KE7001 Biochemistry Lectures

Co-factors, vitamins. Carbohydrates

Reading material: Stryer, Chapter 8.1.1, 8.6, 11

Abstract:
Cofactors
Many enzymes require further components than the amino acid residues of the protein polypeptide chain for their activity. They depend on small molecules called cofactors. Such an enzyme without the cofactor is called an apoenzyme; the complete catalytically active form is called a holoenzyme. Cofactors can be subdivided into two groups: metals and small organic molecules. Loosely bound metal ions may be needed to stabilise the protein structure or to help binding substrates in the right conformation. Some enzymes depend on the reactivity of metal ions that are firmly bound at their active sites, for example iron in many redox enzymes and zinc in carbonic anhydrase. Cofactors that are small organic molecules are called coenzymes. They are further divided into cosubstrates that are loosely bound and prostethic groups that are firmly bound, often covalently. A cosubstrate participates in the reaction and is chemically altered. After the reaction it leaves the enzyme and is regenerated by another enzyme. ATP and NAD⁺ are examples of universal cosubstrates. A prostethic group also participates in the reaction, but does not leave, meaning the enzyme must be able to catalyse also the regeneration of the prostethic group. Examples are FAD and heme.

Vitamins
Vitamins are small biomolecules that are needed in small amounts in the diet of higher animals. The vitamins are classified in two major groups: water soluble and fat soluble. The water soluble are vitamin C, ascorbate, which acts as an antioxidant, and the B-vitamins that are precursors for coenzymes, e.g. thiamine (B₁), riboflavin (B₂) and niacin. The enzyme complex pyruvate dehydrogenase, which is involved in the conversion of pyruvate to acetyl CoA, needs coenzymes derived from four different vitamins. Fat-soluble vitamins, A, D, E, K, participate in diverse processes such as blood clotting and vision.

Carbohydrates
Carbohydrates are the most abundant biomolecules on earth. First, carbohydrates serve as energy stores, fuels and metabolic intermediates. Certain carbohydrates are staple of the human and animal diet, and the oxidation of carbohydrates is the central energy-yielding pathway in most nonphotosyntetic cells. Second, ribose and deoxyribose form part of the structural framework of RNA and DNA. Third, polysaccharides are structural elements of cell walls of bacteria and plants. Fourth, carbohydrates are linked to many proteins and lipids, and mediate the interactions of cells with each other and with the environment.
There are three major classes of carbohydrates: 1) monosaccharides, or simple sugars, consist of a single polyhydroxy aldehyde or ketone unit. 2) Oligosaccharides consist of short chains of monosaccharide units joined together with glycosidic linkages. The most abundant oligosaccharides are the disaccharides, with two monosaccharide units. 3) Polysaccharides consist of long chains having hundreds or more of monosaccharides units. Some polysaccharides, such as e.g. cellulose, have linear chains of monosaccharides , whereas others, glycogen for example, have branched chains.

Key concepts:
Apoenzyme + cofactor(s) = holoenzyme
Coenzyme, cosubstrate, prostethic group
Vitamin C, ascorbate
B-vitamins, coenzyme precursors
Aldose, ketose
D/L isomer, enantiomer, anomer, tautomer
Cyclisation, hemiacetal, mutarotation
Pyranose, furanose, chair, boat
Glycosidic bond, acetal
Mono-, di-, oligo-, polysaccharide
Sucrose, lactose, maltose
Glycogen, starch, cellulose
Protein glycosylation

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