Nutrition: concepts and controversies: Chapter 6

DRI recommendation for protein intake

0.8 gram for each kilogram (or 2.2 pounds) of body weight

denaturation

the irreversible change in a protein's folded shape brought about by heat, acids, bases, alcohol, salts of heavy metals, or other agents.

Amine group

the nitrogen- containing portion of an amino acid.

side chain

the unique chemical structure attached to the backbone of each amino acid that differentiates one amino acid from another.

Uses of amino acids in the body

▪ The amino acid can be used as is to build part of a growing protein. ▪ The amino acid can be altered somewhat to make another needed compound, such as the vitamin niacin. ▪ The cell can dismantle the amino acid in order to use its amine group to build a different amino acid. The remainder can be used for fuel or, if fuel is abundant, converted to glucose or fat.

Sickle-cell disease

hemoglobin, the oxygen-carrying protein of the red blood cells, is abnormal—is an example of an inherited variation in the amino acid sequence. Normal hemoglobin contains two kinds of protein strands. In sickle-cell disease, one of the strands is an exact copy of that in normal hemoglobin, but in the other strand, the sixth amino acid is valine rather than glutamic acid. This replacement of one amino acid so alters the protein that it is unable to carry and release oxygen. The red blood cells collapse from the normal disk shape into crescent shapes (see Figure 6-5). If too many crescent-shaped cells appear in the blood, the result is abnormal blood clotting, strokes, bouts of severe pain, susceptibility to infection, and early death.

antibodies

large proteins of the blood, produced by the immune system in response to an invasion of the body by foreign substances (antigens). Antibodies combine with and inactivate the antigens.

polypeptide

protein fragments of many (more than 10) amino acids bonded together (poly means "many"). A peptide is a strand of amino acids

Difference between protein and other macro nutrients

proteins contain nitrogen atoms in addition to the carbon, hydrogen, and oxygen atoms that all three energy-yielding nutrients contain. Another key difference is that in contrast to the carbohydrates—whose repeating units, glucose molecules, are identical—the amino acids in a strand of protein are different from one another.

Results of protein deficiency

slow growth in children, impaired brain and kidney functions, weakened immune defenses, and impaired nutrient absorption from the digestive tract.

edema

swelling of body tissue caused by leakage of fluid from the blood vessels; seen in protein deficiency (among other conditions).

nitrogen balance

the amount of nitrogen consumed compared with the amount excreted in a given time period.

What happens when protein is deficient in the body

the body slows its synthesis of proteins while increasing its breakdown of body tissue protein to liberate the amino acids it needs to build other proteins of critical importance.

Amino acids

the building blocks of protein. Each has an amine group at one end, an acid group at the other, and a distinctive side chain.

acidosis

the condition of excess acid in the blood, indicated by a below-normal pH (osis means "too much in the blood").

alkalosis

the condition of excess base in the blood, indicated by an above-normal blood pH

Protein composition

20 different amino acid units.

How Proteins Transport substances into and out of Cells

A transport protein within the cell membrane acts as a sort of two-door passageway—substances enter on one side and are released on the other, but the protein never leaves the membrane. The protein differs from a simple passageway in that it actively escorts the substances in and out of cells; therefore, this form of transport is often called active transport.

Final step in how amino acids build protein

Once coiled and folded, the protein may be functional as is, or it may need to join with other proteins, or add a carbohydrate molecule or a vitamin or mineral.

Roles of body protein

Provides Structure and Movement Builds Enzymes, Hormones, and Other Compounds Builds Antibodies Transports Substances Maintain Fluid and Electrolyte Balance Maintain Acid-Base Balance Blood Clotting

Protein digestion in the Stomach

Strong acid produced by the stomach denatures proteins in food. This acid helps to uncoil the protein's tangled strands so that molecules of the stomach's protein-digesting enzyme can attack the peptide bonds.

Importance of protein

The body needs dietary amino acids to grow new cells and to replace worn-out ones.

peptide bond

a bond that connects one amino acid with another, forming a link in a protein chain.

celiac disease

a disorder characterized by intestinal inflammation on exposure to the dietary protein gluten;

First step in how amino acids build protein

a peptide bond is formed between the amine group end of one amino acid and the acid group end of the next.

Inherited Amino Acid Sequences

a standard amino acid specified by heredity. Often, if a wrong amino acid is inserted, the result can be disastrous to health.

collagen

a type of body protein from which connective tissues such as scars, tendons, ligaments, and the foundations of bones and teeth are made.

gluten

a type of protein in certain grain foods that is toxic to the person with celiac disease.

Protein digestion in the Small intestine

alkaline juice from the pancreas neutralizes the acid delivered by the stomach. The pH rises to about 7 (neutral), enabling the next enzyme team to accomplish the final breakdown of the strands. Small Intestine Enzymes from the pancreas and the intestine split peptide strands into tripeptides, dipeptides, and amino acids. Enzymes on the surface of the small intestine's lining and within the absorptive cells split tripeptides and dipeptides. The intestinal cells absorb and transfer amino acids to the bloodstream.

factors that influence a protein's quality

amino acid composition and its digestibility.

essential amino acids

amino acids that either cannot be synthesized at all by the body or cannot be synthesized in amounts sufficient to meet physiological need. Also called indispensable amino acids.

Interactions among amino acids

amino acids whose side chains are electrically charged are attracted to water. Therefore, in the body's watery fluids, they orient themselves on the outside of the protein structure. The amino acids whose side chains are neutral are repelled by water and are attracted to one another; these tuck themselves into the center away from the body fluids. All these interactions among the amino acids and the surrounding fluids fold each protein into a unique architecture, a form to suit its function.

conditionally essential amino acid

an amino acid that is normally nonessential but must be supplied by the diet in special circumstances when the need for it exceeds the body's ability to produce it.

bases

compounds that accept hydrogens from solutions.

buffers

compounds that help keep a solution's acidity or alkalinity constant. blood proteins as buffers to maintain the blood's normal pH.

acids

compounds that release hydrogens in a watery solution.