are linked by peptide bonds between a carbon of a carboxyl group of an amino acid and a nitrogen of an amino group of the subsequent amino acid. Depending on the number of amino acids, it can be called dipeptide, tripeptide, oligopeptide, or polypeptide. An oligo- or a polypeptide can form a primary, secondary, tertiary, or quaternary structure of a protein amino acids can be classified as essential, conditionally essential, and non-essential amino acids based on their availability and needs in an animal body.

Aliphatic amino acid → Valine, Aromatic amino acid → Tryptophan

Match amino acids to a correct category Aliphatic amino acid Answer 1 Correct Aromatic amino acid Answer 2 Correct

proteins

Monogastric (or simple stomached) animals need to receive proteins from dietary sources and digest them into amino acids before they can be used in an animal body. Ruminant animals, however, can receive proteins from both dietary sources and microbes in the rumen. Unlike lipids, proteins cannot be stored in an animal body but rather are continuously synthesized and degraded. Excessive dietary protein would, therefore, cause increased amino acid catabolism in an animal body. In the mean time, insufficient protein intake would cause impaired growth and function of the body.

systems

Most of amino acids will be absorbed by carrier-mediated transporters called 'Systems'.

Rumen-undegradable protein (RUP)

digested by the microorganisms. It enters and leaves the rumen relatively unchanged.

Rumen-degradable protein (RDP)

digested by the microorganisms. The microorganisms produce a variety of proteolytic enzymes that hydrolyze peptide bonds in the protein forming smaller peptides. The peptides are hydrolyzed by peptidase enzymes producing amino acids. The amino acids can be used by the microorganisms to form their own proteins.

gluconeogenesis

emaining carbon skeleton can also be used for synthesis of glucose

nonprotein nitrogen

entering the rumen is hydrolyzed, producing ammonia. Ammonia is another source of nitrogen that the microorganisms use to synthesize amino acids which can be incorporated into proteins.

conditionally essential amino acids

essential depending on species, age, or physiological status of animals. Where they are needed by certain species, these are species that lack the metabolic pathways to synthesize the amino acid. In the cases where they are needed at certain ages or in specific physiological states, it is because the animal cannot synthesize enough to meet their requirements at these times. Examples of these conditions are shown below. Conditionally essential amino acids include: arginine (Arg) for pregnant animals, neonates (newly born animals), feline, poultry, and fish; glutamine (Gln) for sick animals; glycine (Gly) and proline (Pro) for poultry; taurine (Tau) for feline. Taurine does not become a part of protein but is essential for feline because they cannot synthesize Tau.

Acidic amino acids

include Asp and Glu as their side chains include additional carboxyl groups (COO-)

Aliphatic amino acids

include Gly, Ala, Val, Leu, and Ile as their side chains have lipid-like characteristics.

Basic amino acids

include His, Lys, and Arg as their side chains include additional amino groups (NH3+)

Sulfur-containing amino acids

include Met and Cys as their side chains include S.

Aromatic amino acids

include Phe, Tyr, and Trp and their side chains include benzene rings.

enzyme

proteins functioning as catalysts in metabolic reactions

ketogenesis

remaining carbon skeleton can also be used for synthesis of ketone bodies (ketogenesis)

role of protein and amino acids

roteins have various functions in an animal body. Common functions of proteins include their roles in growth and maintenance including cell proliferation and wound repair. Proteins with functions in forming part of the structure of an animal body are collagen (skin, cartilage, and bone), elastin (connective tissue and arteries), actin and myosin (muscle), and keratin (hair, nail, wool, and fur). Among these, collagen is the most abundant protein (at least 20%) in an animal body. Collagens are mainly composed of Gly (about 30%) as well as Pro, OH-Pro (hydroxyproline), and OH-Lys (hydroxylysine)

