Proteins ~ Sugars ~ Nucleotides

Proteins are one of the three macronutrients, along with fats and carbohydrates, which we get in large amounts from the foods we eat. Proteins are crucial to survival as they perform several functions in the body. Along with water they are the main components of muscles, organs and other tissues. Therefore, they are necessary both to build and to repair tissues. Proteins are also a large component of antibodies, some hormones and enzymes. Since the function of enzymes is to regulate bodily processes, proteins are essentially indispensable in ensuring healthy functioning.

Proteins are large molecules which are made from a chain of amino acids. There are 22 different amino acids which can be found in proteins. Nine of these are classified as essential because they cannot be manufactured in the body and need to be acquired through the food we eat. The remaining 13 are classified as nonessential and can be manufactured by the body from other substances.

The body is constantly breaking down and building proteins. Therefore, amino acids continually enter the amino acid "pool" and are then taken up again to create new protein molecules. Some amino acids are also excreted, thereby depleting the pool. We need to get protein in our diet in order to replenish the "pool" as well as to provide amino acids for growth and repair. The body does not store excess protein and any amino acids that are not used up are converted to fat or are excreted.

Proteins are basic constituents in all living organisms. Their central role in biological structures and functioning was recognized by chemists in the early 19th century when they coined the name for these substances from the Greek word proteios, meaning "holding first place." Proteins constitute about 80 percent of the dry weight of muscle, 70 percent of that of skin, and 90 percent of that of blood. The interior substance of plant cells is also composed partly of proteins. The importance of proteins is related more to their function than to their amount in an organism or tissue. All known enzymes, for example, are proteins and may occur in very minute amounts; nevertheless, these substances catalyze all metabolic reactions, enabling organisms to build up the chemical substances--other proteins, nucleic acids, carbohydrates, and lipids--that are necessary for life.

Proteins are sometimes referred to as macromolecular polypeptides because they are very large molecules and because the amino acids of which they are composed are joined by peptide bonds. (A peptide bond is a link between the amino group [-NH2] of one amino acid and the carboxyl group [-COOH] of the next amino acid in the protein chain.) Although amino acids may have other formulas, those in protein invariably have the general formula RCH(NH2)COOH, where C is carbon, H is hydrogen, N is nitrogen, O is oxygen, and R is a group, varying in composition and structure, called a side chain. Amino acids are joined together to form long chains; most of the common proteins contain more than 100 amino acids.

Sugars are carbohydrates, an important source of energy for the body. Other carbohydrate-rich foods include fruits, root vegetables (including potatoes), rice, noodles and bread. However, before the carbohydrates in these foods can be used for energy, they must be digested and broken down into sugars. Sugars are carbohydrates: this means that they contain the elements Carbon, Hydrogen and Oxygen, and that there is twice as much Hydrogen as there is Oxygen. Hydrogen and Oxygen atoms are in the ratio of two to one as in water molecules. The simplest sugars are called monosugars or monosaccharides. The two shown on this page each have six atoms of Carbon so they are called hexose sugars. Deoxyribose (in DNA) and Ribose (in RNA) only have five atoms of Carbon so they are called pentose sugars.

Dietary carbohydrates also include the complex carbohydrates starch and fiber. During digestion all carbohydrates except fiber break down into sugars. Sugars and starches occur naturally in many foods that also supply other nutrients. Examples of these foods include milk, fruits, some vegetables, breads, cereals, and grains. Americans eat sugars in many forms, and most people like their taste. Some sugars are used as natural preservatives, thickeners, and baking aids in foods; they are often added to foods during processing and preparation or when they are eaten. The body cannot tell the difference between naturally occurring and added sugars because they are identical chemically.

In biochemistry, a sugar is the simplest molecule that can be identified as a carbohydrate. These are monosaccharides and disaccharides. Sugars contain either aldehyde groups (-CHO) or ketone groups (C=O), where there are carbon-oxygen double bonds, making the sugars reactive. Most sugars conform to (CH2O)n where n is between 3 and 7. A notable exception is deoxyribose, which as the name suggests is "missing" an oxygen. As well as being clasified by their reactive group, sugars are also classified by the number of carbons they contain. Derivatives of trioses (C3H6O3) are intermediates in glycolysis. Pentoses include ribose and deoxyribose, which are present in nucleic acids. Ribose is also a component of several chemicals that are important to the metabolic process, including NADH and ATP. Hexoses include glucose which is a universal substrate for the production of energy in the form of ATP. During photosynthesis plants produce glucose which is then stored as starch.

