An exception for mammals is the ability of nursing animals to digest lactose. Note that this ability is generaly lost at the age of weaning, at which time the animal becomes lactose intolerant.
Can have both homo- and heteropolysaccharides. [Table 8.2] We will focus on homopolysaccharides as most central, but will mention some heteropolysaccharides to illustrate their functions. Homopolysaccharides have a single type of residue. Most common polysaccharides contain glucose. Used for energy (food) storage (starches and glycogen) and structure (cellulose).
Starch (energy storage in plants). Two kinds
Glycogen: animal starch. Just like amylopectin, but more highly branched (every 8-12 residues). This allows more free ends for more rapid breakdown-important in animals.
Cellulose: beta-1,4 linkages, thus resistant to breakdown (including acid hydrolysis) as want for structure (don't want to digest self). [Figure 8.25] Multiple, extended strands come together as fibrils held together with H-bonds [Figure 8.26], laid down in cell wall in criss-cross pattern, glued together with polyalcohols (lignin).
Chitin: Serves similar role to cellulose, but in animals (crustaceans and insects), fungi, and some algae. Homopolymer of N-acetyl-D-glucosamine. Like cellulose , it has beta-1,4 linkages, and is thus resistant to breakdown. [Figure 8.27]
Among the heteropolysaccharides are the glycans such as Hyaluronic acid, an alternating polysaccharide of D-glucuronic acid and N-acetyl-D-glucosamine; [Figure 8.28] MW to 5,000,000 which serves as a lubricant in joints and is a component of the vitreous humor. Again we see beta-1,4 linkages.
Also very important are the glycans conjugated to proteins and peptides to give proteoglycans. [Figure 8.29]
Catabolism: degradation of molecules to provide energy.
Anabolism: reactions using energy to synthesize new molecules for growth etc.
Sequences of consecutive enzyme catalysed reactions which are readily studied and traced. A more rational definition is that of Newsholme and Leach (Biochemistry for the Medical Sciences, Wiley, 1983: pg.42)
"[A] metabolic pathway is a series of enzyme-catalyzed reactions, initiated by a flux-generating step and ending with either the loss of products to the environment, to a stored product (a metabolic 'sink') or in a reaction that precedes another flux-generating step (that is, the beginning of the next pathway)." Where a flux generating step is a non-equilibrium reaction that generates the flux going through the pathway and to whose rate all other reactions of the pathway conform. Note that by this definition some pathways may be inter-organ while others may take place in single compartment. We will explore this definition/concept as we look at metabolism.
The flux through a metabolic pathway is invariably controlled or regulated, most commonly by Feedback Inhibition, but also through Feed-forward activation. Regulation is one of the things that makes biochemistry "biological" and it will be a focus in our study.
© R. A. Paselk 2010;
Last modified 6 March 2013