metabolism of amino acids in an animal body

After absorption, amino acids are transported in a free form in the blood circulation. Amino acids are transported to target tissues mainly for protein synthesis. The amino acids are also used for energy supply and other biological functions. Once amino acids are delivered to target tissues, they enter cells via Systems existing on the cell membrane. After transport, amino acids can be used for synthesis of proteins needed for maintenance and production. Extra amino acids remaining after protein synthesis are catabolized (or oxidized). The first step of amino acid catabolism is the removal of the amino group. In mammals, the removed amino group will be transported to liver to build urea, which is secreted as a part of urine in the kidney. The remaining carbon skeleton can be completely oxidized to provide energy. The remaining carbon skeleton can also be used for synthesis of glucose (gluconeogenesis), ketone bodies (ketogenesis), or lipid (lipogenesis). The glucose, ketone bodies, or fatty acids that are formed can all be used as energy sources. Synthesis of glucose, ketone bodies, or fatty acids will depend on the type of amino acid and the metabolic situation in the animal. For example, glucose is synthesized to maintain glucose homeostasis. Fatty acids are synthesized to store energy when there is more energy available then the animal needs at the moment.

peptide bond

Amino acids can form a peptide when they are linked by peptide bonds between a carbon of a carboxyl group of an amino acid and a nitrogen of an amino group of the subsequent amino acid.

urea cycle

Amino groups from amino acid catabolism will be converted to ammonia (NH4+), which will be used by urea cycle to generate urea mostly in the liver (and some in the kidney). Ornithine becomes citrulline by taking a mole of ammonia. This reaction is catalyzed by an enzyme called ornithine transcarbamylase (OTC). Citrulline also takes a mole of ammonia, becoming argininosuccinate catalyzed by argininosuccinate synthetase. Argininosuccinate becomes arginine by losing fumarate catalyzed by argininosuccinate lyase. Finally, arginine releases a mole of urea and is converted to ornithine catalyzed by arginase. This will be repeated to generate urea and this cycle is called the urea cycle (Figure 5). Synthesized urea in the liver is dissolved in the blood and delivered to the kidney where urea is excreted as a part of urine. In avian species (like chickens), ammonia is converted to uric acid instead of urea as a form of nitrogen excretion from the body.

digestible amino acids

Animal feeds contain proteins providing essential and nonessential amino acids. When there are essential amino acids that are deficient in the diet (not meeting the requirements for an animal), these amino acids become limiting factors for the growth of the animal and thus are called limiting amino acids. When a feed is formulated using cereal grains and legume seed meals, typically Lys is the first limiting amino acid, followed by Thr and Trp as the second and third limiting amino acids, respectively. In the case of poultry, typically Met is the first limiting amino acid followed by Lys and Thr as the second and third limiting amino acids, respectively. In order to improve growth of animals, it is a common practice to supplement these limiting amino acids in the diet. There are Lys, Met, Thr, Trp, and Val commercially available as feed additives.

protein synthesis and degradation

Continuous synthesis and degradation of proteins (protein turnover) occurs in an animal body. A protein has a relatively short life compared with other nutrients in an animal body (such as fat). With a short life or a fast turnover, it is feasible to regulate concentrations of specific proteins such as enzymes and peptide hormones for the rapid adaptation to different metabolic situations. Hepatocytes (liver cells) may need a completely different set of enzymes in response to changing metabolic status. For example, the metabolic situation may require a change from gluconeogenesis to glycolysis within a short time period. Enzymes and peptide hormones are good examples of proteins with a short life whereas structural proteins in muscle and connective tissues are good examples of proteins with relatively long life. Gain or loss of protein in the body is a result of the net balance between protein synthesis and degradation. Protein synthesis occurs on the ribosomes in the cytoplasm. Proteins are synthesized with a particular amino acid sequence through the translation of information encoded in messenger RNA (mRNA), which is called codon. Amino acids are specified by codons in the mRNA. This process requires adapter molecules, the transfer RNA (tRNA), which recognizes specific codons and inserts amino acids into their appropriate sequential positions in the polypeptide.

ideal protein

Dietary proteins that can provide an amino acid ratio that matches the animal's needs

PEPT

Dipeptides and tripeptides are absorbed into enterocytes by peptide transporters called 'PEPT'. After digestion and absorption, amino acids will be delivered to target cells.

