Biochemistry for Sport and Exercise Metabolism

Donald MacLaren, James Morton

• Publisher: Wiley
• Year: 2011
• ISBN:
  9781119967828
• Edition: 1
• Topic: Medicine
• Subtopic: Sports Medicine

How do our muscles produce energy for exercise and what are the underlying biochemical principles involved? These are questions that students need to be able to answer when studying for a number of sport related degrees. This can prove to be a difficult task for those with a relatively limited scientific background. Biochemistry for Sport and Exercise Metabolism addresses this problem by placing the primary emphasis on sport, and describing the relevant biochemistry within this context.

The book opens with some basic information on the subject, including an overview of energy metabolism, some key aspects of skeletal muscle structure and function, and some simple biochemical concepts. It continues by looking at the three macromolecules which provide energy and structure to skeletal muscle - carbohydrates, lipids, and protein. The last section moves beyond biochemistry to examine key aspects of metabolism - the regulation of energy production and storage. Beginning with a chapter on basic principles of regulation of metabolism it continues by exploring how metabolism is influenced during high-intensity, prolonged, and intermittent exercise by intensity, duration, and nutrition.

Key Features:

• A clearly written, well presented introduction to the biochemistry of muscle metabolism.
• Focuses on sport to describe the relevant biochemistry within this context.
• In full colour throughout, it includes numerous illustrations, together with learning objectives and key points to reinforce learning.

Biochemistry for Sport and Exercise Metabolism will prove invaluable to students across a range of sport-related courses, who need to get to grips with how exercise mode, intensity, duration, training status and nutritional status can all affect the regulation of energy producing pathways and, more important, apply this understanding to develop training and nutrition programmes to maximise athletic performance.

Part One

Basic Muscle Physiology and Energetics

Chapter 1

Energy Sources for Muscular Activity

Learning outcomes

After studying this chapter, you should be able to:

• outline the key energy sources for exercise;
• distinguish between anaerobic and aerobic sources of energy;
• describe the essential structure of ATP;
• draw and explain the components of the energy continuum;
• describe the role of PCr in ATP synthesis;
• explain how PCr is resynthesized;
• describe the involvement of carbohydrates and fats as energy sources for exercise;
• explain reasons why an athlete is unable to sprint a marathon;
• describe the amounts and sources of energy in the body and their rates of energy formation;
• discuss how amino acids can be used as an energy source during exercise.

This chapter presents a brief overview of the energy sources used by muscles in order to engage in various activities. It is a 'taster' that will (hopefully) encourage you to delve a bit more deeply into the basic biochemistry of the macronutrients which provide energy, as well as to gain an understanding of the likely regulation of the processes which produce energy. From this perspective, this chapter examines the energy-yielding processes from a superficial level in addressing issues of energy for sprinting and for more prolonged events.

Key Words

adenosine diphosphate (ADP)

aerobic

anaerobic

anaerobic alactic

anaerobic glycolytic

anaerobic glycolysis

anaerobic lactic

adenosine triphosphate (ATP)

carbohydrate

CK (creatine kinase)

creatine

dephosphorylation

energy continuum

fatty acid

glycogen

glycogenolysis

glycolysis

lipid

lipolysis

mitochondria

oxidation

PCr (phosphocreatine)

phosphorylation

protein

protein degradation

protein synthesis

1.1 Adenosine Triphosphate: The Energy Currency

In order for our muscles to contract and provide movement, energy is required. Such energy is provided by adenosine triphosphate (ATP) and is the only energy capable of being used for muscle contraction in humans. Figure 1.1 provides the structure of an ATP molecule. As you can see from this diagram, ATP consists of a base (adenine) attached to a sugar (ribose), to which is attached three phosphate molecules. The phosphates are attached by 'high energy' bonds which, when removed, provide energy.

The process is reversible, which means that ATP may be re-formed from adenosine diphosphate (ADP) as long as there is sufficient energy to restore the missing phosphate molecule on to the ADP. The latter can be achieved by phosphocreatine (PCr) or by processes such as anaerobic glycolysis, and aerobic processes.

Figure 1.1 Adenosine triphosphate (ATP)

The stores of ATP in muscle tissue are rather limited, so there is a constant need to resynthesize it for survival, let alone movement. The amount of ATP in a muscle cell amounts to 25 mM/kg dry muscle or about 40–50 g in total, which is sufficient to enable high intense activity for around 2–4 seconds if it is the only useable source of energy available. This is not a great amount—hence the importance of resynthesis of ATP at rates sufficient to enable appropriate levels of exercise to ensue, i.e. fast rates of resynthesis for sprinting and slower rates for prolonged exercise.

1.2 Energy Continuum

The major energy sources for exercise are dependent on the intensity and duration of the activity. Examination of Figure 1.2 highlights that there appears to be three such sources, i.e. PCr, glycolytic and aerobic. These energy-producing processes predominate exercise from 1–10 seconds, 10–60 seconds and beyond 60 seconds respectively.

Figure 1.2 Energy continuum

Another way of expressing the energy continuum is represented in Figure 1.3, which shows the major energy sources for running events of varying distances. Note that short, highly intense sprinting bouts lasting 1–10 seconds use PCr predominantly, while events such as the 400 metres mainly use anaerobic glycolysis, and thereafter aerobic metabolism predominates.

Figure 1.3 Primary energy sources for different running distances

1.3 Energy Supply for Muscle Contraction

ATP is not stored to a great degree in muscle cells. Therefore, once muscle contraction starts, the regeneration of ATP must occur rapidly. There are three primary sources of ATP; these, in order of their utilization, are PCr, anaerobic glycolysis and aerobic processes.

Energy from ATP derives from cleaving the terminal phosphate of the ATP molecule. The resulting molecule is adenosine diphosphate (ADP). Phosphocreatine converts ADP back to ATP by donating its phosphate in the presence of the enzyme creatine kinase (CK), and in turn the PCr forms creatine (Cr), i.e. the dephosphorylated form of PCr.

The reaction of PCr with ADP to form ATP is very rapid, but is short-lived since the cell does not store high amounts of PCr (the muscle concentration of PCr is about 80 mM/kg dry muscle or 120 g in total). However, during short, high-intensity contractions, PCr serves as the major source of energy. This form of energy generation is often called anaerobic alactic, because it neither produces lactic acid nor requires oxygen. It is of paramount importance in sports requiring bursts of speed or power, such as sprints of 1–10 seconds.

Figure 1.4 provides a schematic to show the synthesis of ATP from ADP using PCr at the muscle crossbridge, and also the regeneration of PCr from Cr by ATP at the mitochondria.

Figure 1.4 PCr shuttle

Thus, Cr is formed from PCr during intense bouts of exercise, while Cr is re-phosphorylated to PCr by ATP produced in the mitochondria during aerobic recovery. Oxygen is needed for recovery of PCr, as can be seen in Figure 1.5, which clearly demonstrates that recovery of exercise-depleted PCr only happens when the blood supply to the exercising muscle is not occluded, i.e. there is an intact blood supply taking oxygen to the cells. If the blood supply is occluded (e.g. via a tourniquet), then PCr resynthesis fails. As a consequence, you should appreciate the need for a low level (so called active) recovery in between bouts of intense exercise.

Figure 1.5 Resynthesis of PCr after exercise with and without an occluded blood supply (adapted from Hultman et al., 1990)

The enzyme CK, which regulates PCr activity, exists in a number of forms known as isoforms (this will be dealt with later). Note that not only is there a CK which favours the formation of ATP from PCr, but there is also another form, CK

, which is present at the mitochondria...