Many pentoses and hexoses are capable of forming ring structures. In these closed-chain forms the aldehyde or ketone group is not free, so many of the reactions typical of these groups cannot occur. Glucose in solution exists mostly as a ring at equilibrium, with less than 0.1% of the molecules in the open-chain form.

Monosaccharides in a closed-chain form can form glycosidic bonds with other monosaccharides, creating disaccharides, such as sucrose, and polysaccharides such as starch. Glycosidic bonds must be hydrolised or otherwise broken by enzymes before such compounds can be used in metabolism. The term "glyco-" indicates the presence of a sugar in an otherwise non-carbohydrate substance: for example, a glycoprotein is a protein to which one or more sugars are connected. Simple sugars include sucrose, fructose, glucose, dextrose,maltose and mannose.

Sucrose can be converted by hydrolysis into fructose and glucose, producing what is called inverted sugar. This resulting sugar is sweeter than the original sucrose, and is useful for making confections sweeter and softer in texture.

Nucleotides are monomers consisting of a phosphate group, a five carbon sugar(either ribose or deoxyribose) and a one or two ring nitrogen containing base. Nucleotides are building blocks of the nucleic acids, perhaps the most fundamental and important constituents of the living cell. The nucleic acids were probably the first biomolecules to evolve and life could only begin with their evolution because they are the only biological substances that carry the potential for self-duplication. The blueprint for a living organism is encoded into its DNA or RNA (the two types of nucleic acids) and is passed on from generation to generation. The key to understanding how this important exchange of information takes place was discovered in the structure of DNA, which scientists James Watson and Francis Crick first described in 1953. DNA exists in the form of a double helix, two strands of the unusual molecule winding around one another into a spiral. Each strand, as Watson and Crick discovered, is like a very long piece of string consisting of monomer nucleotides, each comprised of a deoxyribose sugar, a phosphate group, and a nitrogenous base. The number of different nucleotides in DNA is limited because there are only four different bases in the nucleic acid (adenine, guanine, cytosine, and thymine), but variations in the ordering of the bases results in the tremendous amount of diversity found among living organisms.

Similar to DNA, RNA is comprised of nucleotides, but the bases found in the organic molecule are slightly different than those in the other nucleic acid, uracil taking the place of thymine. Also, RNA nucleotides contain ribose sugars instead of deoxyribose. Ribose differs from deoxyribose by the presence of a hydroxyl group rather than a hydrogen atom attached to it 2' carbon atom. The number of phosphate groups contained in nucleotides may vary, but in both DNA and RNA only a single group is present per nucleotide. Each of these groups is linked to the sugar molecule of the adjacent nucleotide in the chain. It is also important to note that the nucleotides of one nucleic acid strand have a specific association with nucleotides in the corresponding strand. Due to the chemical affinity of the bases, in DNA nucleotides containing adenine are always paired with nucleotides containing thymine, and nucleotides containing cytosine are always paired with nucleotides containing guanine. In RNA, however, nucleotides containing uracil are paired with those that feature guanine. The complementary bases are linked to one another by weak chemical bonds known as hydrogen bonds.

Nucleotides are widely available in foods and easily obtained in the diet, but there are also biosynthetic pathways and salvage processes that ensure a complete balance of these important biochemicals. In addition to heredity, nucleotides are also involved in a number of intermediary metabolism biochemical reactions that have little or nothing to do with the storage of genetic information. ATP, for example, is heavily involved in most biosynthetic pathways as an important energy-storage and transfer molecule. Other purine and pyrimidine nucleotides and analogs are also involved in important biochemical reactions. Adenosine is coupled to a nicotinamide analog nucleotide to produce NAD and NADP, two coenzymes that are used quite frequently in electron-transfer biochemical mechanisms. Likewise, adenosine is also a part of the Coenzyme A molecule, an acetyl transfer reagent, and cobalamine (vitamin B-12), a methyl group donor to biochemical reactions. Other notable functions of nucleotides include their ability to act as structural substituents for important coenzymes and vitamins, and as metabolic regulators and signal molecules, making the substances a very important class of biomolecules.

Nucleotide names are abbreviated by first indicating whether it is a ribonucleotide (r) or deoxyribonucleotide (d), indicating the nitrogenous base included (G,A,T,C,U), the number of phosphates (Mono-, Di-, Tri-) and the presence of a phosphate (P). For example, deoxy-cytosine-triphosphate is abbreviated as dCTP. Nucleotides are the monomers of nucleic acids and also play important roles in cellular energy transport and transformations (notably ATP and NAD+/NADH) and in enzyme regulation (see for example, protein kinase).

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