functional amino acids

During protein synthesis and degradation, excess amino acids will be deaminated for oxidation (catabolism). However, some amino acids can be converted to other metabolites for specific functions in the body. Good examples are Trp, Met, Phe, Arg, and Gln

role of functional amino acids in animal feed

During protein synthesis and degradation, excess amino acids will be deaminated for oxidation (catabolism). However, some amino acids can be converted to other metabolites for specific functions in the body. Good examples are Trp, Met, Phe, Arg, and Gln

procedure of absorption of amino acids and small peptides

End products of protein digestion are amino acids, dipeptides, tripeptides, oligopeptides, and polypeptides. Among them, amino acids, dipeptides, and tripeptides can be absorbed into enterocytes in the small intestine, whereas oligopeptides and polypeptides would pass to large intestine. There are at least 15 types of Systems and each System would transport specific amino acids. The process of amino acid absorption by Systems is either sodium dependent transport requiring energy or facilitated diffusion. Dipeptides and tripeptides are absorbed into enterocytes by peptide transporters called 'PEPT'. After absorption into the enterocytes, dipeptides and tripeptides are digested to free amino acids by dipeptidase and tripeptidase enzymes synthesized in the enterocytes. The majority (at least 50%) of total absorption is done by PEPT.

limiting amino acids

If a certain amino acid is deficient at the time of protein synthesis, that amino acid becomes a limiting factor for the synthesis of that specific protein.

translation

Proteins are synthesized with a particular amino acid sequence through the translation of information encoded in messenger RNA (mRNA), which is called codon

lipogenesis

The remaining carbon skeleton can also be used for synthesis of lipids

d. Ideal protein

What is it called when dietary proteins provide an amino acid ratio that matches to the animal's needs? Select one: a. Best protein b. Perfect protein c. Required protein d. Ideal protein

a. Codon

What is the name of genetic information contained in messenger RNA that is copied from DNA? Select one: a. Codon b. transferRNA c. Genome d. GMO Feedback

ideal protein in animal feeding

When the ratio among available amino acids matches the needs for protein synthesis, then protein synthesis can be maximized. When the ratio does not match the needs for protein synthesis, all the excess amino acids will be catabolized (oxidized or deaminated). Dietary proteins that can provide an amino acid ratio that matches the animal's needs are called ideal protein. Use of the ideal protein concept in formulating diets for animals can maximize protein synthesis in the body and minimize unneeded amino acid catabolism, resulting in reduced feed cost and nitrogen waste to the environment.

c. Leucine

Which amino acid is considered ketogenic when degraded? Select one: a. Asparagine b. Methionine c. Leucine d. Histidine

b. Chief cell

Which cell secretes pepsinogen in the stomach? Select one: a. Parietal cell b. Chief cell c. Pacreatic alpha-cell d. Oxyntic cell

c. Chymotrypsin

Which enzyme digests dietary proteins producing peptides ending with aromatic amino acids (Phe, Trp, and Tyr) at the carboxyl ends? Select one: a. Elastase b. Trypsin c. Chymotrypsin d. Amylase

c. An aldehyde group

Which is NOT a common feature of an amino acid? Select one: a. Zwitterion at pH 7 b. A carbon center called α (alpha)-carbon c. An aldehyde group d. Amphoteric

a. Bicarbonate

Which is a secretion from pancreas neutralizing acid digesta in duodenum of the small intestine? Select one: a. Bicarbonate b. Hydrochloric acid c. Hydrosulfuric acid d. Ammonia

b. Lysine

Which is an essential amino acid? Select one: a. Glutamate b. Lysine c. Asparagine d. Serine

a. Lysine

Which is the first limiting amino acid for a pig fed corn and soybean meal base diets? Select one: a. Lysine b. Tryptophan c. Alanine d. Glutamine

System

Which is the name of a transporter of amino acids in enterocytes? Select one: a. PEPT b. GLUT c. SGLT d. System

hormone

called peptide hormones which are composed of amino acids or amino acid derivatives. These include insulin, glucagon, cholecystokinin, leptin, and somatostatin.

trypsin

is secreted in an inactive form, trypsinogen, which is activated by an enzyme called enterokinase secreted from the mucosa of the small intestine. Activated trypsin also further activates trypsinogen to trypsin. Trypsin digests dietary protein and products of pepsin and rennin digestion producing peptides ending with basic amino acids (Arg, His, and Lys) at the carboxyl ends.

transcription

mRNA is produced from DNA transcription in the nucleus of a cell.

procedure of protein digestion

major goal of protein digestion is to break down dietary protein into single amino acids for absorption. stomach - Pepsin is the major proteolytic enzyme secreted from the stomach Digesta is slowly released to duodenum where bicarbonate (HCO3-) is secreted from the pancreas to neutralize acidic digesta. There are three key proteolytic enzymes secreted from the pancreas. Trypsin is secreted in an inactive form, trypsinogen, which is activated by an enzyme called enterokinase secreted from the mucosa of the small intestine. Activated trypsin also further activates trypsinogen to trypsin. Trypsin digests dietary protein and products of pepsin and rennin digestion producing peptides ending with basic amino acids (Arg, His, and Lys) at the carboxyl ends. Chymotrypsin is secreted as an inactive form, chymotrypsinogen, which is activated by trypsin. Chymotrypsin digests native (in its original form) protein and products of pepsin and rennin digestion producing peptides ending with aromatic amino acids (Phe, Trp, and Tyr) at the carboxyl ends. Elastase is secreted in an inactive form, proelastase, which is activated by trypsin. Elastase will digest native protein and products of pepsin and rennin digestion producing peptides ending with aliphatic amino acids (Ala, Gly, Ile, Leu, and Val) at the carboxyl ends.

pepsin

major proteolytic enzyme secreted from the stomach. Pepsin is secreted in an inactive form called pepsinogen from chief cells (also called peptic cells) in the fundic glands (also called gastric glands) in the fundic region of the stomach. Pepsinogen will be activated to pepsin under acidic pH (1.5 to 2.5) when hydrochloric acid (HCl) is secreted from the parietal cells (also called oxyntic cells) in the fundic glands. Pepsin will digest dietary protein to polypeptides which can be further digested as they pass to the small intestine. For young animals, a major enzyme for protein digestion in the stomach is rennin which digests milk protein under acidic pH (3.5 to 4.9).

codon

messenger RNA (mRNA) 3 letter codes; Messenger RNA contains genetic information, called codons, transcribed from DNA. Codons consist of three bases each. There are specific codons to initiate translation (called start codon) and to end the translation (called stop codon). Ribosomal RNA (rRNA) attaches to mRNA and allows tRNA to bind to mRNA. There are 20 types of tRNA corresponding to each of the 20 amino acids. Each tRNA binds to its corresponding amino acid and transfers it to rRNA.

chymotrypsin

secreted as an inactive form, chymotrypsinogen, which is activated by trypsin. Chymotrypsin digests native (in its original form) protein and products of pepsin and rennin digestion producing peptides ending with aromatic amino acids (Phe, Trp, and Tyr) at the carboxyl ends.

elastase

secreted in an inactive form, proelastase, which is activated by trypsin. Elastase will digest native protein and products of pepsin and rennin digestion producing peptides ending with aliphatic amino acids (Ala, Gly, Ile, Leu, and Val) at the carboxyl ends.

nonessential amino acids

synthesized in the body in adequate amount and include alanine (Ala), aspartate (Asp), asparagine (Asn), cysteine (Cys), glutamate (Glu), serine (Ser), and tyrosine (Tyr).

glycolysis

the breakdown of glucose by enzymes, releasing energy and pyruvic acid.

essential amino acids

those not synthesized in an animal body thus they are needed from dietary sources. Essential amino acids include histidine (His), isoleucine (Ile), leucine (Leu), lysine (Lys), methionine (Met), phenylalanine (Phe), threonine (Thr), tryptophan (Trp), and valine (Val).